Hydrogen detection by electron energy loss spectroscopy

HYDROGEN DETECTION BY ELECTRON ENERGY LOSS SPECTROSCOPY D. MASSIGNON,

F. PELLERIN*

and C. LE GRESSUS

LXvision de chimie, D&artement de Physico-Chimie, CEN Saclay, BP No. 2, F-911M Gif-sur-Yvette, France * Centre d’Etudes de Chimie Mktallurgique, CNRS, 15 rue G. Urbain, F-94400 Vitry-sur-Seine, France

Hydrogen adsorbed at room temperature on tantalum, platinum, and silicon surfaces was analyzed by means of reflexion Energy Loss Spectroscopy (EIS). When a molecular hydrogen pressure higher than IO-‘Torr was introduced in the apparatus, an EIS peak located at 13 eV below the elastic peak appeared on all samples. This peak was attributed to the excitation of the molecular hydrogen bond. Furthermore, on silicon, a second peak corresponding to the adsorption of atomic hydrogen was observed. This technique could be applied to the detection of hydrogen trapped in the wall of nuclear fusion devices.

1. Introduction When a clean surface is covered with a gas or with a solid impurity, the energy distribution of electrons emitted from the surface under electron bombardment changes drastically. The intensity of the background is modified and new peaks appear (secondary electron peaks (1) Auger peaks (2), and energy loss peaks (3), fig. 1). In previous studies of carbon segregation [l] and of oxygen adsorption [2] on aluminum, we showed that a very good correlation existed between the Energy Loss Spectra (ELS) and the Secondary Electron Spectra (SES). ELS peaks correspond to primary electrons which have lost a discrete amount of energy in an electronic transition (interband transition, molecular excitation, plasmon excitation). SES peaks were

attributed to the relaxation of the excited states probed by ELS with transfer of the released energy to a valence electron. Several studies of hydrogen adsorption on Ni t3,41, Pt PI, ‘A’t61, and Si [7] have been performed using ELS in the past. ELS peaks of various energies are reported. They are attributed to the excitation of the metal-hydrogen bond. In a previous paper [8] we reported that a new ELS peak appears when Ta and Pt are exposed to pressures of molecular hydrogen higher than 10M4Torr. The energy of this loss peak seemed to be independent of the substrate. It was attributed to ISi+ 18: and lX:+II:, transitions of molecular hydrogen. In this paper we extend these results to silicon. Silicon was chosen because its electronic structure is very different from Ta and Pt. Our intent was to check the independence of the 13 eV loss peak on the substrate. Furthermore, an attempt is made to separate the atomic from the molecular hydrogen contributions to the ELS spectrum. 2. Experimental

6

Eloetren Enorgy Fig. 1. Energy distribution of electrons emitted under electron bombardment (schematic). (1) True secondary electron spectrum (SES). (2) Auger peaks (AES). (3) Energy loss spectrum (ELS). (4) Elastic peak.

Journal of Nuclear Mater&

The experiments were performed in an ultra high vacuum vessel (base pressure 5 x lo-” Torr). The electron energy analyzer was a cylindrical mirror (CMA) equipped with a coaxial electron gun. The spectra were recorded: (i) in the EN(E) mode by means of wehnelt modulation and lock-in detection [9], and (ii) in the second derivative mode -d2N(E)/dE2 by modulating the

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D. Massignon et al. I Hydrogen detection by ELS

812

deflection voltage (lV,,) and observing the second harmonic. Primary electron energies (E,) in the range 100-200 eV were chosen in order to enhance the surface sensitivity of the technique, to work at a sufficiently good energy resolution (l? = 0.5 eV), and to be near the maximum of the cross-sections for excitation of electronic transitions. The primary electron current Ip was kept smaller than 5 x lo+ A. In these experimental conditions the spot diameter was greater than 1001.~rn. The primary current density was always smaller than 5 x 10T4A * cm-*. Thus, the electron-induced modifications of the surface (heating, local electric field, desorption) were minimized. The samples of polycrystalline Ta and Pt and monocrystalline Si (111) were mechanically pol-

