MEASUREMENT OF PRlMARY ELECTRON INTERACTION COEFFICIENTS (500 TO 1500 eV REGION)

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MEASUREMENT OF PRlMARY ELECTRON INTERACTION COEFFICIENTS (500 TO 1500 eV

REGION)

Burton Henke

To cite this version:

Burton Henke. MEASUREMENT OF PRlMARY ELECTRON INTERACTION COEFFICIENTS (500 TO 1500 eV REGION). Journal de Physique Colloques, 1971, 32 (C4), pp.C4-115-C4-123.

�10.1051/jphyscol:1971421�. �jpa-00214622�

MEASUREMENT OF PRIMARY ELECTRON INTERACTION COEFFICIENTS (500 TO 1500 eV REGION)

BURTON L. HENKE

Department of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii

Rbum6. - Etant donne que les electrons d'energie incidente comprise entre 500 et 1 500 eV n'ont d'interaction apprkciable qu'avec peu de couches atomiques dans 1es solides, des mesures precises et l'application de leurs coefficients d'interaction sont difficiles. La statistique des interac- tions et la sensibilite a la disposition gkometrique de la cible nkcessitent une attention particulikre.

Nous presentons ici une m6thode pour l'ktude de telles interactions, baske sur la deposition contrb- lee d'une couche monoatomique de cations m6talliques bivalents comme cible, sur un support forme d'une double couche de sttarate du type de Langmuir-Blodgett. Pour vQifier la mkthode, nous avons mesure les sections efficaces des cations de barium a 466 eV, 706 eV et 1 349 eV, et celles du zinc et du plomb a 706eV. Les valeurs mesurCes sont en accord avec des mesures recentes de faisceau atomique sur le barium et avec un modkle theorique simple. Nous considkrons que des mesures de ce type sont d'une grande importance dans le dkveloppement de l'analyse chimique quantitative de surface par spectroscopie d'klectrons et de rayons X de basse Bnergie.

Abstract. - Because electrons of incident energy in the 500-1 500 eV range interact appreciably within only a few atomic layers in solids, precisely measuring and applying their interaction coefficients becomes difficult. The statistics of the interactions and the sensitivity to the geometric arrangement of the target atoms require special attention. A method for the study of such interac- tions is presented here that is based upon the controlled deposition of monatomic layers of bi- valent metal cations as targets within the framework of stearate double layers of the Langmuir- Blodgett type. To demonstrate the method, cross sections of the barium cation have been measured at 466 eV, 706 eV and 1 349 eV, and those of zinc and lead at 706 eV. The measured values are shown to be consistent with recent atomic beam measurements on barium and with simple theore- tical models. Measurements of this type are considered to be of great importance in the development of quantitative surface chemical analysis by low energy X-ray and electron spectroscopy.

1. Introduction. - This work is concerned with the measurement of primary electron interactions in solids for incident electron energies in the 500-to- 1 500 eV region. Such electrons have an associated wavelength of about 0.5 A. Their mean free paths within condensed matter are generally less than 100 A.

Although a considerable amount of theoretical work on the passage of electrons through matter has been presented and from a variety of approaches, very few experimental measurements have been repor- ted for kilovolt-electron interactions in solids [l].

Interesting questions remain to be answered as to the relative roles of elastic and inelastic scattering, and of collective and individual electron energy loss mechanisms.

The ionization of matter by keV electrons should be similar in both mechanism and magnitude to that by MeV protons, for example [2]. Important new insights might be gained by comparing measurements with electrons in this region to the relatively large amount of data reported in recent years on heavy particle cross sections in the MeV region.

Finally, important analytical techniques based

upon chemical and structural analysis by electron and X-ray spectroscopy demand a precise knowledge of X-ray and electron cross sections. Although many important applications involve radiations in the 100-to-1 000 eV region, it has been only relatively recently that low energy X-ray cross sections have become available to any extent [3, 41. There remains an appreciable need for further work on low energy X-ray interaction coefficients and an even greater need for work on the low energy electron cross sec- tions.

