Dynamics of hydrogen absorption into the N(111) bulk : spectroscopic identification and chemistry of subsurface hydrogen

DYNAMICS OF HYDROGEN ABSORPTION INTO THE Ni(111) BULK: SPECTROSCOPIC IDENTIFICATION AND CHEMISTRY OF

SUBSURFACE HYDROGEN

by

ANDREW DAVID JOHNSON B.Sc. University of East Anglia

(1985)

SUBMITTED TO THE DEPARTMENT OF CHEMISTRY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY February (1991)

@Massachusetts Institute of Technology, 1991 All rights reserved

Signature of AuthorSignature

redacted

Department of Chemistry

Signature redacted

Signature redacted

MASSACHUSEMIS 1fST 1TE

OF TECHN'' OGY A

1991

UBFMIQ-S

This thesis has been examined by a committee of the Department of Chemistry as follows:

Signature redacted

Professor Sylvia T. Ceyer

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_____

_,_ ______

_

Thesis Supervisor

Professor Irwin Oppenheim

_______________

Signature redacted

___ _

/1

Signature redacted

Professor Robert J. Silbey

---v ...

DYNAMICS OF HYDROGEN ABSORPTION INTO THE Ni(111) BULK: SPECTROSCOPIC IDENTIFICATION AND CHEMISTRY OF

SUBSURFACE HYDROGEN by

ANDREW DAVID JOHNSON

Submitted to the Department of Chemistry on February 5, 1991 in partial fulfillment of the requirements for the Degree of Doctor of Philosophy

in Chemistry ABSTRACT

Two mechanisms are reported whereby hydrogen is absorbed into the bulk of nickel: direct absorption of gas phase hydrogen atoms and the collision induced absorption of hydrogen chemisorbed on the Ni(111) surface. Absorption sites beneath the

Ni(111) surface are populated with hydrogen atoms by exposure of the crystal at 130

K to atomic H. The subsurface or bulk H is characterized by a new loss feature at

800 cm-1 in the HREEL spectrum. This loss feature at 800 cm-1 is assigned to a vibrational mode of subsurface hydrogen on the basis of the isotope frequency shift and the electron energy dependence exhibited by the intensity of this feature. Because the bulk vibration is excited by the short range impact scattering mechanism, the electrons penetrate the crystal and hence the intensity of this feature is determined by the electron mean free path in nickel. The intensity of the 800 cm-1 loss feature decays monotonically to zero as the electron impact energy is increased from 8 to 20 eV with the same energy dependence exhibited by the electron mean free path in nickel. Above an electron energy of 20 eV, the intensity of the bulk hydrogen mode is zero because plasmon excitation is the dominant electron energy loss mechanism. Absorbed H atoms are believed to occupy the bulk octahedral interstices of nickel. The frequency shift from 850 cm- to 800 cm-1 as the absorbance is increased from 0.4 ML to 1.0 ML is shown to be a result of a 5% expansion of the fcc nickel lattice, which accompanies the hydrogen absorption. As much as 8 equivalent monolayers have been absorbed into the nickel bulk. Subsurface hydrogen atoms recombine and desorb as molecular H2 between 150 K

and 220 K.

Subsurface hydrogen is shown to be produced by another mechanism: collision induced absorption. The impacts of 2.5 to 6.0 eV Xe, Kr and Ar atoms are observed to induce the bulk absorption of hydrogen atoms that are chemisorbed on the Ni(111) surface. The hydrogen so absorbed is characterized by HREELS and H2

thermal desorption experiments and the results are in agreement with those following the absorption of gas phase H atoms. The cross section for collision induced absorption has been measured as a function of the kinetic energy, angle of incidence and mass of the inert gas atoms. It is found that, above a threshold of 2.5 eV, the collision induced absorption cross section scales linearly with the energy associated with the inert gas atom momentum normal to the surface. For all incident energies, the Xe atom impacts are more efficient than those of Kr or Ar atoms at inducing hydrogen absorption. This dynamical behavior suggests a mechanism in which gas-solid energy transfer is important. The impacted surface atom is driven into the lattice, opening up pathways for the adsorbed H to diffuse into the subsurface sites.

A kinematic model, which extends the hard cube model to include multiple collisions

and energy dissipation to the lattice, quantifies the amount of gas-solid energy exchange. There is good agreement between model predictions and the experimental results.

The impacts of Xe atoms are also observed to induce the recombinative desorption, as molecular H2, of hydrogen atoms chemisorbed on Ni(111). The cross section for

collision induced desorption increases by a factor of four as the Xe kinetic energy is increased from 4.5 to 6.0 eV and predominates when the Xe atoms are incident 350 away from the surface normal. Collision induced desorption verifies that the loss feature at 800 cm~1 in the HREEL spectrum following atomic H absorption is a bulk vibrational mode. Because the bulk H is not subject to the Xe atom flux, the 800

cm-1 loss feature is unperturbed by the Xe atom bombardment.

The chemistry of subsurface hydrogen has been investigated. In particular, it is shown that subsurface hydrogen is responsible for the hydrogenation of chemisorbed

CH3 and subsequent methane formation over nickel. It is proposed that the absence

of bulk hydrogen at low pressure can explain why some hydrogenation reactions, which are facile at high pressure, fail to proceed under UHV conditions.

A mechanism for CO bond activation has been identified: collision induced

dissociation. The impacts of energetic Xe atoms are observed to induce the dissociation of CO that is molecularly adsorbed on the Ni(111) surface. The collision induced dissociation cross section is largest, not when Xe is incident along the surface normal but for a 200 angle of incidence, suggesting that the origin of the dissociation barrier is a requisite tilting of the CO molecule which is bound upright on the surface.

Thesis Supervisor: Sylvia T. Ceyer

TABLE OF CONTENTS

ABSTRACT... 3

List of Figures ... 10

List of Tables ... ... 15

Dedication and Acknowledgement ... 16

Introduction ... 17

Chapter 1: Bulk Absorption of Hydrogen by Ni(111): Detection of Subsurface Vibrational Modes by HREELS ... .19

1.1 INTRODUCTION ... 20

1.2 EXPERIMENTAL APPARATUS ... 27

1.2.1 Overview of the Molecular Beam - Ultra High Vacuum Chamber Apparatus ... 27

1.2.2 Generation of a Hydrogen Atom Source ... 40

1.2.2.1 Determination of the H Atom Flux ... .45

1.3 EXPERIMENTAL PROCEDURE AND RESULTS ... .48

1.3.1 Observation of Hydrogen Absorption into the Bulk of Nickel .... 49

1.3.1.1 Experimental Procedure for Observing Bulk Hydrogen Absorption ... 49

1.3.1.2 Control Experiments Eliminating Artifactual or Additional Sources of the Enhanced H2 Uptake ... 55

1.3.2 Spectroscopic Identification of Subsurface Hydrogen ... .59

1.3.2.1 Detection of Subsurface Hydrogen by HREELS ... 61

1.3.2.2 Deuterium Isotope Frequency Shift ... .63

1.3.3 Measurement of the Probability of Atomic Hydrogen Absorption into the Subsurface Sites of Ni(111) ... . ... ... . 64

