Surface Science 87 (1979) 501-509 0 North-Holland Publishing Company
A “HIGH-PRESSURE”
ELECTRON SPECTROMETER
FOR SURFACE STUDIES
Richard W. JOYNER and M. Wyn ROBERTS * School of Chemistry, University of Bradford, Bradford BD 7 lDP, England and Kenneth
YATES
V.G. Scientific Ltd., The Birches rnd~stria~ Estate, ~mber~orne Lane, East Gr~nstead, Sussex, RHI 9 1 Q Y, Erzgkmd
Received 6 March 1979; manuscript received in final form 18 May 1979
A photoelectron spectrometer which permits the study of solids in the presence of gas at up to 1 Torr pressure has been developed and the main factors considered in its design and construction are described. The inelastic mean free path of 890 eV kinetic energy electrons through argon at 1 Torr has been measured and shown to be 4.5 f 1 mm. The X-ray induced valence band spectrum of liquid mercury is also reported. It is in good agreement with calculation (S.C. Keeton and T.L. Loucks, Phys. Rev. 152 (1966) 548), but shows some differences from that of condensed mercury monolayers at 163 K (S. Svensson et al., .I. Electron Spectrosc. 9 (1976) 51).
1. Introduction When solid surfaces are examined by physical techniques such as electron spectroscopy and low energy electron diffraction the in-situ gas pressure is limited to less than about lo-’ Torr (1 Ton = 133.3 Pa), Experiments in heterogeneous catalysis are usually performed at pressures up to and frequently greater than 1 atm. Because of this considerable difference in pressure between studies in “surface science” and “catalysis” it could be argued that results from these physical techniques cannot give insight into catalytic problems. As a means of exploring this question we have designed and built an electron spectrometer which allows the study of surface species in a gas pressure of up to about 1 Torr. This approach should lead to a better understanding of those surface species which are weakly held and therefore most likely to be active in heterogeneous catalysis. In this paper we describe the important factors in the design and performance of the spectrometer. We illustrate its application to the determination of the mean free * Present address: Department of Chemistry, University College, Cardiff CFl lXL, Wales. 501
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path of -890 eV kinetic energy electrons through argon and in the study of the X-ray induced valence band spectrum of mercury. An example of its advantage over conventional electron spectrometers in the study of chemisorption has recently been published [4].
2. Design and construction The factors governing the design of any spectrometer are the needs for adequate resolution and sensitivity. In the present application, where it is desired to maintain the highest possible gas phase pressure, this means reducing the attenuation of the signal from the solid, by the gas phase, to a minimum. The path of the photoelectrons through the high pressure region of the gas should therefore be as short as possible and to achieve this we employ a feature familiar in electron spectrometers for the study of gases, namely differential pumping. At the same time the spectrometer must be capable of maintaining a solid surface free of contamination during the course of an experiment. The attenuation of an electron beam passing through a layer of gas 1 mm thick is given by: I attenuated = Zvac exp(-P/V,
(1)
where P is the pressure in Torr and h is the inelastic mean free path of electrons through the gas at 1 Torr pressure. Values of h are not readily available although Dushman [l] suggests that, at low electron energies they can be calculated from: x, = 4&C+,
(2)
where up is the mean free path of the gas molecules themselves at pressure P. Eq. (2) suggests that X, may be about 1 mm at 1 Torr pressure and this was the value assumed in designing the present electron spectrometer. Eq. (1) indicates that, with a 1 mm path length about 14% of the electrons would pass elastically through the gas and into the electron spectrometer at a pressure of 1 Torr. The spectrometer was therefore designed with a view to operating with a gas pressure of 1 Torr above the solid sample being studied. The configuration of the sample/gas cell reflects a compromise between several factors and a section through the cell is shown in fig. la. An important feature is the electron lens between the gas cell and the retarding element of the electron analyzer. This is essential to allow adequate differential pumping as well as sample access. It also acts as a broad pass filter. The sample is held in a horizontal plane to facilitate the study of powders and liquids. The path length between the sample and electron exit hole was increased to 2 mm to allow access of photons to samples of up to 3 mm in width. It is intended inter alia to study single crystals and 3 mm was considered the acceptable minimum width for characterisation of the surface by low energy electron diffraction. Both X-ray and UV sources are incorporated, each
R. W. Joyner et al. /High-pressure TO LENS
electron spectrometer
503
ELECTRON & ANALYZER
a
TO
1
PUMPS
TRAPS& PUMPS
b
GAS
IN
GAS
IN
GAS
IN
Fig. 1. (a) Section through sample/gas cell, showing configuration and position of photon sources etc. fb) General diagram of the spectrometer: A = argon ion gun, D = differentially pumped region, EL = electron lens, G = gas cell, HSEA = hemispherical electron analyzer, LO = 2 grid LEED optics, LV = leak valve, M = longe travel rotatable manipulator, P = Pirani gauge, S = sample, TSP = titanium sublimation pump, W = window, X = twin anode X-ray source. The location of the W source (not shown above) can be clearly seen in (a).