2. Electron spectroscopy of thin films. - The electron spectrograph which is used for the present work is shown in figure 1 and has been described previously [5]. It employs hemispherical plates as the electrostatic analyzer. These have inner and outer diameters of 18.5 and 21 inches. For this work the slits were set at 0.3 % energy resolution and to receive only photoelectrons which leave nearly normally from the surface of the sample (maximum angles off normal are plus-minus 30). AI-K, photons (1 487 eV) were used to excite the samples as generated by an

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971421

BURTON L. HENKE

SAM'LE CHAMBER

F

U L T R A S O F T

X-RAY SOURCE Cu-Be ELECTRON

MULTIPLIER

SUBLIMATION PUMP

I ^{ROUGHING L I N E}

VACION PUMPS

I I

LITERS / S

U

FIG. l . - Schematic of the electron spectrograph which employs a hemispherical lens electrostatic analyzer. For the work reported here, an eight-position holder is used. The 80 literls pump is used only for pump-down. Its magnet is removed when the spectrograph is operated. Helmholz coils are employed to

eliminate stray fields.

aluminum anode, demountable source and filtered by a 6-micron aluminum foil. The natural width of this X-ray line is about one electron volt. All photo- electron counting rates presented here are for an X-ray source operation of 200 mA and 6 kV.

Figure 2 presents a model for indicating the factors which determine the intensities of the primary photo- electron lines. The fraction of the electrons that might be reflected back into the sample from the top surface is assumed negligible. Also, the decrease in the inci-

S O L I D A N G L E SUBTENDED BY SLITS

h'3NOCHROMATIC E F F E C T I V E

X-RAY SOU SAMPLE AREA-A

INTENSITY- AMPLE THICKNESS-L

L - A T O M I C CROSS SECTION FOR { - L E V E L I O N I Z A T I O N n , - N O . OF A T O M S / C M ~ E M I T T I N G i-PHOTOELECTRONS

Xi - MEAN FREE PATH OF i - PHOTOELECTRONS

-

PI - SURVIVING PHOTOELECTRONS REACHING SLITS

-

FIG. 2. - Illustrating the parameters which determine the intensity of photoelectron emission lines.

dent X-ray intensity, Q , within the effective source volume for the photoelectrons is considered negli- gible. (For the corresponding primary Auger peak intensities, the appropriate yield factor must be included.)

Fabrication, by vacuum deposition methods, of thin film samples which are homogeneous and of uniform and precisely known mass thickness is extre- mely difficult in the 50 A thickness region. The thin samples which are being used in the studies reported here are built up from monomolecular, insoluble organic films (as soaps) following techniques first developed by Langmuir and Blodgett [6, 71. The tank which has been developed in this laboratory for succes- sively depositing monomolecular layers is described in figure 3.

TEMPERATLRE CONTRC:

WATZR L I N E 5 REVERSlNG MOTOR

CONSTANT S P E E D DRIVE

I O N ISOLATICH

JEWEL BEARING

FIG. 3. - Illustrating method developped for the dipping of Langmuir-Blodgett multilayers, applying the surface pressure, controlling substrate temperature and vibration isolation. The apparatus is mounted on a clean bench at positive pressure of

filtered air.

A known amount of stearic acid, for example, dissolved in n-hexane, may be deposited on the surface of doubly distilled and filtered water which contains about 10-4 molar of some bivalent metal ion, M. The hydrophobic, 25 A carbon chains orient nearly vertically out of the water surface ; the hydrophi- lic, carboxyl ends of the stearic molecule react at the water surface to form a monolayer of metal atoms.

A 7.5 X 18 cm glass slide forms one end of the tank and a floating barrier the other. In order to condense the film, a surface pressure (of the order of 20 dynes per cm, depending upon the metal salt being construc- ted) is applied by the barrier.

It has been found that one layer can be deposited upon the glass slide as it is slowly lowered into the surface, and another as it is raised out again. The double layer thus formed consists of a monolayer of metal ions of double density with 25 A chains of CH, groups projecting normally outward on each side. The area occupied on the solid substrate is slightly greater than the corresponding area on the water surface (-- 4 % greater). The first in-and-

out double layer sample film must be deposited upon a smooth surface that is hydrophobic. Float glass or picture frame glass can be sufficiently smooth.