1.3.3.1 Procedure for Measuring the Absorbance as a Function of Exposure ... ... 66

1.3.3.2 TDS as a Measure of the Absolute Amount of Hydrogen Absorbed ... ... ... ... .. 68

1.3.3.3 Calculation of the Hydrogen Absorption Probability ... .71

1.3.3.4 Effect of Crystal Temperature on the Absorption Probability: Measurement of the Absorption Probability at a Crystal Temperature

6

of 75 K ... 73 1.3.4 The Mechanism for the Recombinative Thermal Desorption of

Subsurface Hydrogen ... 74

1.3.4.1 Displacement of Surface Adsorbed Deuterium by Atomic H ydrogen ... 76

1.3.4.2 Recombinative Desorption of Subsurface Deuterium Through a Monolayer of Surface Hydrogen ... 77 1.4 DISCUSSION ... 83 1.4.1 Assignment of the HREEL spectrum ... 85

1.4.1.1 Electron Scattering Mechanism: Angular Profile of the 800 cm-1 Loss Intensity ... 85

1.4.1.2 The Probe Depth of HREELS: The Dependence of the HREEL Intensity on Electron Impact Energy ... 88

1.4.1.3 The Geometry of the Bulk Absorption Site ... 99

1.4.1.4 The Symmetry of the Bulk Absorption Site: The Selection Rules for HREELS ... 102

1.4.1.5 Comparison to Incoherent Inelastic Neutron Scattering Studies of Transition Metal Hydrides . .. .. . . ... ... 103 1.4.2 The Extent of Lattice Expansion Following Hydrogen Absorption: The

Absorbance Dependence of the Subsurface Hydrogen Vibrational

Frequency ... 106

1.4.2.1 The Surface Structure of Ni(111) Following Hydrogen Absorption ... 112

1.4.3 Estimation of the Electron Inelastic Mean Free Path in Nickel ... 114 1.4.3.1 The Intensity of the Bulk Hydrogen Loss Feature as a Function

of Absorbance ... 114 1.4.3.2 Estimation of the Electron Mean Free Path from the Plot of

HREELS Intensity Versus Hydrogen Absorbance ... .117

1.5 CONCLUSIONS ... 120

1.6 REFERENCES ... . . . . .... . .... . .... 121

Chapter 2: Collision Induced Absorption of Chemisorbed Hydrogen by Ni(111)125

2.1 INTRODUCTION ... . ... .. . 126 2.2 EXPERIMENTAL APPARATUS ... . ... .131 2.3 EXPERIMENTAL PROCEDURE AND RESULTS ... 145

2.3.1 Observation of the Collision Induced Absorption of Surface Chemisorbed Hydrogen by Energetic Xe Atoms ... .146

2.3.1.1 HREELS Detection of Subsurface Hydrogen Following the Xe

Atom Bombardment of a Hydrogen Monolayer ... 146 2.3.1.2 Observation of Low Temperature Molecular H2 Desorption

Following Collision Induced Absorption: The Requirement of a Hydrogen Saturated Surface ... 153

2.3.1.3 Control Experiments Demonstrating That Energetic Xe Atoms Induce the Bulk Absorption of Surface Adsorbed Hydrogen ... .157

2.3.2 Comparison of Collision Induced Absorption and Atomic Hydrogen Absorption ... 159 2.3.2.1 Recombinative Desorption of Subsurface Hydrogen ... .160 2.3.2.2 HREEL Spectroscopy of Subsurface Hydrogen ... 163 2.3.3 The Dynamic Exchange of Subsurface and Surface Hydrogen During

the Xe Atom Bombardment ... 167 2.3.3.1 Control Experiments Demonstrating that the Xe Atom

Bombardment has Induced the Diffusion of Bulk Hydrogen to the Surface ... 171

2.3.4 Measurement of the Cross Section for Collision Induced Absorption: Dependence on Xenon Kinetic Energy and Angle of Incidence ... 174 2.3.4.1 Procedure for Measurement of the Collision Induced Absorption

Cross Section: Maintaining a Hydrogen Monolayer During the Bombardment ... 174 2.3.4.2 Dependence of ECI on the Xenon Kinetic Energy and Angle of

Incidence ... 185 2.3.4.3 Effect of the Colliding Partner Mass: Collision Induced

Absorption of Surface Hydrogen by Energetic Krypton and Argon A tom s ... 187

2.3.4.4 Absence of an Isotope Effect to zcA: The Collision Induced Absorption of Surface Deuterium ... 192

2.4 DISCUSSION ... 193 2.4.1 The Mechanism for Collision Induced Absorption of Surface Adsorbed

Hydrogen ... 195

2.4.1.1 Summary of the Experimental Results ... .195

Mechanism: The Importance of Gas - Solid Energy Transfer . ... 196

2.4.2 Kinematic Model of Gas - Solid Energy Exchange: Extension of the Hard Cube Model to the Scattering of Massive Gas Atoms from a Solid Substrate ... 203

2.4.3 Comparison of the Kinematic Model Predictions with The Experimental Results ... 212

2.4.3.1 Quantitative Comparison ... 212

2.4.3.2 The Mechanism for Collision Induced Absorption ... 221

2.5 CONCLUSIONS ... 224

2.6 REFERENCES ... 225

Chapter 3: Collision Induced Recombinative Desorption of Chemisorbed Hydrogen ... 227

3.1 INTRODUCTION ... 228

3.2 EXPERIMENTAL APPARATUS ... 230

3.3 EXPERIMENTAL PROCEDURE AND RESULTS ... 234

3.3.1 Observation of the Collision Induced Recombinative Desorption of Chemisorbed Hydrogen ... 234

3.3.1.1 Depletion of the Hydrogen Monolayer During the Inert Gas Atom Bombardment ... 234

3.3.1.2 Control Experiments Demonstrating that the Hydrogen Monolayer is Depleted as a Result of Molecular H2 Desorption Induced by Xe Atom Bombardment ... 240

3.3.2 Measurement of the Collision Induced Recombinative Desorption Cross Section: Dependence on Xe Kinetic Energy and Angle of Incidence ... 243

3.3.2.1 Results of ECIRD at Low eH ... 246

3.4 DISCUSSION ... 251

3.5 CONCLUSIONS ... 260

3.6 REFERENCES ... 261

Chapter 4: The Chemistry of Subsurface Hydrogen: Catalytic Hydrogenation of CH3 Adsorbed on Ni(111) ... 263

4.2 EXPERIMENTAL APPARATUS ... 267

4.3 RESULTS AND DISCUSSION ... 269

4.3.1 Hydrogenation of Methyl Radicals by Subsurface Hydrogen ... .270

4.3.1.1 General Procedure for Synthesizing Methane under UHV C onditions ... 270

4.3.1.2 The Unreactivity of Surface Hydrogen with Adsorbed Methyl R adicals ... 279

4.3.2 Synthesis of the Deuterated Analogues of Methane: Reduction of Chemisorbed CH3 by Subsurface D Atoms ... 282

4.3.2.1 Reduction of Chemisorbed CH3 Radicals by Subsurface D A tom s .... ... 284

4.4 CONCLUSIONS ... 290

4.5 REFERENCES ... 291

Chapter 5: Collision Induced Activation of CO Dissociative Chemisorption on Ni(111) ... 293

