Journal of Electron Spectroscopy and Related Phenom&a, 6 (1975) 391-395 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

ON THE PRESSURE EFFECTS OF GASEOUS SAMPLES ON PHOTO- ELECTRON KINETIC ENERGY

K. KIMURA, T. YAMAZAKI and K. OSAFUNE

Physical Chemistry Laboratory, Institute of Applied Electricity, Hokkaido University, Sapporo 060

(Japan) (Received 5 November 1974)

ABSTRACT

Effects of the pressure of gaseous samples on photoelectron kinetic energies have been studied for Xe, Ar and 0, with a high-resolution photoelectron spectro- meter using the 584 A He line. The following conclusions have been obtained on the pressure-dependent shift in the kinetic energy. The kinetic energy is shifted to lower energy with increasing pressure. The shifts are dependent on the type of gaseous sample and approximately proportional to sample pressure and light intensity, but independent of absolute photoelectron kinetic energies. In a mixture of Xe and Ar, the shifts of the Xe and Ar peaks are the same as each other under a given mixed condition, but depend on their partial pressures. These pressure-dependent effects on the kinetic energy may be interpreted in terms of Coulomb forces due to positive ions constantly present inside the ionization chamber.

INTRODUCTION

in vacuums ultraviolet photoelectron spectroscopy’ of gaseous samples, the calibration of ionization energies is usually carried out with standard substances such as rare gases, which show sharp photoelectron peaks whose ionization energies are known with high precision from optical spectroscopy. In the course of our recent gas-phase photoelectron experiments using a He(I) source, we have noticed that the kinetic energies of photoelectrons are appreciably shifted towards lower energy with increasing pressure.

Siegbahn et a12’ 3 have indicated in gas-phase ESCA studies that the nitrogen and neon 1 s lines are shifted towards lower kinetic energy when the pressure increases. Johansson et a1.4 have also described procedures for the energy calibration of electron spectra in an ESCA study with X-rays.

In the field of vacuum ultraviolet photoelectron spectroscopy in the gaseous

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phase, however, there seem to have been no studies of pressure effect on photoelectron kinetic energy. We considered it important to investigate quantitatively the pressure dependence of photoelectron energy in the gaseous phase in UV PES. In the present study, we used Xe, Ar and 0, as gaseous samples, since the kinetic energies for the photoelectron peaks of these gases can be determined with high precision.

EXPERIMENTAL

Measurements of the kinetic energy of photoelectrons in the gaseous phase were carried out with a JASCO high-resolution spectrometer model PE-IA, which contains a hemispherical electrostatic analyzer (used in our previous studies5).

The pressure in the spectrometer tank was lo- 6 Torr before the measurements and 1 to 5 x 10e5 Torr during the measurements. The gaseous sample was supplied into the ionization chamber through a variable leak valve, and its true pressure was determined from the scale reading of an Edwards pirani gauge with pressure calibra- tion curves. The sample pressure was also monitored with an ionization gauge attached to the photoelectron spectrometer. The ionization chamber is of cylindrical type (14 mm in diameter and 27 mm long).

Photoelectrons entering into the electron analyzer were accelerated or de- celerated. The analyzer was operated at several different voltages between +0.5 and +2.0 V. Acceleration/retardation potentials U were measured at different gas pressures for the 2P3,2 peaks of Xe and Ar as well as for the following four peaks of oxygen: (1) the z, = 0 peak at 12.07 eV in the first band, (2) the 2, = 1 peak at 16.25 eV in the second band, (3) the 0 = 0 peak at 18.17 eV in the third band and (4) the u = 0 peak at 20.29 eV in the fourth band. In each case, the kinetic energy at zero pressure was estimated by extrapolation from the U values measured at different pressures, and then the shifts AK in photoelectron kinetic energies were evaluated.

RESULTS

From the measurements of the peak positions of Xe, Ar and O,, we have obtained the following several results of the pressure dependence on the kinetic energy. The photoelectron kinetic energy is approximately linearly shifted to lower energy with increasing sample pressure, as shown in Figure 1, in which the observed shifts AK in the kinetic energy for the Xe, Ar and O2 peaks are plotted against the pressurep. It is also seen from Figure 1 that for Xe IAKl is larger than for Ar and 0, at an identical pressure.

The shift is approximately proportional to the intensity of the He resonance line as shown in Figure 2, which shows the p-AK lines for different light intensities. Figure 2 also indicates that the AK is independent of the analyzer voltages used.

