INFRARED SPECTROSCOPY OF ADSORBED MOLECULES : SOME EXPERIMENTAL ASPECTS

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INFRARED SPECTROSCOPY OF ADSORBED MOLECULES : SOME EXPERIMENTAL ASPECTS

R. Ryberg

To cite this version:

R. Ryberg. INFRARED SPECTROSCOPY OF ADSORBED MOLECULES : SOME EXPER- IMENTAL ASPECTS. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-421-C10-427.

�10.1051/jphyscol:19831085�. �jpa-00223543�

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JOURNAL DE PHYSIQUE

Colloque C10, supplbment a u n012, Tome 44, d k e m b r e 1983 page C10-421

I N F R A R E D SPECTROSCOPY OF ADSORBED MOLECULES : SOME E X P E R I M E N T A L ASPECTS

R. Ryberg

Department of Physics, ChaZmers University of Technology, 5-412 96 GUteborg, Sweden

Resum6

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Cet article passe en revue la situation exp6rimen- tale en spectroscopie infrarouge des mol6cules adsorb&es, en particulier celles adsorbges sur une surface m6tallique d'un monocristal. Les conditions expgrimentales concernant ce type d16tude sont definies ; certaines difficultes inh6- rentes 3 la spectroscopie infrarouge sont discutees. Nous d6crivons ensuite les differents montages exp6rimentaux utilis6s dans le domaine. A titre d'exemple de spectromgtre infrarouge destine 2 la physique des surfaces, nous decri- vons un spectrom6tre sous vide avec modulation de longueur d'onde. Pour finir, nous discutons quel est le spectromStre infrarouge le mieux adapt6 2 differents types d'etudes.

Abstract

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This paper reviews the experimental situation in infrared spectroscopy of adsorbed molecules, particularly molecules adsorbed on a single crystalline metal surface.

The experimental conditions for this kind of study are de- fined and some special difficulties with infrared spectroscopy are discussed. This is followed by a review of the different experimental setups that have been used in the field. As an example of an infrared spectrometer dedicated for surface physics work an evacuated wavelength modulation spectrometer is described. Finally, the optimum infrared spectrometer for different kinds of study is discussed.

INTRODUCTION

A general introduction to and the history of infrared spectroscopy of adsorbed molecules have most recently been given by Dignam and

Fedyk ( 1 ) and Hollins an6 Pritchard (2). All work published in the field up to August 1 9 8 2 has been tabulated by Darville (3). In this paper I will restrict myself primarily to a discussion of work on single crystalline metal surfaces under well-characterized conditions.

The physical problem is that we want to excite vibrational modes in molecules adsorbed on a metal surface. We know that infrared radiation can only excite modes that are dipole active and to which we can associate a dynamical dipole nonent

8 .

From previous work by Persson and Ryberg (4) it follows that the quantity of interest to measure is the absorptance, because

where I? and I are the intensities of the incident and reflected light respect~vely, n is the number of adsorbed molecules,-riw the photon energy and a the complex polarizability of the adsorbate layer. For a single adsorbed molecule there is a simple relation between a and

u

but for an adsorbed layer, where different kinds of interaction may exist, the expression for a is more complicated (4).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831085

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C10-422 JOURNAL DE PHYSIQUE

We must also consider the so called "infrared selection rule", which states that if the substrate is a good conductor the electric field in the metal surface is screened out. This means that it is only the component of the field normal to the surface, Ep, that can excite vibrational modes in the adsorbed molecules and simultanously that only modes which have a non vanishing component of their dynamical dipole moment

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normal to the surface can be excited.

The other important restriction is, as was first shown by Greenler (5)

,

that the probability for exciting molecular vibrational modes is strongly dependent on the angle of incidence of the light with a maximum typically around 85O. All in all we end up with an experimental situation shown in figure 1. I may add that for small molecules there usually exist rather few dipole active modes distributed over a large energy region (typically 300-3000 cm-1) and with a peak width less than 50 cm-1.

Fig. 1

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A typical experimental situation for infrared spectroscopy of adsorbed molecules.

THE RELUCTANT INFRARED

Anyone used to optical spectroscopy finds many new problems arising when going into the infrared, and I will briefly discuss the major ones here.

Light sources. The most used infrared light source is the globar

,

a Sic rod heated to about 1500 K. Even if the temperature is low the emissivity is high so there is a considerable output. The globar may be used in an ordinary atmosphere. It is also possible to use a tungsten filament at about 3000 K in vacuum or an inert gas. The drawback is the low emissivity ( a 0.2) which however, can be increased up to 0.7

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0.8 by making some kind of cavity of a tungsten foil (6).