On this a substrate of about twenty double layers of another stearate system is deposited. This 0.1 micron ( ( p a d ) ) forms an ideal hydrophobic substrate. It integrates out submicroscopic irregularities of the glass surface. It also forms the carrier for the appro- priate metal ion sources of photoelectron emission lines that are used for the absorption measurements.

The photoelectron emission lines from one, two or three monatomic layers of metal atoms within steara- tes can be measured when these are deposited upon substrates not containing the particular metal. Such measurements on three-layer systems of the stearates of zinc, barium and lead are shown in figure 4, with

ZINC STEARATE C S 1 , - 3970

1,-3970 It-3960 1,-3760

I , = I , ( I - T " ) 1230 t n o ~ F I

FIG. 4. - X-Y plotter scans of the LIII, MIV-Mv, and Nvr NVII lines of zinc, barium, and lead, respectively, as emitted from three-double-layer systems of their stearates. Also is indicated typical intensities as measured from one-and two-layer systems and from thick systems. These photoelectron emission lines were chosen for the emission and absorption measurements

reported here.

X-Y plotter scans of the L,,,, M,,-M, and N,,- N,,, lines, respectively. The intensities correspond to a monatomic area density of metal ions of about 5 X 10i4 atoms per cm2. The intensities that are measured for one and two double layer systems are also indicated.

The carbon-K photoelectron emission signal is illustrated in figure 5 as measured from a thick stearic

1 1

PHOTOELECTRON 4 ( 1 4 8 7 eV)

./

I I

I STEARIC ACID

MULTILAYER

-CH

A

1 2 ' f

I CARBON SlGNAL/ATOMlC LAYER 1 1 0 0 1 2 0 0 1 3 0 0 Ic = (A/ A) I,= 310 c/s FIG. 5. -The carbon-K (AI-K,) photoelectron signal from a thick sample of stearic acid multilayer. Assuming that the photo- electrons are generated essentially in the CH2 chains, an estimate is made that each layer of carbon atoms (5 ^X 1014/cmz) emits an intensity as received by the spectrograph detector of 310 c/s.

chain system within a stearic acid multilayer sample.

The photoelectrons are generated within a surface layer of effectively one mean free path depth, which, for 1 200 eV electrons, is approximately 90 A for this system.

3. Measurement of film transmission - T. ^- 3.1 BY EMISSION MEASUREMENTS. - AS may be noted in figure 4, the emission intensity for a given photoelec- tron line rapidly reaches a constant value upon adding layers of thickness of the order of a mean free path.

This growth curve may readily be shown to be descri- bed by the following equation :

where I, is the photoelectron emission intensity from n double layers, each characterized by a transmission probability, T. I, is the corresponding emission inten- sity for a thick sample. This equation leads to the working relations which permit the determination of the transmission per double layer, T , in terms of the measured I,, I, and I,,

and

3.2 BY ABSORPTION MEASUREMENTS. - By measu- ring from the same one, two and three double layer samples, but on a photoelectron emission line origi- nating from a common substrate material, the trans- mission per double layer can be determined through the absorption equation,

where I, is now the photoelectron intensity after passing through n double layers and I, is that from the substrate directly. This absorption measurement of T is illustrated in figure 6 for the M , line (706 eV) of barium as absorbed by stearate double layers contai- ning cations other than barium.

Equation (4) may be written as

In I,, = In I, - n In ( 1 f T ) ; ( 5 ) so T may be determined from the negative slope,

- K, of a In I, vs n plot by

For the results presented below for T, a least squa- res criterion was used for combining the two values of T derived from ( 2 ) and (3) for emission intensities I,, I, and I,, and for the best fit for the absorption data I,, I,, I, and I, to yield the slope value K.