5.1 INTRODUCTION ... 294

5.2 EXPERIMENTAL APPARATUS ... 298

5.3 RESULTS AND DISCUSSION ... 300

5.3.1 Observation of CO Dissociation Chemisorption ... 300

5.3.2 Observation of Atomic Carbon and Oxygen Recombinative Desorption Following the Collision Induced Dissociation of Chemisorbed CO ... 306

5.3.3 Measurement of the Collision Induced Dissociation Cross Section: Dependence on Xe Kinetic Energy and Angle of Incidence ... 308

5.4 CONCLUSIONS ... 318

10 List of Figures

Chapter 1

Figure 1 HREEL spectra of hydrogen chemisorbed on Ni(111): (a) 0.5 ML

coverage. (b) 1.0 ML coverage . ... 22

Figure 2 Potential energy diagram of the hydrogen - nickel interaction . . .. 26

Figure 3 Schematic diagram of the molecular beam -UHV apparatus ... .28

Figure 4 Auger spectrum of the clean Ni(111) crystal ... .39

Figure 5 Schematic diagram of the H atom source ... .41

Figure 6 Estimation of the H atom flux ... .47

Figure 7 Thermal desorption spectrum following a molecular H2 exposure . 50 Figure 8 Series of thermal desorption spectra after an exposure to both atomic and molecular hydrogen ... 52

Figure 9 Absolute hydrogen absorbance as a function of H atom exposure . 54 Figure 10 Multiplexed 2, 3 and 4 amu thermal desorption spectra following exposure to both atomic H and D ... 57

Figure 11 Thermal desorption spectrum after exposure to a H2 molecular beam through the hot filament in front of the crystal ... .60

Figure 12 HREEL spectra after exposure to: (a) molecular H2. (b) atomic H. 62 Figure 13 HREEL spectra after exposure to: (a) H atoms. (b) D atoms. . . . . 65

Figure 14 Series of thermal desorption spectra following H atom exposures that are in the limit of zero absorbance ... 69

Figure 15 Bulk hydrogen absorbance as a function of H atom exposure .... 72

Figure 16 Thermal desorption spectra after exposing a surface deuterium monolayer to 3.1x1015 H atoms cm-2 . . . 78

Figure 17 H2, HD and D2 thermal desorption spectra after a D then H atom exposure: 1 ML D is absorbed in bulk and 1 ML H is surface adsorbed ... . 80

Figure 18 (a) HREEL spectrum following D then H atom exposure so that 1 ML D is absorbed and 1 ML H is adsorbed. (b) After heating to 240 K. 81 Figure 19 HREELS angular profiles of the subsurface H and surface H vibrational modes . ... 87

Figure 20 Intensity of the surface and bulk hydrogen features in the HREEL spectrum as a function of electron impact energy . ... 91 Figure 21 Calculated HREELS intensity of bulk hydrogen as a function of

electron impact energy ... 95

Figure 22 Geometry of hydrogen occupying: (a) Ni(111) surface 3-fold hollow site, (b) subsurface octahedral site and (c) subsurface tetrahedral site . .. . .. . . .... ... . .. .. ... ... . .. ... .. .. .. . . . . .. . 100

Figure 23 Real space electron scattering geometry for the HREEL spectra of this study ... 104 Figure 24 HREEL spectra after exposing a hydrogen monolayer to a H atom

flux. Absorbances are: (a) 0.4 ML, (b) 0.5 ML and (c) 1.0 ML . . 107

Figure 25 LEED pattern of (a) the clean Ni(111) crystal and (b) following a bulk hydrogen absorbance of 1.2 ML ... 113

Figure 26 HREELS intensity of bulk hydrogen as a function of absorbance .. 116

Chapter 2

Figure 1 Potential energy diagram of the hydrogen - nickel interaction .... 128

Figure 2 Schematic diagram of the molecular beam - UHV apparatus ... .132

Figure 3 Xe atom time of flight (TOF) spectrum ... 139

Figure 4 Flux weighted kinetic energy distributions for the Xe atom beams of this study ... 141 Figure 5 (a) HREEL spectrum of a hydrogen monolayer. (b) after

bombardment by 138 kcal/mol Xe. (c) 0.5 ML surface coverage . . 147 Figure 6 H2 thermal desorption spectrum following the Xe atom bombardment

of a hydrogen monolayer ... ... .150

Figure 7 (a) HREEL spectrum of a H monolayer that has been bombarded with Xe. (b) after resaturation of the surface via a H2 exposure ... 152

Figure 8 (a) HREEL spectrum of a H monolayer that has been bombarded by Xe atoms (b) after raising the crystal temperature to 273 K ... .155 Figure 9 H2 thermal desorption spectrum after bombarding a H monolayer with

Xe atoms then resaturating the surface via a molecular H2 exposure 156

Figure 10 H2 thermal desorption as a function of the hydrogen absorbance

induced by Xe atom impacts ... 162

Figure 11 H2 thermal desorption as a function of absorbance following an atomic

H exposure ... ... 164 Figure 12 HREEL spectra after the production of subsurface hydrogen by: (a)

collision induced absorption (b) absorption of gas phase H atoms . 166

with D (b) after bombardment by 126 kcal/mol Xe ... 170

Figure 14 Net hydrogen absorbance as a function of H2 partial pressure during

the Xe bombardment ... 179

Figure 15 Thermal desorption spectra as a function of exposure to a 104 kcal/mol Xe atom beam ... 182

Figure 16 Absolute hydrogen absorbance versus 104 kcal/mol Xe atom exposure ... 184 Figure 17 Absolute cross-section for collision induced absorption as a function of

Xe normal energy ... 186

Figure 18 Absolute cross section for Ar, Kr and Xe collision induced absorption as a function of normal energy ... 189

Figure 19 Thermal desorption spectra following the Xe collision induced absorption of deuterium ... 194 Figure 20 Schematic diagram of the adsorbate collision mechanism ... 198

Figure 21 Schematic diagram of the substrate collision mechanism ... 202 Figure 22 Schematic diagram summarizing the kinematic model of gas - solid

energy transfer ... 206

Figure 23 Mass ratio Amin required for 2,3 and 4 gas-solid collisions versus the fraction of the Ni momentum retained during its recoils from the lattice ... 211 Figure 24 Calculated gas-solid energy transfer when the effective mass of Ni is

58.7 g ... 214 Figure 25 Calculated gas-solid energy transfer when the effective mass of Ni is