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illuminating about 1 mm* of the sample. Mg Ka! photons, with up to 500 W power, enter through a vacuum tight aluminum foil window. The He I radiation through the differentially pumped region surrounding the sample cell and enters the gas cell through a small hole. The photon sources are standard V.G. components with only small modifications. The differential pump~g must be wuch that the pressure in the electron analyzer does not exceed 5 X IO-’ Torr with 1 Torr in the gas cell, the pressure in the region between the gas cell and the electron analyzer should not exceed 2 X 10e3 Torr. To achieve this the sample/gas cell is situated directly over an Edwards 4 inch diameter, polyphenyl ether oil diffusion pump, separated from it only by a water cooled baffle. The electron analyzer is pumped, through a large cold trap (V.C. CCT loo), by another 4 inch diameter oil diffusion pump. The overall layout of the spectrometer is illustrated in t?g. lb. The ultimate vacuum of each part of the stainless steel system is at least 2 X 10-r’ Torr. The right-hand preparation chamber, equipped for single crystal work with specimen cleaning facilities and LEED, includes a titanium sublimation pump and can achieve pressures lower than 6 X 10-r ’ Torr. A glove box is attached to the left-hand preparation chamber to facilitate studies of catalyst preparation and pretreatment, and also of atmosphere sensitive samples. The uhv gas handling system, serving both preparation chambers and the gas/sample cell is not shown in fig. 1b. The electron analyzer is of the concentric hemisphe~c~ design, 200 mm diameter, with a 6 element Einzel lens input system. This is now marketed by V.G. Scientific Ltd. as “Escalab”.
3. Performance Under ultra-high vacuum, with Mg Kol radiation at 500 W power the count rate for the silver Sd,,, peak is 100 X 1O3 count/s at 1.6 eV full width at half maximum
, 535,o
, BINDING
539.0 ENERGY
,
5L3,O
wrt
, VACUUM
5&7,0
eV
LEVEL
Fig. 2. The oxygen Is peak for molecular oxygen. Mg Ka radiation at 500 W, 1 X IO3
C/S
fsd.
R. W. Joyner et al. /High-pressure
505
electron spectrometer
(FWHM) (analyzer pass energy 50 eV, slits fully open). At 5 eV pass energy the count rate for the same peak is 5.00 X lo3 c/s with 0.87 eV FWHM. These values may be compared to 350 X IO3 c/s at 1.6 eV for the standard V.G. ESCA 3 spectrometer, showing that the introduction of differential pumping and reduction of sample size cause some loss in sensitivity at comparable resolutions. The spectrometer can also be used (with no sample present), to observe photoelectron spectra of gases. Fig. 2 shows the O(ls) peak for molecular oxygen. UV induced spectra of good quality can also be observed. For argon, with minimum analyzer slit widths and He I excitation, the FWHM observed for the 2p3,, peak was -21 meV.