E.n(E) (a4

ished and introduced in the apparatus. They were heated at 600°C and ion sputtered (Ar+, 3 keV, 10d5A * cm-*) for several days. After such a treatment neither carbon nor oxygen were detected by Auger Electron Spectroscopy (AES). Furthermore, for Si, the surface plasmon loss peak intensity was characteristic of an extremely pure surface. This cleanliness test is much more sensitive to surface contamination than AES

[lOI. Molecular hydrogen was introduced into the main chamber through a leak valve. Atomic hydrogen was produced by introducing molecular hydrogen (-5 x 10e6Torr) in a differentially pumped ion gun with the filament “on” but with no voltage applied on the electrodes. The percentage of atomic hydrogen obtained by this method is not known. This hydrogen flows into the main chamber (-3 x lo-‘Torr) by the differential pumping aperture and reaches the sample with a very low kinetic energy. The purity of hydrogen was monitored by means of a quadrupole residual gas analyzer. The main impurities were Ar and CO. For low hydrogen exposures (< 10s6Torr), the partial pressure of CO was lo-‘OTorr. For higher exposures the ion pump released some CO and its partial pressure rose to 10e7Torr.

3. Results 3.1. Tantalum Fig. 2a shows the ELS spectrum of clean Ta. When hydrogen at a pressure of 10m4Torr was introduced in the apparatus, a new 13 eV loss peak appears (fig. 2b). After pumping down, this loss peak partially disappears (fig. 2~). A short period of heating at 300°C is needed to remove it completely (fig. 2d).

d 300 ‘C heating

I

I

40

I

30

1

20

I

ImqE W

10

0

Fig. 2. EIS spectra of molecular hydrogen adsorbed on Ta [EN(E) mode]: (a) clean Ta; (b) under 10-4Torr hydrogen pressure; (c) after pumping down; (d) after heating.

3.2. Platinum The ELS spectrum of clean Pt is shown in fig. 3a. Under various hydrogen pressures (figs. 3bd), a new 13 eV loss peak is detected. This peak disappeared when hydrogen was pumped down.

D. Massignon et al. / Hydrogen detection by ELS

En(E)

EpcQOdV

E+OO&

425.3

(a-u)

P’

’ P&d

_d4 -._._ 4#4 ---__ 7&4

Fig. 3. ELS spectra of molecular hydrogen adsorbed on Pt [EN(E) mode]: (a) clean Pt; (b), (c), (d) under various hydrogen pressures. LOSS

3.3. Silicon (111) Fig. 4a shows the loss spectrum of the clean Si (111) surface. When the clean surface is exposed to atomic hydrogen (3 x 10e7Torr, 45 mn), and after pumping down (fig. 4b), an 8.6 eV loss peak is detected. The surface is then exposed to molecular hydrogen. A sharp peak at 12.8eV increases with hydrogen pressure (fig. 4c) and disappears when hydrogen is pumped down (fig. 4d). The 8.6 eV loss peak also disappears and a 6.2 eV loss peak is observed (figs. 4c and d).

4. Discussion The origin of the structures observed on clean Pt, Ta, and Si (111) surfaces will not be discussed here since we are only interested in the modifications of these spectra under hydrogen adsorption. For more information see [ll-133. We will discuss only the hydrogen-induced features.

ENERGY , eV

Fig. 4. ELS spectrum of atomic and molecular hydrogen adsorbed on Si (111): (a) clean Si (111); (b) exposure to 3 x lo-‘Torr of atomic H for 45 mn and pumping down; (c) under 7 x 10e4Torr of molecular hydrogen; (d) after pumping

4.1. Origin of the 8.6 eV loss peak observed on Si (111) Ibach and Rowe [7] studied the adsorption of atomic hydrogen on Si by ELS. They report that an ELS peak appears at 8.5 eV after atomic hydrogen adsorption. They attribute this loss peak to the excitation of the Si-H bond. Our 8.6 eV loss peak probably has the same origin. 4.2. Origin of the 13 eV loss peak The 13 eV loss peak observed on all samples (Ta, Pt, and Si) has the following characteristics: (i) this peak is observed at the same energy for all samples within the energy resolution of the analyzer (R = 0.5 eV);

