THE KINETICS AND INFRARED SPECTROSCOPY OF C1 HYDROCARBONS ADSORBED ON CLEAN AND OXYGEN-MODIFIED Mo(100)

Abstract

The surface infrared spectra (700–2300 cm -1) of CH 3 I , CD 3 I , CH 2 I 2 and CD 2 I 2 adsorbed on Mo(100) at 80 K have been observed and assigned. The strongest infrared absorption of CH 3 I is the δs( CH 3) mode at 1236 cm -1, which shifts significantly on heating the surface to 135 K, yielding a new peak at 1106 cm -1 indicating the formation of a surface- CH 3 species. This new feature dominates the spectrum after annealing to 160 K and disappears at 235 K, where the TPD results show the desorption of methane. This chemistry was also studied on oxygen-covered Mo(100) with oxygen coverages of 1.0 and 1.5. These were not found to substantially affect the surface chemistry. When CH 2 I 2 is adsorbed on Mo(100), the ω( CH 2) mode at 1107 cm -1 is the strongest feature, and when one heats the sample to 135 K, a new peak appears at 1061 cm -1 which is ascribed to the formation of a surface- CH 2 I species. This peak disappears on heating at about 200 K, where previous ultraviolet-photoelectron-spectroscopic data showed the formation of a surface- C 1 species. No RAIRS data have been reported for adsorbed-methylene species, because of either a low-adsorption cross section or the lack of appropriate symmetry of these modes. The deuterated molecules show the same effects except that peaks are shifted as expected so that ν(H)/ν(D) ~1.35.