The Role of Substrate Photoelectrons in the Ultraviolet Photochemistry of Physisorbed Methyl Halides.

The ultraviolet surface photochemistry over the range 351nm to 193nm and adsorption behavior of several physisorbed methyl halides was studied on Ni(111), with the particular goal of understanding the role of substrate photoelectrons. Comparison of fragment time of flight yields, as a function of coverage and laser wavelength, with known gas phase behavior, clearly showed that the photochemistry of some of these molecules was dominated by dissociative attachment of substrate photoelectrons. Direct photofragmentation was observed at coverages above 1 monolayer for laser wavelengths which the molecules absorb in the gas phase. CH_3Cl was the most thoroughly studied molecule, and exhibited a dissociative attachment of substrate electrons which, at its maximum, had a cross section 4 orders of magnitude larger than the analogous process in the gas phase. Both the direct photolysis and the attachment process are inhibited when close to the metal substrate. CH_3Br, CF _3Cl, CF_3Br, CF_2Cl_2, and CCl_4 were studied in less detail, and found for the most part to behave similarly, except that their attachment cross sections were nearer the gas phase values. Photolysis of CH_3Cl and some of the other methyl halides was studied on top of spacer layers of water, xenon, and hexane. The spacer was found to affect the energy of the ejected fragments only slightly, but affected the yield of fragments profoundly. The work function of the Ni/spacer system is shown to correlate with the yield: if the work function is above the energy of the incident photon no fragmentation is observed, if it is below this energy fragmentation is observed. The fragment intensity was observed to increase markedly as the photon energy increases beyond the work function. The adsorption behavior and its effect on the work function was studied for most of these molecules. CH _3Cl was found to have the largest effect on the work function (DeltaPhi = -1.25 eV) because of its large dipole. CF _3Cl was found to have the smallest effect (DeltaPhi = -.2 eV), and showed no photochemistry at 248nm. When this same molecule was placed on top of a water spacer, the work function was reduced to below the 248nm photon energy and photolysis was observed.