Recently tunable infrared lasers using Raman shifts in H2 gas have be- come comercially available. One nanufacturer (Lamda Physics) claims that by using an Excimer pumped dye laser and the third Stoke's shift in H2 one can reach beyond 10 pm with a mean power of 10-2 W at 10 Hz.

It has since long been known that synchrotron radiation, in addition to being a dedicated X-ray or UV-source, also can be used in the infrared and four ir-spectrometers are presently being set up at different storage rings.

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I I 1 1 I I I I I I I I 1 1 1 1 1 2

- - - -

- - - - - - - -

1 I I I I I I I I I I I 1 1 1 1 1

100 1000 10000

WAVENUMBER (cm")

Figure 2. Emitted power from a alobar (----I, a tungsten cavity (----I and a synchrotron (-a-.-)

In figure 2 I compare the emitted power from a globar (1500 K , emissivity 0.9)) a tungsten cavity (3000 K emissivity 0.7) assuming a slit height of 10 mm and a f-number of 3.5 with synchrotron radiation from a small ring (MAX at Lund ( 7 ) ) with a current of 0.5A and a beam di- vergence of 40 mrad, all with a constant bandwidth of 5 ern-'. One finds that for energies above 800 cm-1 the blackbodies are equal or superior, while below 800 cm'l the synchrotron is better. This means, that if you want to study intermolecular vibrational modes you can stick to your blackbody, but if you are interested in the metal-molecule vibration or the metal phonon modes you are better (in the latter case much better) off with a synchrotron. How about the laser? Well, even if the mean power is at best three orders of magnitude higher, the laser spectrometer will be limited by source noise while the other arrangements are limited by detector noise. So special precautions must be taken and laser setups must be tested before an definite opionion can be male.

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S l i t widths. As we have choosen to work on single crystalline surfaces it is difficult and expensive (although possible) to make samples larger than say 1 2 x 20 mm. With the configuration in figure 1 one finds that the widest slit one can focus on the sample is about 1 mm.

This means that for longer wavelengths one gets an unnecessarely high resolution and a correspondingly low power with standard gratings.

Therefore one may have to order special gratings with large groove spacings together with a small blaze angle. From this aspect the synchrotron and the laser are superior with their very narrow beam angles!

OptieaZ components. The problem with windows transparent at long wavelengths is that these materials (such as CsI) are very hygroscopic.

A good solution to this is to build a vacuum spectrometer with the windows sealed off with gatevalves, never exposing them to the atmosphere after mounting. A more radical solution is to build a completely UHV compatible spectrometer with no windows at all ( 8 ) .

It has previously been difficult to obtain grid polarizers with good extinction. However, one can now obtain ion beam eched ~olarizers with an extinction of 5.1 o - ~ beyond 1 0 pm which would allow the con- struction of precise polarimetric spectrometers.

Detectors. The sensitive area of an infrared detector is usually rather small, so when focusing onto the detector one gets a large field of view (typically around 600). This means that much room temperature background radiation is seen by the detector. To overcome this a cooled bandpass filter may be placed in front of the detector, trans- mitting just in the region where you want to measure. For multiplex

spectrometers this can not be made equally efficient and one may cool the whole surrounding areas instead. Little is however gained unless the sample is also at low temperature. Note again the advantages of synchrotrons and lasers, which give a much smaller field of view.

DIFFERENT EXPERIMENTAL APPROACHES

Here 1 discuss most of the different infrared spectrometer setups that have been used for studies of adsorbed molecules.

Dispersive with external light source.

Single beam reftectometer. Simplest possible approach. Can only be used for high coverages of strongly absorbing species like CO ( 9 ) .

Double beam reftectometer. Difficult to realize in connection with a vacum' chamber and it can also be difficult to prevent adsorption on the reference sample ( 1 0)

.

EZlipsometer. I f the polarizers are good enough one can of course set up some kind of ellipsometer. Fedyk et al. (11) have used this

approach to measure both R /R, and the phase, but as stated in the introduction it is only the gorrner quantity that is important. The great advantage of the ellipsometer is its surface sensitivity, that is it can detect adsorbed molecules with the sample in high ambient gas pressures. The drawback is its complexity and intensity losses in the many optical components.

PoZarizat7;on modulation. This method gives a pseudo-double beam instrument, using the fact that it is only Ep that excites the vibrational modes. A chopper gives alternating I and Is so Rp/Rs can be re-

covered. This setup is simpler than the ellipsometer but measures directly the quantity of interest and has still the high ambient gas pressure adventage. A good sensitivity has been reached with this

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approach (12,131.