4. Attenuation
of signal from the solid in the presence of gas
Critical to the successful application of the spectrometer is the need to achieve a satisfactory intensity from the solid surface in the presence of gases at pressures up to 1 Torr. To investigate this we chose to examine the attenuation of the silver 3d s/2 core level in the presence of argon at various pressures. Argon pressures were measured with a Pirani gauge (V.G. Model PIR 2) situated in the gas cell, the calibration was checked using the vapour pressure of nitric oxide, 0.1 Torr at 78 K. Fig. 3 shows both the experimental points and a theoretical curve calculated using eq. (2) assuming the mean free path of the electrons (h) to be =4.5 mm in argon at 1 Torr pressure. Two points are noteworthy, firstly that attenuation of signal is less than 50% at 1 Torr argon pressure, corresponding to a decrease in signal to noise ratio of 30%. Secondly, the experimental points follow the theoretical curve well at argon pressures less than 1 Torr, suggesting that the value of X is 4.5 + 1 mm. Above this pressure the count rate drops more rapidly than predicted by theory and falls to zero at -2 Torr. This is thought to reflect decreasing efficiency,
op
0,L
0,6
O,B
I,0
1s
1-4 ,
16
18
20
22
Pergon'torr
Fig. 3. (a) The attenuation of silver 3ds/2 signal by argon: (0) experimental points; ( -) culated from eq. (1) assuming h = 4.5 mm at 1 Torr pressure.
cal-
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R. W. Joyner et al. /High-pressure electron spectrometer
and ultimately failure, of the pumping in the intermediate region between the sample and analyzer, leading to an increase in the path length through gas at higher pressure. The diffusion pump was observed to stall at P*r = 2.0 Torr. The inelastic mean free path of electrons through a gas has not, to our knowledge, been measured. There have, however, been many measurements of total ionization cross section S, a closely related quantity [2]. S is defined by:
where I’ is the positive ion current produced by an electron current I- travelling on a path, length L through a gas, concentration N atoms cme3. From eq. (3) L,, the mean distance between ionizing collisions, can be calculated at any pressure since L, = (SN)-’ cm. The data of Schram et al. for argon [3] yield a mean free path between ionizing collisions of 3.4 mm at 1 Torr pressure for electrons of 900 eV kinetic energy. Given the errors introduced into our pressure measurement by the presence of the electron exit hole just above the sample, and the extrapolation of the results of Schram et al. over several orders of magnitude in pressure, there is good agreement between the different quantities. The results suggest that, as might be expected, ionization of the gas phase represents the predominant mode of electron energy loss. Fig. 3 indicates that the spectrometer adequately fulfils the design specification. Above 1 Torr the magnitude of the signal is limited by the decreasing effectiveness of the differential pumping rather than by considerations of the electron mean free path. With improved pumping it should thus be possible to achieve adequate signal/ noise ratios from the solid at gas phase pressures up to about 5 Torr.
5. Relation
between spectra from the gas and solid phases
The success of the instrument depended on whether or not it was possible to differentiate between spectra from the surface and from the gas phase which arise at high pressures. Spectra from gaseous oxygen for example, are clearly visible at pressures >5 X lo-* Torr. We have explored this during a study of oxygen interaction with a silver foil which is fully described elsewhere [4]. Three surface species are observed, with binding energies 528.3, 530.0 and 532.5 eV. The gas phase oxygen peak, by contrast is observed with a binding energy of 538.3 eV with respect to the spectrometer Fermi level. There is therefore a separation of almost 6 eV between the highest energy surface species and the gas phase peak. There is some overlap of the surface 0 Is peak at -530 eV with the Ka,,, satellite spectrum of the gas phase. This contribution is however only about 15% of the total peak area and can easily be removed numerically (see below).
