J . Phys. Chem. 1990, 94, 6852-6854
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toms. The C C bond scission is promoted on Pd( 1 IO)( 1X2)-Cs because this surface is electron-rich, and thus, the r* antibonding orbital associated with the CC bond of ethynyl species may easily be filled with electrons. The saturation C2H4 coverage on the Pd( 1 IO)( 1 X2)-Cs surface at 90 K is less than half of that on the Pd(l10) clean surface = 0.23 and 0.58, respectively). This is attributed mainly to the existence of the ( 1 X2) structure. Every other [ IT01 row of the Pd(l I O ) surface is missing on the Pd(l IO)(lX2)-Cs surface according to the missing-row m ~ d e I . ~ ~ - * ~
V. Summary A combined vibrational EELS, TDS, and LEED study has been performed on the interaction of ethylene with the Pd( 1 lo)( 1 X 2)-Cs surface. Some of the important results are as follows: I . At 90 K, ethylene is predominantly *-bonded to the Pd( 1 IO)( 1 X2)-Cs surface. A small amount of physisorbed ethylene exists for a large exposure (2 langmuirs). The fractional ethylene coverage is 0.23 at the saturation.
2. Physisorbed C2H4 (C2D4) is desorbed by heating to 110 ( 1 10) K. A part of *-bonded C2H4(C2D4) is desorbed intact at 200 (210) K, and additionally at 270 (280) K by the recombinative process. On heating to 300 K, methylidyne (CH, CD) species, and H (D) and C adatoms are formed. On heating up to 500 K, only carbon adatoms exist on the Pd( 1 lo)( 1 X2)-Cs surface. 3. Compared with ethylene on the P d ( l l 0 ) clean surface, ethylene on the Pd( 1 IO)( 1 X2)-Cs surface is more weakly bonded to the surface at 90 K. The dehydrogenation and CC bond scission processes are promoted on the Pd( 1 IO)( 1 X2)-Cs surface. The CC bond scission occurs at 300 K. Effects of Cs adatoms on the surface reactions are discussed in section IVD. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (Research Program “Surfaces as New Materials”) from the Ministry of Education, Science and Culture, and by a Grant-in-Aid from the Foundation for Promotion of Material Science and Technology of Japan.
Correlation of Photoelectron Yields and Photodissociation Rates of CH,CI on Pt( 111) and Carbon-Covered Pt( 111) Sam K. Jo and John M. White* Department of Chemistry. The University of Texas at Austin, Austin, Texas 78712 (Received: February 16, 1990)
Photoelectron yields, as a function of coverage, were measured during UV irradiation (