VaveZength modulation. This technique is based on the fact that the adsorption peaks are sharp. A modulated wavelength is produced by a vibrating mirror in the monochromator. The signal at twice the modulation frequency is proportional to d 2 ~ / d x 2 , which enchances sharp structures whereas slow background variations are depressed. The advantage of this approach is that it is optically simple, uses all the available intensity and gives no problem with room temperature radiation from a chopper. Disadvantages are that it can generally not take high ambient gas pressures as it is not surface sensitive and it can not measure the absolute value of the absorptance. However, when built as a vacuum spectrometer it has been shown to be the most sensitive method (14) and to have the largest energy range capable of recording spectra down to 1000 cm-1 ( 1 5)

.

Multiplex with external light source.

Fourierspectroscopy is an often used technique in the infrared becuase of its two advantages over dispersive instrument.

a) The multiplex advantage, because one measures over a large energy region simultanously, which gives more power on the detector (the detector noise often dominates in dispersive setups). However, as stated in the introduction the absorbtion spectra of small adsorbed molecules often consist of a few narrow peaks distributed over a large energy range. This means that in such cases the multiplex spectrometer spends much of the measuring time in regions where the absorp- tion is zero!

b) The high throughput advantage, which means that the spectrometer has a very low f-number. However, it is the experimental situation shown in figure 1 that limits the f-number of the whole system, so one can only use a few percent of the maximum throughput of the spectrometer.

Baker and Chesters (16) showed that the method worked in a straight forward setup. Combined with polarization modulation one can also make the spectroscopy surface sensitive (17). A very special approach has been taken by Bailey et a1 (18) using the sample as detector

(bolometer) coupled to a Fourier spectrometer. The sample, an eva- porated film, was held at 1.6 K , making contamination problems severe.

Emissivity measurements

Dispersive. I f the sample is kept at room temperature or above it can be used as the light source, that is in an emission experiment.

Chiang et al. (19) have tested this approach in a very nice way. The demands are that the whole surrounding has to be cooled and that special low noise detectors have to be used.

MuZtipZex. Allara et al. (20) have used the same approach with a Fourier spectrometer, also with very promising results.

Laser spectroscopy.

The only laser work I am aware of is by Chabal and Sievers (21) using a fixfrequency C02 laser.

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THE VACUUM WAVELENGTH MOCULATION SPECTROMETER

As an example I show here schematically an evacuated wavelength modulation spectrometer connected to an UHV chamber. The pressure in the spectroneter is 1 0 - 6 torr.

L = cavity shaped tungsten light source M = 0.25 rn, f = 3.5 grating monochromator VM = vibrating mirror at 400 Kz

S. = slit GV = gate valve

W = o-ring sealed CsI-window EM = off-axis elliptical mirror CF = cold bandpass filter D = semiconductor detector

THE OPTIMUM INFRARED SPECTROMETER

Does there exist an optimum infrared spectrometer that is best for the job in question? From the discussion above it should be clear that which approach one chooses depends on what kind of investigation is

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intended. One may divide the field into three major groups.

Surface physics. By surface physics I mean studies of small molecules and the behaviour of specific vibrational modes under different well characterized conditions, including UHV an2 low temperatures. Here the evacuated wavelength modulation spectrometer has proven to be the most sensitive spectrometer and to have the largest accessible energy range. However, polarization modulation could also be considered.

For energies below 800 cm-1 the use of synchrotron radiation should be favourable.

Surface chemistry a t Low pressures. The chemistry stands for larger molecules with more dipole-active vibrational modes. The previous modulation techniques are still competitive but a multiplex approach should also be considered.

Surface chemistry a t high pressures. Now one needs a surf ace specific technique, which means some kind of polarimetric spectrometer, either dispersive or multiplex. As the sample will probably be kept at room temperature or above, an emission approach could be advantageous.

REFERENCES

1. M.J. Dignam and J. Fedyk, Appl. Spec. Rev.

14

(1978) 219.

2. P. Ilollins and J. Pritchard, Vibrational spectroscopy of P.dsorbates, Ed. Willis (Springer in Chem. Phys. 1980).

3. J. Darville, Vibrations at surfaces, Ed. Brundle and Morawitz (Elsevier 1983).

4. B.N.J. Persson and R. Ryberg, Phys. Rev. B

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(1981) 6954

.

5. R.G. Greenl-er, J. Chem. Phys.

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(1966) 310.

6. J.H. Taylor, C.S. Rupert and J. Strong, J. Opt. Soc. Am.

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⁽¹⁹⁵¹⁾

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3rd Intern. Conf. Solid Surf. (Vienna 1977).

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R. Ryberg, Phys. Rev. Lett.

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(1982) 1579.

M.D. Baker and M.A. Chesters, Vibrations at Surfaces, Ed. Caudano et a1 (Plenum 1982).

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S. Chiang, R.G. Tobin and P.L. Richards, Vibrations at Surfaces, Ed. Brundle and Morawitz (Elsevier 1983).

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