It is important to note that in these transmission measurements, the intensities appear as ratios and that any reduction in the individual values I, due to a contamination layer common to all samples would

BURTON L. HENKE

\

'THICK' BARCM STEARATE SUBSTIUSIE-EXCITED W 1487ev FiiOTcNS

FIG. 6. -Illustrating the absorption method used for deter- mining the transmission- T for absorber double-layers of stearates which contain cations other than barium. Here the absorption of the barium My photoelectron line emitted from a thick barium stearate multilayer as substrate is being measured.

therefore effectively cancel out. Since all layer systems have the same carbon chain, hydrophobic structure on the outer surface, and since each is handled simi- larly in and out of the spectrograph, it is most proba- ble that any contamination layer would be the same for each sample of a particular series.

Also, it should be noted that (1) and (4) are based upon the assumption that successive layers are posi- tioned randomly with respect to each other ; therefore, alignment, or tunneling effects can be assumed negli- gible. The test for the desired absence of both the contamination and the alignment effects would be the equality of the transmission values as derived from (2) and (3), and the linearity of the In I, vs n absorption curves. These conditions prevailed for most of the measurement series and only such data were used for the results reported below.

4. Characteristics of stearate multilayers. - 4.1 MOLECULAR STRUCTURE. - The model [S, 91 which is used in these studies for the stearate monomole- cular layer systems is presented in figure 7 and figure 8. In a double layer, the opposing stearic CH,- chains have a common axis, and the cations form a single monatomic layer, regularly spaced between the chain axes. When sufficient surface pressure is used to form a cr stifS)) film in the tank, the area occupied by each stearate unit (e. g., the square indicated in figure 8-F) is nearly the same for all cations and is approximately 20.5 A2.

4.2 STABILITY OF FATTY ACID MULTILAYERS. -

Thick crystals ( W 50 double layers) of lead laurate, lead stearate, lead lignocerate and lead melissate have been used as very effective long wavelength X-ray analyzing crystals in this laboratory for many years [10]. These have 2 d-spacings of 70, 100, 130 and 160 A, respectively. No significant effect of the

I

d=2.5(n+4)+E - M - [ c n 3 ( C H 2 h C j M

--- ---

0 0 ²

C - ^{I n=16} - ^CH2 - ²

-

FIG. 7. - Schematic of the fatty acid molecule. For a stearate, M is a bi-valent cation and n = 16.

FIG. 8. - Packing model of a multilayered crystal : (A) Structure perpendicular to the plane of the substrate. Cylinders represent stearate chains and spheres represent cations. (B) Structure parallel to the plane of the substrate. Open circles represent stearate chains and black circles represent cations.

(C) Diagram of the end of a fatty acid chain, showing a carboxyl group and several CH2 groups from the side. The carbon atoms are coplanar (carbon atoms shaded). (D) Section of the carboxyl group as indicated. The carbon and oxygen atoms are all copla- nar. (E) Section of the fatty acid chain as indicated. (F) The indicated square, as in B, represents the area to associate with each stearate molecule. The shaded circles are the cations, in the plane of the figure. The open circles and the black circles are the oxygen atoms, below and above the plane of the paper, respectively. Geometrically, the circle surrounding the oxygen group is of area 10 A2. The cation area (maximum without overlap) is then 6 A2. The total area per stearate molecule is

about 20.5 A2.

X-ray vacuum spectrograph environment on these crystals has ever been observed when they are at riom temperatures. However, X-ray reflection effi- ciency measurements are not sensitive to changes in the first few double layers as involved in the electron

spectrographic measurements here. Many sets of absorption curves were measured over two- and three-day periods in order to detect possible changes in the sample under vacuum (10-' torr) and X-ray excitation. Typical results are shown in figure 9, which

2 3

DOUBLE LAYERS

-

PIG. 9. - Illustrating typical changes observed in the absorp- tion curves resulting from having the samples under high vacuum and X-radiation for relatively long timeperiods. Similar measure- ment on some other samples showed no detectable change in the

absorption curve slope value-K.

presents absorption curves taken at 4 hours and at 70 hours on a stearic acid multilayer and on a stearate.

In some other sample runs, no change could be detec- ted. All the transmission results given below were first runs, with a maximum time under vacuum of overnight.

