Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

7B/2003G178

Carbon-induced changes in the chemical property of a W(110) surface
Kensuke TONO,* Junichi IKEUCHI, Hiroshi KONDOH, and Toshiaki OHTA
Department of Chemistry, the University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
Introduction
Carbide-layer formation on tungsten surfaces induces
drastic changes in the chemical properties of the surfaces.
Our recent study has shown that a W(110)(15×3)R14°–C
surface adsorbs hydrogen molecules much more slowly
than a clean W(110) surface [1]. The present study
extends the investigation of chemical reactions between
the carbide-modified W(110) surface and gas molecules
such as carbon monoxide (CO) and oxygen (O2) by meansof angle-resolved photoelectron spectroscopy (ARPES).
Experimental procedure
ARPES experiments were performed with the vacuum
ultraviolet beam line, BL-7B, of KEK-PF. The W(110)
surface was cleaned by oxygen treatment and subsequent
flashing to 2200 K. The W(110)(15×3)R14°–C surface
was prepared by 10-Langmuir (L) exposure of the cleansurface to ethylene and subsequent flashing to 1500 K.
Photoelectron spectra were measured at an electronemission angle of 90° from the surface (normal emission).
The energy and angular resolutions are estimated to be
0.2 eV and 2°, respectively, in the present experimentalcondition.
Results and discussion
Figure 1(a) shows a normal-emission photoelectron
spectrum of the W(110)(15×3)R14°–C surface measured
with the photon energy of 21 eV. The origins of thespectral features observed have been discussed in ourprevious report [2]. The spectrum did nod change after20-L exposure of the surface to CO at the temperature of295 K [see Fig. 1(b)]. Therefore, it is found that theW(110)(15×3)R14°–C surface does not adsorb CO
molecules at 295 K. However, this surface adsorbs the COmolecules at the lower temperature of 100 K. Figure 1(c)shows the spectrum measured after 5-L exposure to CO at
100 K. This spectrum is completely different from that ofthe intact W(110)(15×3)R14°–C surface. The spectralfeatures at about 7.5 and 11.5 eV are ascribed to themolecular orbitals of CO (5σ+1π and 4σ, respectively),and thus, indicate molecular adsorption of CO.
On the other hand, the W(110) surface adsorbs the COmolecules even at 295 K; a photoelectron spectrum of thissurface significantly changed after 5-L exposure to CO at
295 K as shown in Figs. 1(e) and (f). In the spectrum
measured after the CO exposure [Fig. 1(f)], there are no
spectral features that originate from the molecular orbitalsof CO, that is, the CO molecules on the surface arereadily dissociated at 295 K. These results indicate thatthe W(110) surface is more reactive with CO than theW(110)(15×3)R14°–C surface.

58

Users' Report

Figures 1(d) and (g) show normal-emission
photoelectron spectra of the W(110)(15×3)R14°–C andW(110) surfaces measured after 10-L exposure to O2 at
295 K. The O2 exposure induces only slight change in thespectrum of the former surface [see Figs. 1(a) and (d)],while considerable change in the spectrum of the lattersurface[see Figs. 1(e) and (g)]. Therefore, it issuggested that the W(110) surface is oxidized much moreeasily than the W(110)(15×3)R14°–C surface.
The present results lead us to conclude that the carbide
layer on the W(110) surface is less reactive than the bareW(110) surface.

Fig. 1. Normal-emission photoelectron spectra of theW(110)(15×3)R14°–C and W(110) surfaces measured
with the photon energy of 21 eV: (a)–(d)W(110)(15×3)R14°–C surface. (e)–(g) W(110) surface.

References[1] J. Ikeuchi et al., PF Activity Report 2003 part B, 86(2004).
[2] K. Tono et al., PF Activity Report 2003 part B, 87(2004).
* tono@chem.s.u-tokyo.ac.jp