88.0 g ... 217

Figure 26 Calculated cross sections from the kinematic model compared to experimental results ... 219 Figure 27 Relative cross section for Xe and Ar collision induced absorption as a

function of kinetic energy ... 222 Chapter 3

Figure 1 H2 thermal desorption spectrum after exposuring a hydrogen

monolayer to 151 kcal/mol Xe atoms incident 400 away from normal 236 Figure 2 H2 partial pressure change during collision induced desorption of a

hydrogen monolayer by 151 kcal/mol Xe atoms ... 238

as a function of Xe kinetic energy ... 247 Figure 4 Absolute cross section for collision induced recombinative desorption

as a function of Xe angle of incidence and kinetic energy ... .248 Figure 5 Absolute cross section for collision induced recombinative desorption

at a coverage of 0.5 ML as a function of Xe angle of incidence ... 250 Figure 6 Schematic diagram of proposed collision induced recombinative

desorption mechanisms ... 254

Figure 7 Schematic diagram of impact parameters blocked by neighboring H atom s ... 255 Figure 8 HREEL spectra of (a) H monolayer in presence of 1.5 ML bulk H and

(b) after exposure to 138 kcal/mol Xe incident 400 from surface

norm al ... 259

Chapter 4

Figure 1 Thermal desorption spectrum following an atomic H and molecular H2

exposure ... . ... 272

Figure 2 CH4 and H2 thermal desorption spectra after absorbing 1.4 ML of H

into the bulk and depositing 0.15 ML of CH3 on the surface ... .277

Figure 3 16 amu and 15 amu thermal desorption spectra after absorbing 1.4 ML of H into the bulk and depositing 0.15 ML of CH3 on the surface . 278

Figure 4 Hydrogenation of CH3 by (a) subsurface H and (b) surface H ... . 280

Figure 5 CH4 and H2 thermal desorption spectra after depositing 0.15 ML of

CH3 and a saturation coverage of H on the surface ... . ... 283

Figure 6 HREEL spectrum after absorbing 1 ML of D into the bulk and depositing 0.15 ML of CH3 on the surface ... 286

Figure 7 4, 16, 17, 18, 19 and 20 amu thermal desorption spectra after absorbing

D (1.1 ML) into the bulk and depositing CH3 (0.15 ML) on the

surface ... . ... 288 Chapter 5

Figure 1 Potential energy diagram for the dissociative chemisorption of CO 297 Figure 2 C(272 eV) and Ni(848 eV) Auger emission following the Xe atom

bombardment of a CO monolayer ... 302

Figure 3 Thermal desorption spectrum of molecularly adsorbed CO .... . . 307 Figure 4 Thermal desorption spectrum following the Xe atom bombardment of

a CO m onolayer ... 309

Figure 5 Relative cross section for the collision induced dissociation of CO as a function of Xe kinetic energy ... 312

Figure 6 Relative cross section for the collision induced dissociation of CO as a function of the angle of incidence of the 151 kcal/mol Xe atom beam ... 313

Figure 7 Schematic diagram of a collision between a Xe atom and molecularly adsorbed CO ... 315

List of Tables Chapter 1

Table I Assignment of the (2x2)2H HREEL spectrum ... 21 Table II Optical mode frequencies of hydrogen atoms in bulk tetrahedral

Dedication and Acknowledgement

I would first like to thank Sylvia Ceyer for her guidance and support over the past

51/2 years. Sylvia was always ready to discuss problems that arose in the lab whether in the afternoon or early evening -2 am was probably the optimal time though. The opportunity to attend conferences and present papers I very much appreciated and this greatly enriched my graduate school career.

Much of this work was undertaken with Kevin Maynard. Kevin and I were not only good lab partners but together with his wife Anabella, formed a valued friendship. Many an enjoyable time was spent catching a movie, seeking an ever stronger curry or taking day trips. It is Kevin, who in a bold move, forced my concession in the great Hot and Sour soup battle. Sean Daley is to be thanked for his help in data collection and also for tolerating my maniacal phase of thesis terminus. It has been a pleasure to work with the other members of the group, Yulin Li, Dave Pullman, Julius Yang and Jerry Cain. Welcome also to the newest group members Ted Trautman and Art Utz. I wish the present team every success with their future endeavors, scientific and otherwise.

The group has now matured, with the emeritus members outnumbering the current members. I have been fortunate to know all those who have gone before. Special thanks to John Beckerle whose patient lessons on everything from computers and electronics to machining have served me well. Many a libation was enjoyed with Myung Lee and Dave Gladstone and I am glad we have kept in touch since their graduation. Thanks also to Qingyun Yang, Michelle Schulberg, Marianne McGonigal and, the present wearer of the green jacket, Ken Laughlin, for lively discussions on everything from politics to sailing and especially the Red Sox.

Many people outside the lab made my time at MIT memorable. I will remember in particular times spent with Eric Lucas, Jeff Shorter, Joanne Hetzler and Chris Fell at the Muddy (or Ear), Red Sox games and getting sunburned on the esplanade or softball field. It is sad that the parallel lives Mark Schofield and I had been leaving are now diverging. Mark has been a good friend throughout the years we spent together at UEA, UMass and MIT. Although our Seiko watches will now be set for different time zones, I know we shall keep in touch. Many an enjoyable time has been spent with Richard and Sandy Andrews, usually accompanied by good food. Congratulations to Richard and Sandy on the birth, yesterday (just in time to make it into these acknowledgements), of their daughter Fiona. I haven't met Fiona yet but knowing her parents, she has to be adorable.

Over the years the love and support of my Nana is something I have always been able to count on. Sometimes it was necessary to escape for a while. Thanks to Gary, Marlene and Monte for all the happy times spent together in Minneapolis (and Utah,

L.A., Alberta, Arizona and Vermont). It is to my Mum and Dad, who have encouraged me at every stage of my academic career, and my wife Jenni, whose love has made these years so enjoyable, that this thesis is dedicated.

Introduction

The ultrahigh vacuum surface science discipline is motivated by a desire to understand, on a fundamental level, the interactions between a gas and a solid. The reason these studies are performed at low pressure is to take advantage of the insightful surface sensitive techniques that have been developed over the past 20 years. However, these techniques are only useful if there is a measurable rate of reaction. Much important chemistry that is catalyzed at the solid interface fails to proceed in the UHV environment. This thesis focuses on a particular example of disparate low and high pressure behavior: the absorption of hydrogen into the bulk of nickel. That hydrogen can cross the gas - solid interface is not in question. This bulk absorption is important in such diverse technological problems as metal embrittlement and hydrogen fuel storage. Under UHV conditions, although extensive study of the metal - hydrogen interaction has resulted in a solid understanding of dissociative chemisorption, direct evidence of bulk absorption is lacking.

In chapters 1 and 2, two mechanisms are identified by which hydrogen can cross the vacuum interface and become absorbed into the nickel bulk: direct absorption of gas phase H atoms and the collision induced absorption of H that is chemisorbed on the surface. Having mimicked the high pressure environment, it is now possible to study bulk hydrogen with the surface sensitive technique high resolution electron energy loss spectroscopy (HREELS). The topic of chapter 1 is the sensitivity of HREELS to bulk or subsurface vibrational modes. Although usually thought of as a surface sensitive technique, bulk hydrogen is detectable by HREELS due to the

long electron mean free path at the energies typically employed. By showing that the intensity of an 800 cm~1 loss feature in the HREEL spectrum has the energy dependence exhibited by the electron mean free path in nickel, this feature is assigned to a vibrational mode of bulk hydrogen.