4.3 UNIFORMITY OF STEARATE MULTILAYERS. -

A small fraction, H, of holes in the stearate multilayer structure could be present in several ways. It has been observed [l11 that free stearic acid molecules can occur in place of some of the stearate molecules. The transmission, T,, that is measured would then be related to the transmission, T, of the pure stearate double layer through the equation

T,= HT,

+

-

where T, is the transmission per double layer of pure stearic acid.

The hole fraction, H, has been measured by other workers by infrared absorption analysis [l21 and by polarographic analysis [13]. The method developed for this work is based upon the fact that the stearic acid component can be effectively dissolved out by

rinsing the sample in benzene or n-hexane (10 to 30 S).

This process has been well documented by Langmuir and Blodgett and was called by them <<skeletoniza- tion ^D. Measurement of a sample series that has been thus treated yields a transmission value, T:, which is related to T by

Combining (7) and (8) gives the H-value

Calculated curves for In I vs n for different values of H and for typical values of T and T, (0.2 and 0.5) are shown in figure 10 for samples before and after

FIG. 10. - The stearate double layer samples may have a small fraction of free stearic acid as a component. This is detected by dissolving out this component and observing the change in the absorption curve. Also illustrated here is the characteristic nonlinear shape of the absorption curve that appears when the

multilayer deposits in an << island >> structure.

skeletonization. Some examples of measurements for the stearate fraction-S (S = 1 - H ) are shown for Zn, Ba and Pb stearates in figure 11. Over all of the measurements reported here, the variation in S was in the range 90-100 %. With H measured in this way, the stearate transmission per double layer may then be determined from (7) by

T, - HT, T =

l - H '

C4-120 BURTON L. HENKE

AT-CHANGE IN TRANSMISSION T, -STEARiC ACID TRANSMISSION

(1349 ev) BARIUM

- ZINC

-

S= 100%

I ^-

+ STEARIC DISSOLVED OUT

I

FIG. 11. - Typical changes in the absorption curves upon

<< skeletonization D. Such data are used to determine a corrected value for the transmission to associate with a pure stearate.

As a test of this method of correcting for the stearic acid fraction, measurements for T were made on several sample series for which H varied considerably due to changing the p H of the tank solution. (Hcould be made as high as 0.2 for Ba stearate.) For such measurements, it was found that the T values as calculated from (10) for each stearate type were independent of H and constant well within experi- mental error limits.

Also shown in figure 10 are calculated curves for In I vs n for the case when the hole fraction, H, repre- sents the possible straight-through holes between cc islands of pure stearate multilayer structure.

This model is described by the equation for I as

The possibility of such (( island structure occur- ring has been made evident by actual measurements which display the characteristic nonlinear shape of the In I vs n curves shown in figure 10. These occurrences were usually related to the associated observation of reduced and unsymmetric floating barrier displace- ments in the in-and-out deposition of the particular double layer system.

5. Sample preparation and measurement. - A sample series consists of eight pieces, 2.5 X 5 cm each cut from the large glass slide on which 20 double layer substrates have been deposited. One pair is used for the I. measurement, a second pair is covered with one double layer of the absorber, a third with two double layers, and the fourth with three double layers. One member from each pair is washed in n-hexane to remove possible free stearic acid.

As described above, transmission values can be obtained from such a sample series by both emission and absorption line measurements. (The transmission value by emission would be for a different electron energy than that by absorption.) Each pair of measu- red transmission values, for washed and unwashed samples, is used to determine the hole fraction, H, and corrected transmission values are then deter- mined. An eight-position sample holder is used to permit the measurements to be made in a single run.

Samples are maintained at room temperature. A small laboratory computer is used to carry out the least- squares averaging for T as the measurements are taken.

Certain parameters are recorded for each sample series. These include tank conditions, such as surface pressure, pH, and surface barrier displacements, along with any variation in the chemistry. As a check on the quality of the substrates, a sample of each is placed in a vacuum X-ray spectrograph for an X-ray reflection efficiency measurement. It is already evident that these studies can be very helpful in the optimization of multilayer systems for X-ray analysis.

