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The surface photochemistry of monolayer and submonolayer CH3Br on Pt(lll) has been studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) to diagnose the results of continuous low-intensity ultraviolet irradiation. For wavelengths of 300 nm or less, and surface temperatures near 100 K, evidence is presented for the nonthermal cleavage of the C-Br bond and the formation of adsorbed Br atoms and CH3 groups. The wavelength dependence is strong and red-shifted compared to the gas-phase electronic excitation spectrum of CH3Br. By quantitative XPS, the maximum photolysis yield of retained Br from one monolayer of CH3Br is 60 ± 5%. While there is some slow thermal desorption, there is no detectable photodesorption of the parent during irradiation. Excitation mechanisms and product formation channels are discussed.

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The photochemical carbon–halogen bond cleavage of CH3 Cl and CD3 Br, adsorbed on Pt(111) at 100 K, has been studied. These two adsorbates, which do not decompose thermally, are readily photodissociated, either alone or coadsorbed, when irradiated with the full spectrum of a 100 W high-pressure Hg arc. Just as in the gas phase, the dissociation products are methyl groups and halogen atoms. These species are identified using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). With wavelengths below 300 nm removed with a cutoff filter, irradiation of coadsorbed layers breaks only C–Br bonds. Using substrate temperatures of 65 K and coadsorbed Xe, the absence of significant local heating effects is clearly demonstrated. These results show that the photodissociation process is molecule specific and that the time scale for excited state quenching is, for methyl chloride and methyl bromide, not significantly faster than the time scale for significant bond elongation.

The coverage dependent diffusion rate of CO on Ni(100) has been studied by laser-induced desorption methods. The CO diffusion rate varies with the morphology of the adlayer. At low coverages, where the CO is disordered, the rate is relatively low but constant. At high coverages where a c(5√2×√2)R45 structure is known to prevail, the rate is significantly faster but again constant with coverage. Between these two regions (0.3<θCO <0.5), where a c(2×2) structure develops, the rate is intermediate and tends to increase with coverage. The activation energy Ed decreases but the preexponential term D0 also decreases when the rate increases. This compensation effect between Ed and D0 is used to anticipate surface phase changes.