In the second chapter, the impacts of energetic inert gas atoms are observed to induce the absorption of H that is chemisorbed on the nickel surface. The bulk H so produced is characterized by HREELS and the results are in agreement with those following the absorption of gas phase H atoms. The dynamics of hydrogen absorption are probed by measuring the collision induced absorption cross section as a function of kinetic energy, incident angle and mass of the inert gas atom. A mechanism by which surface H is absorbed, suggested by these measurements, is supported by a kinematic model of gas-solid energy exchange developed and applied to this system.

The impacts of the inert gas atoms are also observed to induce the recombination of chemisorbed H atoms and their subsequent desorption as molecular H2. Collision

induced recombinative desorption verifies, in chapter 3, that the 800 cm-1 loss feature following the absorption of H atoms is a bulk vibrational mode.

The absence of any bulk hydrogen at low pressure can explain why some hydrogenation reactions that are facile at high pressure fail to proceed under UHV conditions. In chapter 4, it is demonstrated that subsurface hydrogen is responsible for the hydrogenation of chemisorbed CH3 and subsequent methane formation.

Bulk Absorption of Hydrogen by Ni(111):

1.1 INTRODUCTION

Motivated by the role played by hydrogen in the embrittlement of metals [1,2], and also by the technological importance of metal hydrides as fuel storage media [1,2], the interaction of hydrogen with metals has received considerable attention. In recent years, the powerful techniques of surface science have been brought to bear on these problems [3], yielding insight into the dynamics of hydrogen dissociative chemisorption as well as the ultimate structure of the adsorbate. It is well known that molecular H2 dissociatively chemisorbs on Ni(111) up to

a saturation coverage of 1.0 ML (1.0 ML = 1.9 x 1015 cm-2) [4,5]. A dissociation

probability of 0.05 has been measured for ambient H2 on a clean Ni(111) surface

[6]. This low initial dissociation probability drops precipitously as the hydrogen coverage increases. At a coverage of 0.15 ML, the H2 dissociation probability is a

factor of two lower than that on a clean Ni(111) surface [7]. This low dissociation probability means that it is difficult to saturate the Ni(111) surface with hydrogen by

exposure to molecular H2.

At all coverages, hydrogen occupies a three - fold hollow site on the surface [4,8]. The Ni(111) surface contains two types of hollow site: HCP and FCC. A HCP adsorption site differs from a FCC site owing to the presence of a substrate atom immediately below the surface. It is concluded from a study employing high resolution electron energy loss spectroscopy (HREELS) that both HCP and FCC sites

are occupied when the hydrogen coverage is less than 0.5 ML [9]. The assignment of the vibrational spectrum (Figure 1.1a), which was collected from 0.5 ML of hydrogen adsorbed as an ordered (2x2)2H monolayer on Ni(111), is shown in Table

1.

Table I Assignment of the (2x2)2H HREEL spectrum

Also shown in Figure 1.1 is a vibrational spectrum collected from a Ni(111) surface that is covered with 1.0 ML of hydrogen. The symmetric and antisymmetric stretches have shifted upwards in frequency from 755 cm-1 and 1170 cm-1 respectively (Figure

1.1b). The vibrational frequency shifts exhibited by chemisorbed hydrogen result

from adsorbate interactions [9] and are therefore a sensitive probe of surface coverage. Ion channeling techniques have suggested that only one type of three fold site is occupied when the hydrogen coverage is 1.0 ML [8].

Hydrogen recombinatively desorbs as molecular H2 from Ni(111). Below a

coverage of 0.5 ML, a single feature is observed at 380 K in the H2 thermal

Site A Site B Assignment

745 cm~1 790 cm-1 Antisymmetric (E) Stretch

1080 cm-1 1105 cm-1 Symmetric (A,) Stretch

1260 cm-1 1400 cm-1 E Overtone 2180 cm-1 A, Overtone

Chapter 1: Atomic Hydrogen Absorption 500.0 1000.0 1 00

(a)

Energy Loss (cm-1)

HREEL spectrum of hydrogen chemisorbed on Ni(111). The surface

(a) 0.5 ML (b) 1.0 ML. Spectra were collected 120 off - specular with an impact energy of 9.7 eV.

1. 000 0.800 + 0 x U U,) z 0 U 0. 600 + 0. 400 1170

(b)

c-n

desorption spectrum [4,10]. The area under this H2 desorption feature saturates

at hydrogen coverage of 0.5 ML. For coverages above 0.5 ML, a second H2

desorption feature is observed to grow in at a temperature of 342 K [4,10]. The two H2 desorption features at 342 K and 380 K have been shown to result from adsorbate

interactions and not the desorption of hydrogen from sites having different binding energies [4,11].

The phenomena of metal embrittlement and hydrogen storage involve the dissolution of hydrogen into the bulk of the solid. Whilst most surface science experiments have focused on H2 dissociative chemisorption [3], it has also been

proposed that hydrogen can occupy sites below the surface of a number of transition metals. Most attention has been focused upon Pd owing to its unique propensity, amongst the group VIII metals, of forming an exothermic hydride [1]. Population of sites beneath the (110), (100) and (111) facets of Pd has been invoked to explain the hydrogen desorption from this metal [12,13,14,15]. The diffusion of

chemisorbed hydrogen into subsurface of Pd(110) is also thought to drive a surface reconstruction that is observed by low energy electron diffraction (LEED) [16].

Surface science experiments have suggested that hydrogen is also absorbed into the bulk of a number of transition metals besides palladium. For example, a reconstruction of Cu(110), observable by LEED but not He atom scattering [17] is also attributed to hydrogen occupying sites below the metal surface. Because of

their very different mean free paths in metals, He atom diffraction is not sensitive to the bulk reconstruction observable by LEED. Implantation of high energy (>80 eV) H2 ions into Ni(111), results in the appearance of a low temperature (-200 K)

feature in the 2 amu thermal desorption spectrum that has been assigned to the recombinative desorption of bulk hydrogen [18].

That hydrogen can be absorbed into the bulk of a metal is not in question. Indeed, the seminal work on activated dissociative adsorption supplied hydrogen to the surface by permeation from the rear of the sample [19]. This was accomplished by supplying a high H2 pressure to one end of a tube whose other end

was sealed off with polycrystalline crystals of Ni, Pd and Fe. After an induction period, some of the hydrogen from inside the tube makes its way to the front face of the crystal, where it recombinatively desorbs as molecular H2. It is obvious that the

hydrogen must have been present in the bulk prior to its reaching the front surface. What has been a subject of recent debate is the observation of subsurface hydrogen under UHV conditions. Despite solid, evidence that hydrogen can occupy sites below the metal surface, the evidence is indirect. Direct spectroscopic evidence has remained elusive.