From the experiments with 02, the four oxygen peaks were found to be equally shifted within a standard deviation of + 16 meV. This indicates that

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Figure 1. The pressure-dependent shifts in the photoelectron kinetic energies for the ‘P3/2 peaks of Xc and Ar and the 18.17 eV peak of oxygen. The lines are drawn through the experimental data shown by solid circles.

0.01 p (torr)

Figure 2. The effect of the light intensity on the pressure-dependent shift in the photoelectron kinetic energy for the Xe 2Ps/~ peak. It is also shown that the shift is independent of theanalyzer voltages used.

0.06-

3 0.06- ?! ZG 9 O.OL- ,

I . % I , , 0.005 0.010 pxe (lorrl

Figure 3. The pressure-dependent shifts for mixtures of Xe and AT, plotted as a function of the partial pressure of Xe at a constant partial pressure of Ar, PAr = 0.0025 Torr. The open and solid circles indicate the shifts obtained for the Xe and Ar peaks, respectively.

the AK’s are independent of the absolute kinetic energies of photoelectrons. For a mixture of Xe and Ar, the AK’s of the Xe and Ar peaks are the same as

each other, but depend on their partial pressures. Figure 3 shows the variation in AK as a function of the partial pressure of Xe at a constant partial pressure of Ar.

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DISCUSSION

Average times of photoelectrons to reach the wall of the ionization chamber or to escape through the slit are roughly less than several nanoseconds, whereas the ionized species in the chamber are estimated to have lifetimes of about 33, 18 and 16 ,US for Xe, Ar and 0,, respectively, from their average speeds (209,380 and 425 m/s, respectively). Consequently, positive ions should constantly remain to some extent inside the ionization chamber. The density of the positive ions in the chamber should depend on the photoionization cross section as well as on the mass of the ions. The linear relationships between AK and p in Figures l-3 may be associated with the number of the positive ions present in the ionization chamber_

The observed pressure-dependent shift in kinetic energy may be explained in terms of the attractive Coulomb forces of the positive ions acting on a photo- electron escaping through the slit. From the experimental AK values, let us estimate the stationary number of positive ions in the chamber. Generally speaking, the kinetic energy of an electron emitted from a sphere containing N positive ions is reduced by ZVe2/r, that is the energy required for removing the electron from the radius y. to infinity;

AK= - ivt?/r,

Putting Y* w 7 mm and using AK = -0.120 and -0.047 eV obtained for Xe and Ar, respectively, at p = 0.01 Torr (Figure I), the number of the ions may be estimated to be N = 5.8 x lo5 and 2.3 x 105, respectively, under these experimental con- ditions.

The present study clearly indicates that the photoelectron measurements under a mixed condition of a gaseous sample and a standard substance should‘always be necessary for obtaining correct ionization energies. Otherwise, appreciable pressure- dependent shifts in ionization energies may occur.

p (torr)

Figure 4. The effects of gas pressure on the photoelectron intensities of the W/z peaks of Xe and Ar and the 18.17 eV peak of oxygen.

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We also measured the intensities of the photoelectron peaks at various pres- sures. The effects of the sample pressure on photoelectron intensity are shown in Figure 4, approximately indicating the difference in photoionization cross-section. The intensity ratio of the 2P 3,2 peaks (Ar/Xe) is about 1.5 at the same pressure. This ratio is considerably close to the ratio of the available photoionization cross-sections of Ar and Xe, (cJc~J = 1.3 at 21 eV6.

ACKNOWLEDGEMENT

We wish to thank Dr. S. Katsumata for helpful discussion:

REFERENCES

1 D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley, New York, 1970.

2 K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Hedbn, K. Hamrin, U. Gelius, T. Bergmark, L. 0. Werme, R. Manne and Y. Baer, ESCA Applied To Free Molecules, North- Holland, Amsterdam, 1969, pp. 19 and 27.

3 l-J. Gelius, E. Basilier, S. Svensson, T. Bergmark and K. Siegbahn, J. Electron Spectrosc., 2 (1973) 405.

4 G. Johansson, J. Hedman, A. Berndtsson, M. Klasson and R. Nilsson, J. Electron Spectrosc., 2 (1973) 295.

5 T. Yamazaki, S. Katsumata and K. Kimura, J. Electron Spectrosc., 2 (1973) 335; K. Kimura, S. Katsumata, T. Yamazaki and H. Wakabayashi, J. Electron Spectrosc., 6 (1975) 41.

6 A. R. Samson, Advm. At. Mol. Phys., 2 {1966) 223.