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D. Massignon et al. / Hydrogen detection by ELS

(ii) there is no saturation of the intensity of this structure: it always increases with hydrogen pressure; and (iii) its intensity is reversible when hydrogen is pumped down. For all these reasons the 13 eV loss peak is attributed to the excitation of the molecular hydrogen bond. The most likely electronic transitions are lZ:+ 1C: (11.4 eV) and ISi+ III. (12.4eV). These transitions have been observed in electron energy loss experiments performed in gaseous H2 [14]. There is some evidence of carbon contamination of our samples after the introduction of hydrogen pressures higher than 10m4Torr: we observed 6 and 20 eV loss peaks at the end of the adsorption cycle (figs. 2d, 3d, 4d). These loss peaks were previously attributed to carbon contamination [l]. This contamination is probably due to the CO released by the ion pump when it is turned off during exposures to high pressures of hydrogen. This CO could react with hydrogen adsorbed on the surface and lead to carbon deposition. This mechanism might then explain the disappearance of the 8.6 eV loss peak on Si (111) after pumping down the molecular hydrogen and the appearance of the 6.2eV loss peak. The role of this CO is under investigation and the results will be published later. These experiments will be repeated with a liquid Nz cryopumping of the apparatus during the complete adsorption cycle. The first results show the following points. (i) The 13 eV loss peak intensity is not affected by the cryopumping. Thus, this loss peak cannot be attributed to CO. (ii) When the cryopump is operating, the 13eV loss peak does not appear on a clean aluminum surface, but appears on a contaminated aluminum surface under the same experimental conditions. This led us to the conclusion that molecular hydrogen requires either an atomically chemisorbed hydrogen layer or a

contamination surf ace.

layer in order to be adsorbed on a

5. Conclusion The detection of hydrogen by ELS is a very promising technique. It seems possible to disatomically chemisorbed tinguish between hydrogen and hydrogen molecules physisorbed on a surface. Furthermore, it is possible by this method to adjust the depth of analysis dA by changing the primary electron energy E,, since dA is closely related to the mean free path of the analysed electron. For E, = 100 eV, dn = 3-5 A. For E, = loo0 eV, dA = lo-20 A. Such a method could be applied to the detection of hydrogen trapped in the wall of nuclear fusion devices. It may be possible to distinguish between atomic hydrogen bonded to the wall and molecular hydrogen trapped in surface defects. References [l] F. Pellerin and C. Le Gressus, Surface Sci. 87 (1979) 203. [2] F. Pellerin, C. Le Gressus and D. Massignon, Surface Sci., to be published. [3] .I. Kiippers, Surface Sci. 36 (1973) 53. [4] K. Christmann, 0. Schober, G. Ertl and M. Neumann, J. Chem. Phys. 60 (1974) 4528. [S] K. Christmann, G. Ertl and T. Pignet, Surface Sci. 54 (1976) 365. [6] N.R. Avery, J. Electron Spectrosc. Related Phenomena 1.5 (1979) 207. [7] H. Ibach and J.E. Rowe, Surface Sci. 43 (1974) 481. [8] C. Le Gressus, D. Mass&on and F. Pellerin, J. Phys. Letters 40 (1979) L309. [9] C. Le Gressus, D. Mass&on and R. Sopizet, Surface Sci. 68 (1977) 338. [lo] Y. Ishikawa, N. Awaya, T. Ichinokawa, C. Le Gressus and D. Massignon, in: Proc. 8th Intern. Conf. on Solid Surfaces, Cannes, 1980, to be published. [ll] F.P. Netzer and J.A.D. Matthew, Surface Sci. 15 (1975) 352. [12] W.K. Schubert and E.L. Wolf, Phys. Rev. B20 (1979) 1855. 1131 J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. [14] J. Geiger and M. Topschowsky, Z. Naturforsch. 21a (1966) 626.