This chapter describes the in situ observation of subsurface hydrogen in nickel. As described above, molecular H2 dissociatively chemisorbs on Ni(111) and is not

because not only is the heat of solution of hydrogen in nickel 8 kcal/mol endothermic

[1], but there is a barrier of -26 kcal/mol to dissociative absorption (Figure 1.2).

Molecular H2, with an average energy of 1.5 kcal/mol, is not able to surmount the

energy barrier to bulk absorption. It is possible that the H2 molecules that cross this

energy barrier and are absorbed into nickel are the fraction of H2 molecules in the

high energy tail of the Maxwell - Boltzmann distribution. The reason no hydrogen is absorbed under low pressure conditions is that the absolute number of H2

molecules with enough energy to be absorbed is simply too low. Under UHV conditions, the hydrogen absorption rate is below our detection limits. If the picture just described is valid, it should be possible to prepare a hyperthermal beam of H2

molecules and observe the onset of absorption as the kinetic energy is increased above the bulk diffusion barrier. However, hydrogen atoms should be able to access the subsurface sites of Ni(111) because their bulk absorption is exothermic by 48 kcal/mol. In fact, it was noted more than 25 years ago that nickel shows an enhanced uptake of hydrogen atoms relative to H2 molecules [20]. The saturation coverage

of hydrogen on the nickel surface was thought to be higher for an atomic hydrogen exposure. In this chapter, it is shown that this additional uptake is the result of hydrogen being absorbed into the bulk or subsurface sites of nickel. No distinction is made between the terms bulk or subsurface hydrogen.

I

Solid

9

kcal mol

4 kcal/mol

Vacuum

HAg

52 kcal/mol

40

1

10 kcal/mol

/2

Figure 1.2 Schematic one dimensional potential energy surface of the hydrogen -Ni(111) interaction constructed from the known binding energy of chemisorbed hydrogen [4], the barrier to bulk diffusion [1] and the heat of solution of hydrogen in nickel [1].

H2(g)

considered a surface sensitive technique, it was speculated 10 years ago [21] that high resolution electron energy loss spectroscopy (HREELS) should be sensitive to bulk vibrational modes because of the long electron mean free path at the low energies typically employed (<10 eV). In this chapter, subsurface hydrogen is shown to be detectable through HREELS and is readily distinguished from surface chemisorbed hydrogen by the electron impact energy dependence of the scattering cross section. The assignment of a vibrational mode of frequency 800 cm-1 in the HREEL spectrum to subsurface hydrogen is discussed in detail because it shows unequivocally that hydrogen is absorbed in a site below the metal surface.

1.2 EXPERIMENTAL APPARATUS

1.2.1 Overview of the Molecular Beam - Ultra High Vacuum Chamber Apparatus All experiments were performed with an ultra high vacuum (UHV) chamber -supersonic molecular beam apparatus that was designed and built in this laboratory. The design philosophy is well documented [22] and a detailed description of the apparatus presented elsewhere [23,24,25]. The result is a unique and versatile piece of equipment that enables us to study many dynamical aspects of the gas -solid interaction. In this section, the features relevant to the present investigation are briefly reviewed and the modifications made to the apparatus described.

The apparatus (Figure 1.3) consists of a supersonic molecular beam source precisely coupled to a UHV main chamber equipped with a number of surface

H B .L ...

CL

M

Figure 1.3 Schematic diagram of the molecular beam-UHV apparatus: A, nozzle molecular beam source;B, electronic shutter; C, rotating disc chopper; D, to source chamber 10 inch diffusion pump; E, to first differential stage 6 inch diffusion pump; G, to main chamber liquid N2 trapped 10 inch diffusion pump; H, HREEL

spectrometer; I, reverse view LEED; J, quadropole mass spectrometer; K, single pass CMA Auger; L, M, possible crystal positions and orientations for rotatable, liquid He cooled sample manipulator.

analytical tools. The supersonic molecular beam source is aligned along the axis of the main chamber and is thrice differentially pumped so as to collimate the beam and reduce the effusive load on the main chamber. The supersonic molecular beam is formed by an adiabatic expansion of a gas at high stagnation pressure (<200 psi) through a 0.001" orifice in an inconel 600 nozzle. The source chamber is pumped by an unbaffled, 10" silicone oil diffusion pump having a pumping speed of 4000 liters/s and is forepumped by a roots blower and mechanical pump in series. The kinetic and internal energy of the molecules in the beam are varied by inert gas seeding techniques in combination with resistive heating of the nozzle cap up to 1100 K. The nozzle is mounted in a water cooled copper mount such that only the nozzle cap becomes hot when heated. The nozzle temperature is measured with a Chromel -Alumel thermocouple spot welded to the nozzle cap and the desired temperature is

maintained to within 1 K by a temperature controller.

A 1 mm diameter skimmer, mounted 1 cm downstream on the wall of the

source chamber skims off 0.10 rad of the nozzle expansion. The nickel skimmer has sharp edges so as not to introduce any turbulence into the flow. This central portion of the molecular beam is further collimated by slits in the first and second differential chamber walls. During operation, the pressure in the source chamber as measured

by a Bayard-Alpert type ionization gauge is around 4x104 torr.

Chapter 1: Atomic Hydrogen Absorption 30 i/s) untrapped and a 4 inch (1200 I/s) liquid nitrogen trapped diffusion pump respectively. These pumps may be isolated from the differential chambers by O-ring sealed gate-valves atop the diffusion pumps and trap. This differential pumping provides two further stages of collimation and a reduction in the effusive gas load on the main chamber. Mounted in the first differential chamber is an electronic shutter that acts as a molecular beam flag, allowing accurate and reproducible exposure times of the molecular beam on the crystal. Also housed in the first stage is a two phase hysteresis synchronous motor that spins the molecular beam chopper used in the direct measurement of the kinetic energy distribution of the atoms and molecules in the beam by a time of flight (TOF) technique. The motor is mounted in a copper jacket which is water cooled. The shaft of the motor protrudes through the first stage wall into the second stage. The 6 inch diameter stainless steel slotted disc, called a beam chopper, is mounted on the shaft and has a thickness of 0.01 inch and has four radial slots equally spaced along the circumference. The molecular beam chopper fulfills a dual purpose, necessitating that two diametrically opposed slits have a width of 0.055" while the other two slits have a width of 0.100". The larger slits do not physically obstruct the molecular beam and so one of them is positioned coaxially with the beam when TOF analysis is not being performed. The smaller slits provide a narrower gating function to the molecular beam during the TOF measurement. Also mounted in the second differential chamber is a LED/phototransistor unit which

detects when a slit is aligned with the molecular beam. This LED/phototransistor is also used to trigger the multichannel scaler in TOF measurements. For the investigation described in this chapter, knowledge of the molecular beam kinetic energy and flux is unnecessary and so a discussion of the TOF technique is deferred until chapter two.