6. Results and comparison to theory. - The trans- mission per double layer of stearic acid and of barium stearate for primary photoelectrons of energy 466 eV, 706 eV and 1 349 eV were measured. Also, the trans- mission values for zinc stearate and for lead stearate were measured at 706 eV. Each T-value as listed in Table 1 has been based upon several independent sample series runs and has an estimated precision of about plus-minus 5 %.

Transmission - T vs. energy (for 50 A double layers)

466 eV 706 eV 1 349 eV

- - -

Stearic Acid 0.36 0.52 0.55

Zinc Stearate 0.056

Barium Stearate 0.052 0.16 0.31

Lead Stearate 0.26

The probability for a first absorption event can be written as the ratio of a cross section, o, for the stea- rate molecule (or stearic acid molecule) to the area, A, occupied by each molecule. This cross section would be associated with the direction of the electron beam within such an ordered sample. For these measure- ments, the appropriate area, A, is indicated by the square in figure 8-F, and is approximately 20.5 A2.

Also suggested by this figure is that this cross section might be approximated as a simple sum of two contri- butions, one for the stearic acid chain (principally due to the carboxyl group ) and the other for the cation, M. This assumes that the ⁽⁽ overlap >> of these two cross sections as viewed along the electron beam and within one double layer is not appreciable. With this simplifying assumption, the relation between

the transmission probability as measured and the cross sections may be written as

T = l - (0,

+

T, = l

-

.

(a,/A) = T, - T . (1 3) Using (12) and (13), values for a,/A and o,/A have been determined and are listed in Table 11.

Absorption probability - a/A vs. energy

(4 ^N 20.5 A2)

466 eV 706 eV 1 349 eV

- - -

Stearic Acid 0.64 0.48 0.45

Zinc Atom 0.46

Barium Atom 0.44 0.36 0.24

Lead Atom 0.26

In order to gain some first insights for the absorp- tion processes responsible for the observations presen- ted here, it would seem instructive to compare these to the first order theoretical predictions of the quantum mechanics as applied to free atoms. Bethe's Born approximation to his theory for the total ionization cross section should apply for the energy region of interest here, viz., for E U,, where U, is the ioniza- tion energy of the innermost shell involved with the interaction. His equation may be written as [l41

where E is the incident electron energy, ui is the ioni- zation energy for the i-th shell, and ni is the oscillator strength for the i-th shell

(C

For the energy region of interest here, the cross section for elastic scattering by a free atom of atomic number, Z , can be given as

This result is predicted for fast electrons [l51 by the self-consistent field model of Hartree and Fock as well as by the statisticalmodel of Thomas and Fermi, hence should apply for both light and heavy ele- ments. However, it is evident from the measurements presented here and by others [l] that (15) cannot yield a reasonable value for the contribution to the

total cross section for atoms in condensed materials.

(Using (15) to estimate the elastic scattering cross section for barium at 466 eV, 706 eV and 1 349 eV, one obtains 38, 26, and 13 A2, respectively. These values are considerably higher than any elastic scat- tering contribution that can be consistent with the data of Table 11. Also, if such scattering were of appreciable effect, it would seem that the Z-depen- dence of the total cross section would be different from that indicated in Table 11.)

By describing condensed material in terms of a dielectric constant (most completely as by a complex dielectric constant) and by applying quantum mecha- nical dispersion theory, one may also obtain relations for rates of energy loss for primary electrons and the corresponding cross sections [16]. Results of these theories usually predict very similar relations for the cross sections as due to individual electron excita- tions or to collective electron excitations [l71 (e. g., interaction with the conduction electron plasma in metals). The energy losses of this type (characteristic energy losses) are often sharply evident on the low- energy side of electron emission lines. These most probable discrete loss peaks are typically less than about 30 eV. It is interesting to note that such charac- teristic energy loss spectra are very similar for both a metal and its compounds [18].

Although the precise form of the cross section equations vary according to present theories [16, 17, 191, that of Van der Ziel would seem sufficiently descriptive for our purposes here :

where n is the number of electrons per atom which are absorbing a most probable energy value, U.