R. W. Joyner et al. /High-pressure
6. The photoelectron
electron spectrometer
507
spectrum of liquid mercury
Although the main impetus to the design and construction of the spectrometer was the desire to study surfaces and solids at conditions closer to those of catalytic interest there are many other obvious applications. We report one such example, namely a study of liquid mercury. Mercury typifies that class of substances, both solids and liquids, whose vapor pressures are too high for study by conventional electron spectroscopy. The vapour pressure of mercury at 295 K is about 10e3 Torr. The approximately spherical sample (0.005 cm3) of triple distilled mercury was supported in a recess 0.4 mm deep in a stainless steel sample holder. It was introduced onto the sample holder using a syringe. The sample was heated at 340 K to generate a fresh mercury surface, on evacuation to 10m6 Torr the spectrum showed a small carbon peak as the only additional element present. The valence band of the mercury, excited with Mg KCYradiation is shown in fig. 4a. Because of the satellite Kaa.4 spectra from the 5d band the position of the Fermi level is not obvious. After numerical removal of this feature using the X-ray intensity ratios (Van Attekum et al. [5]) however, the Fermi level becomes clearly visible. The shape of the 5d band spectrum is quite different to that observed by Svensson et al. [6] from condensed layers of mercury at 163 K. They observed discrete 5d,,, and 5d3,2 states at 7.8 and 9.8 eV binding energy respectively. These were approximately equal in height, with FWHM values of 0.86 eV for the 5d3,, component and 1.33 eV for the 5d 5,2 peak. For the liquid we observe a broad band, with markedly greater intensity at 7.8 eV BE than at 9.8 eV. We have deconvoluted the spectrum of the liquid phase by an iterative method [7] and the result is shown in fig. 4b. Although deconvolution reveals some additional structure the d levels still show predominant band character. Keeton and Loucks [8] have performed relativistic energy band calculations for crystalline mercury, which suggest the existence of pronounced splitting and bending of the 5d,,, levels due both to crystal field and spin orbit effects. They note that the electronic environment in the liquid is likely to resemble that in the solid. The calculated d band extends from 6.8 to 9.7 eV below Fermi level, in good agreement with observation. Keeton and Lou&s argue that the bending of the 5d,,, bands is due to overlap with the 6sp band and that only a small gap (0.2 eV) exists between the bottom of this dsp band and the lowest binding energy, 5d3,a level. The 5d3,2 bands do not overlap with the 6sp levels and are therefore flat. Our observations are thus in good agreement with theoretical prediction. Mercury can be contrasted with zinc, where no overlap occurs between 3d levels and the 4sp band [8], and discrete d levels are noted by XPS [9]. The binding energies of the 4f peaks, 99.9 and 104.0 + 0.1 eV (calibrated with respect to Ag 3d,,, = 368.0 eV) are the same as those reported for solid mercury by Svensson et al. [6], although different to those noted by Jen and Thomas [lo] for mercury chemisorbed on several substrates. The width of the 4f core levels
508
R. W. Joyner et al. /High-pressure electron spectrometer
a
LI
00 I
o-o
20 I
20
LO I
60
80
100
120 I
BINDING ENERGY
LO
60
1LO IeVl
80 BINDING
100 ENERGY
12.0
15 0
/eV
Fig. 4. (a) X-ray induced valence band of liquid mercury: curve 1 as observed, Mg Ka radiation; curve 2 after removing contribution due to Kor3.4 radiation. (b) Iterative deconvolution [7] of curve 2.
(allowing for differences in analysing conditions) and liquid states.
are similar for the condensed
solid
Acknowledgement We gratefully Council.
acknowledge
the support
of this work by the Science Research
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electron spectrometer
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References [l] [2]
[ 31 [4] [S] (61 [7] [8] [9] [lo]
S. Dushman, Scientific Foundations of Vacuum Technique, 2nd ed. (Wiley, New York, 1962). H.S.W. Massey, Electronic and Ionic Impact Phenomena, Vol. I (Oxford Univ. Press, 1969). B.L Schram, F.J. de Heer, M.J. van der Wiel and J. Kistemaker, Physica 31 (1965) 94. R.W. Joyner and M.W. Roberts, Chem. Phys. Letters 60 (1979) 459. K. Siegbahn et at., ESCA Applied to Free Molecules (North-Holland, Amsterdam, 1969). S. Svensson, N. Martensson, E. Basiler, P.A. Malmqvist, U. Gelius and K. Siegbahn, J. Electron Spectrosc. 9 (1976) 5 1. A.F. Carley and R.W. Joyner, J. Electron Spectrosc. 12 (1978) 411. S.C. Keeton and T.L. Loucks, Phys. Rev. 152 (1966) 548. D. Briggs, Faraday Disc. Chem. Sot. 60 (1975) 81. J.S. Jen and T.D. Thomas, Phys. Rev. B13 (1976) 5284.