An O-ring sealed slide valve mounted on the second differential chamber wall isolates the UHV chamber from the molecular beam source and differential chambers. A hole in this beam valve provides the final collimating aperture and hence determines the cross-sectional area of the molecular beam at the crystal. There are two collimating positions for the beam valve. With the large beam hole collimating the beam, the diameter of the beam at the crystal when it is in front of the CMA (position M) is 1.27 cm and the entire crystal is imaged. The beam diameter when the small beam hole acts as the final collimator is 0.51 cm and therefore only 1/5 of the crystal is imaged when in front of the CMA (position M). The stainless steel UHV main chamber is pumped by a 10" Santovac*-5 poly phenyl oil diffusion pump and a liquid nitrogen trap. The procedure for obtaining a working pressure of 4x10-1 1 torr in the main chamber after the venting to atmosphere is as follows. Molecular sieve sorption pumps, cooled to 77 K, are used to rough pump the vented chamber from atmosphere to a pressure of 1.0 x 10- torr as measured with a thermocouple gauge tube on top of the sorption pumps. When

the main chamber pressure is less than 1 x 10-3 torr, the nude Bayard -Alpert type ionization gauge may be turned on. It is not possible to turn the ionization gauge on if the chamber pressure is higher than this. The UHV chamber is then isolated from the sorption pumps by a metal sealed valve and the O-ring sealed gate valve above the liquid N2 trap opened, allowing the chamber to be pumped by the diffusion

pump. The pumping speed of N2 and 02 is high and the chamber pressure

immediately drops to below 1.0 x 10-7 torr. However, H20 is adsorbed on the walls

of the chamber and its desorption rate at room temperature is very low. Therefore, because the pump out time at this temperature is prohibitively long, the entire chamber is heated to 130 0C by resistive heating strips and tapes on the outside of

the chamber walls to remove H20 and other low vapor pressure contaminants (e.g.

pump oil). A heating strip in contact with a 35.56 cm teflon sealed rotatable lid on top of the chamber is turned on 5 hours after the start of the bake out. This delay is necessary because the lid is thermally isolated from the chamber and heats up at a faster rate than the housing which contains the bearing. The accompanying thermal expansion could damage these bearings. Similarly, at the end of the bake out, the heating strip on the lid is turned off, and the lid allowed to cool for 5 hours before cooling the rest of the chamber. The main chamber pressure initially rises to around

1.0 x 10-7 torr as the chamber heats up, then drops precipitously as the H20 is

falling, indicates that equilibrium is established and the bake out can be terminated.

A typical bake out is of 48 hour duration and the resulting base pressure when the

chamber is cooled to room temperature is 4x10-1 1 torr.

Mounted along the molecular beam axis is a quadrupole mass spectrometer

(QMS) used to measure the kinetic energy of the atoms and molecules of the

molecular beam via a time of flight technique. The QMS is also used to monitor the partial pressure of desorption products during thermal desorption spectroscopy (TDS) and in residual gas analysis (RGA). The major components of the residual gas are H2 and CO with no pump fluid contamination. The chamber is in good condition

when the ratio of the 28 amu signal (CO and N2) to the 18 amu signal (H20) is

larger than 2 : 1. The ratio of the 28 amu signal to the 32 amu (02) signal is usually greater than 30 : 1. An atmospheric leak is indicated when the 28 amu to 32 amu ratio approaches 5 : 1 and the 14 amu (N) signal is larger than that for 12 amu (C). Suspected leaks are located by tuning the QMS to 4 amu and helium leak checking the chamber. The appearance of fragments with a higher m/e ratio than 44 amu

(C02) is an indication of pump oil contamination and mandates that the chamber be

further baked.

A cylindrical mirror analyzer (CMA) detects Auger electron emission following

excitation by a 2 kV electron beam directed at the center of the crystal, along the surface normal. Auger spectroscopy is used as a monitor of the crystal cleanliness

and a low electron beam current at the crystal (5 gA) minimizes any electron beam cracking and subsequent adsorption of the residual gas. The Auger spectrum is collected in the derivative mode by a lock -in amplifier with a 3 eV modulation on the inner cylinder of the CMA and a time constant of 0.3 s. The sweep rate is 0.5 eV/s. Low energy electron diffraction (LEED) techniques ensure that the Ni(111) surface is well ordered and are also used to observe any long range overlayer structure.

A major feature of this chamber, and one that is heavily employed in this

investigation is the high resolution electron energy loss spectrometer (HREELS). The spectrometer [26] consists of two single pass 127 cylindrical deflectors for the incident electron monochromater and scattered electron energy analyzer. The electron beam is incident on the crystal at an angle of 600 and the energy analyzer is usually aligned along the specular direction. Electrons scattered away from specular may be detected by rotating the crystal about an axis perpendicular to the scattering plane that contains the crystal surface. This procedure also changes the incident angle and the scattering angle is therefore twice the angle by which the crystal is rotated. The electron gun filament is usually held at ground potential and the electron impact energy varied by changing the crystal bias. The electron beam current at the crystal is typically 3.0 x 10-10 amps as measured by a floating electrometer. The electron detector is a non -magnetic Galileo channeltron* (model

4028 CSL/NM) electron multiplier operated in pulse counting mode. These current pulses are conditioned with a pulse amplifier discriminator having a TTL output that serves as input for the counting electronics. The lens voltages of the analyzer are swept with a linear voltage ramp generated by the laboratory PDP 11/23 computer. Routine energy resolution of the spectrometer is 5 meV with an elastic count rate of

1x106 cps at the specular angle from a clean crystal.

The nickel single crystal is cut and polished to within 0.20 of the (111) orientation through monochromatic X-ray diffraction. The elliptically shaped crystal has major and minor axis of 1.27 cm and 1.02 cm respectively and so the crystal surface area is 1.02 cm2. The crystal is in good thermal contact with a cryostat tube and may be cooled to 80 K or 8 K by using either liquid nitrogen or liquid helium as the refrigerant. The crystal temperature is monitored by measuring the voltage at a type E Chromel -Constantan thermocouple junction (referenced to an 00C ice bath)

spot welded to the top edge of the crystal. A Helitran* (Model LT-3B-110) transfer line is used to supply the liquid helium to the cryostat. The transfer line is of the continuous flow type where liquid helium is transferred to the cryostat by externally pressurizing (5 psig) the supply dewar. Loss of cryogen fluid is kept to a minimum

by surrounding the incoming helium flow with the returning cold exhaust gas. The

cryostat tube is filled with liquid N2 simply by pouring into a funnel i.e. no transfer

To achieve an ultimate temperature as low as 8 K, indirect heating methods are necessary [27]. Resistive heating of the crystal requires heavy gauge copper wire from a power supply which introduces a large heat load on the crystal. A thoriated tungsten filament immediately behind the crystal allows either radiative heating or electron beam heating when the crystal is biased 600 V positive with respect to the filament, usually held at ground potential. The crystal heater is never hot whenever a gas is introduced into the chamber. Electron beam heating is kept to a minimum because the 600 V electrons readily break chemical bonds. Following a gas exposure, any subsequent warming of the crystal is achieved through radiative heating. This precludes any electron beam induced surface chemistry. The highest temperature that can be achieved by radiative heating is 450 K. Thermal desorption spectroscopy requires the crystal temperature to be ramped from a base temperature to a limiting temperature. It is necessary to supply this temperature ramp by electron beam heating (600 V) because of the high temperatures (1000 K) and higher temperature ramp rates (2 K/s) required. All of the high temperature (1000 K) annealing necessary in the nickel cleaning procedure described below is accomplished with 600 V electron beam heating. By controlling the flow of liquid N2 into the

cryostat, it is possible to maintain the crystal at any temperature between 80 K and 200 K without use of the crystal heater. This is particularly useful when it is necessary to expose the crystal within this temperature range (section 1.2.2.1).