The three types of cross sections described by (14), (15) and (16) should yield straight line plots, singly or as a sum effect, when plotted as oE vs In E. In figure 12, plots of E(a/A) are shown as a function of In E for barium stearate and for stearic acid double layers, and for the contribution of barium alone. If elastic scattering were dominant, these straight line fits should have zero slope.

McFarland [20] has measured the total ionization cross section for barium atoms as ionized in an ato- mic beam by electrons of energy in the few-hundred- volt region. He used a surface ionization detector by which he could detect double ionization events as well as single. His data have been replotted in figure 13 along with those for barium as measured here in stearate double layers. Also plotted here is a theoreti- cal curve calculated by McFarland based upon the Gryzinski [21] semiclassical theory for total ioniza- tion cross sections. It also seems of interest to include here the theoretical curve as predicted cc semiclassi- cally ^B by using the occupancy number of electrons

9

C4-122 BURTON L. HENKE

the measurement of total cross sections for the inter- action of monoenergetic electrons in solids and in the 500-1 500 eV region. Essentially no such data exist in the present literature ; hence there has been essentially no support for the development of theore- tical models for the interaction of low energy elec- trons in solids. The method that has been described here is based upon the measurement of photoelec- tron lines as emitted and as absorbed by monolayer systems. It has been found that these systems can be constructed with sufficiently precise knowledge of density, uniformity, chemistry and structure to permit quantitative measurement at the difficult monolayer- thickness levels. Although the application of only one type of monolayer has been presented here, viz., the metal soaps, it is important to note that there are a large number of insoluble monolayer systems of pure chemical and biological systems that can be

, , , I

6.0 6 5 7.0 7.5 8.0 in this work. _.- -

L n E -+

If, by methods as described here, quantitative know- FIG. 12. - Demonstrating the degree of linearity in the ledge can be obtained for the interaction of kilovolt-

E(o/A) VS In E curves. Most theoretical expressions for cross region electrons i n surface films, two interesting and sections for elastic scattering, for total ionization, and for

important steps then be taken The intensities collective interaction losses predict that such curves will be linear

for the electron energy region of interest here. of photoelectron lines can be used to yield new and needed measures of photoelectric cross sections for

14 - McFARLAND (1967)

0 HENKE 0970)

THEORY THEORY

I ^{I I I I I}

0 300 600 900 1200 1500

E!&)

FIG. 13. - Comparing the cross sections measured in this work for barium (as combined in a stearate double layer) with

those measured ^by McFarland on barium ⁱⁿ an atomic beam.

per shell in the barium atom for n i (instead of the oscillator strengths) in the Bethe relation of (14).

The single electron ionization energies for barium were taken from the Bearden-Burr tables [22].

7. Conclusions. - A method utilizing low energy electron spectroscopy has been demonstrated for

the atomic subshells for atoms combined in a conden- sed phase. And, finally, the quantitative analysis for surface chemistry and structure by low energy electron spectroscopy may follow for sample surfaces of thicknesses less than 100 A and of sample masses less than one microgram. The author believes that such an application of photo-Auger electron spectro- scopy can and should be emphasized along with the present rapidly developing and important applica- tion based on the measurement :of chemical shifts of photoelectron lines for chemical bonding information.

Acknowledgements. - The author gratefully ack- nowledges the invaluable assistance of his technical and st;dent assistants. Among his more advanced students on this current project have been Therese McElhaney, Jerel Smith and R. Srinivasan.

He is also pleased to acknowledge the collabora- tive assistance of Professor Jack C . Merritt in develo- ping the basic approach of using deposited mono- molecular layers as control standards in quantitative surface chemical analysis by low energy electron spectroscopy (1962).

This research is supported by the United States Air Force Office of Scientific Research under their Grant AFOSR 1262-67.

References

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DISCUSSION

CARLSON. -Your study of transmission of low depth to which atoms lie below the surface. To be energy electrons through monolayers of material perhaps overly optimistic, in the future one might may be of tremendous importance in using photo- be able to locate crucial elements within macromole- electron spectroscopy as a tool for examining the cules such as DNA and RNA.