An automatic (proportional-integral-derivative) temperature controller [27] is able to maintain the crystal at any desired temperature within the range of operation. This is accomplished by varying the power to the crystal heater until the measured thermocouple voltage matches that of the set point. The temperature controller is also able to ramp the crystal temperature at a variable rate by ramping the set point from a base to a limiting temperature. In addition, the temperature controller also supplies the power to the hydrogen atom source filament (section 1.2.2).

The crystal and cryostat tube are mounted on a differentially pumped, teflon sealed, rotatable insert that allows the crystal to be rotated by 3600 about a vertical axis that includes the cryostat tube and passes through the front crystal face. This rotatable insert sits on a manipulator assembly with which the crystal can be moved in three orthogonal directions throughout the UHV chamber. Movement of the crystal from the rear (Auger) to the front (HREELS) of the chamber (positions M and L respectively in Figure 1.3) is possible by a 1800 rotation of a 35.56 cm diameter, differentially pumped, Teflon sealed rotatable lid on which the crystal manipulator assembly is eccentrically mounted. The manipulator therefore allows the crystal to be positioned in front of all the surface analytical instruments with which the UHV chamber is equipped. In addition, the manipulator allows the angle at which the molecular beam is incident on the crystal to be varied and the crystal to be raised out of the molecular beam.

Common surface contaminants of nickel are sulfur, carbon and oxygen. The cleaning procedure involves 1 kV Ar* ion sputtering at room temperature for 30 minutes followed by annealing at 1000 K for 5 minutes to repair the resulting surface damage. The emission current of the ion gun is usually 20 yA and an ambient Ar pressure of 5.0 x 10-5 torr is introduced through a leak valve. Typical ion currents at the crystal are 3.5 4A and the crystal is moved throughout the ion beam image to ensure that the entire surface is sputtered. During the high temperature annealing, the cryostat is filled with liquid N2 so that only the crystal becomes hot. Any

immutable carbon or oxygen not removed by this sputter procedure is then chemically oxidized or reduced. After flashing the crystal to 1000 K, it cools to 80 K in a molecular beam of either 02 or H2. The beam is formed by expansion from a room

temperature nozzle having a stagnation pressure of 25 psig and is collimated such that the entire crystal surface is imaged. This procedure allows the crystal to be hot during the oxidation or reduction without the crystal heater being on in the presence

of 02 or H2 gas. The crystal heater is never hot when gas is introduced into the

chamber. The crystal is moved throughout the molecular beam to ensure that the entire crystal is treated. The CO, CO2 or H20 so formed is then desorbed from the

crystal by annealing at 1000 K for 5 minutes. The crystal is considered clean when contaminants are below the Auger detection limit (0.01 ML). Figure 1.4 shows the Auger spectrum obtained from a clean crystal. An Auger spectrum is collected every

0(512 eV)

C(272

eV)

S(152 eV)

Ni(102 eV)

Figure 1.4 Differential Auger spectrum of the clean Ni(111) surface collected at a crystal temperature of 310 K with a 3 eV modulation on the inner cylinder of the

CMA and a time constant of 0.3 s. The 2 kV electron beam has a current at the

0.1" from the top to the bottom of the crystal. The CMA is more sensitive to S (152

eV) Auger emission when the crystal is hot (500 K) because the intense, nickel electron diffraction features at 102 eV are reduced in intensity by the appropriate Debye - Waller factor. The crystal temperature when checking for C (272 eV) and O (512 eV) contamination is about 350 K. Below a temperature of 150 K, the CMA becomes relatively insensitive to C and 0, because of substrate features.

The H2 gas used in these experiments has a purity of 99.9999% with total

impurity concentration of less than 1 ppm. No single impurity has a concentration higher than 0.5 ppm. There are two stages of trapping by liquid nitrogen before the gas is introduced into the UHV chamber. Exposure to a H2 pressure of 2.0 x 10-5

torr for 900 s (18 000 L) deposits no carbon or oxygen detectable by the CMA. In addition, there is no desorption of H20 in the TDS. For the isotope experiments

(section 1.3.2.2), the D2 has a deuterium content of 99.9% with a HD concentration

of 0.2%. This gas is also trapped with liquid nitrogen prior to use.

1.2.2 Generation of a Hydrogen Atom Source

The atomic hydrogen required for this investigation is generated by the thermal dissociation of molecular H2 over a hot tungsten filament [28] positioned

0.25 inch away from the Ni(111) sample. In all experiments, the temperature of the

filament is 1800 K as measured by an optical pyrometer. The filament is fabricated from 0.010" diameter tungsten wire bent into a grid shape (Figure 1.5). Care needs

Chapter 1: Atomic Hydrogen Absorption

CT

RS

FS

CI

FP

C

0

SF

B

Figure 1.5 Schematic drawing of the crystal mount and hydrogen atom source. For a complete description of the liquid He cooled sample manipulator see reference 24.

C, Ni(111) crystal; CS, crystal support to the cold end of the refrigerator; CT, cryostat

tube; RS, radiation shield; SF, H atom source hair pin W filament; FS, horizontal filament support, mounted on a rotatable feedthrough so that the filament can be rotated in a horizontal plane, out of the molecular beam image when not in use; CI, ceramic insert; FP, fiber glass insulated Cu wires supplying power to the filament; MB, H2 molecular beam from the nozzle source

to be exercised when cutting the wire so that it does not splinter. The tungsten wire is coated with a passive oxide that must be removed. Without complete removal of this oxide surface, spot welding is extremely difficult and there are intolerable water contamination problems during operation. Oxide removal is accomplished by electropolishing [29]. The crystal is immersed in 20% by weight NaOH solution while passing alternating current (10 V) through a carbon electrode for 15 s. The resulting tungsten wire has a metallic luster. The oxide free tungsten filament is spot welded to the contacts of a ceramic filament support which isolates the filament from the chamber and ground. A used CMA electron gun filament provides the ceramic filament support. Fiber glass insulated copper wires connect the atom source filament to an electrical feedthrough on the rotatable lid.

The atom source filament (Figure 1.5) is supported by a rotatable feedthrough so that it may be positioned along the molecular beam axis and in front of the nickel crystal or moved out of the beam path. The distance between the crystal and the atom source filament is adjustable by moving the sample manipulator. The tungsten filament must be aligned in front of the nickel crystal. The alignment procedure is most easily carried out while watching the crystal and filament through the 23 inch view port located on top of the chamber, which has its axis directed towards position M (Figure 1.3) and is positioned in the vertical plane that contains the molecular beam axis. With the crystal raised -1" above the molecular beam axis the