PHOTOCHEMISTRY OF ADSORBED MOLECULES .17. PHOTODISSOCIATION AT 193 NMOF CH3BR ADSORBED ON LIF(001) AND NACL(001)

The adsorption and photodissociation of CH3Br(ad) on LiF(001) and NaCl (001) have been studied by temperature programed desorption (TPD) and angle-resolved time-of-flight mass spectrometry. The results of TPD on LiF(001) indicated that CH3Br(ad) formed a first adsorbed layer in wh ich repulsive adsorbate-adsorbate interactions dominated. At high cove rages, the TPD profiles on LiF(001) were characteristic of multilayer (''zero-order'') desorption but at no time were separate monolayer and multilayer TPD peaks observed. In contrast, on NaCl(001) CH3Br(ad) fo rmed a distinct monolayer phase before growth of the second and subseq uent layers, Methyl radicals were produced by photodissociation of CH3 Br(ad) at 193 nm on both surfaces at coverages from submonolayer to > 10 monolayers. The CH3(g) translational energy distribution, P(E-T'), exhibited two coverage-dependent photodissociation channels; the first , termed the ''direct'' (DIR) channel, observed at multilayer coverage s, had a P(E-T') centered at 2.4 eV, and the second, termed the ''indi rect'' (IND) channel, had a P(E-T') less than or equal to 1.8 eV. The mean energy and width of the DIR methyl, P(E-T'), was similar to that from CH3Br(g) photodissociation. This DIR channel peaked at approximat ely 25 degrees off-normal for LiF(001) and 38 degrees off-normal for N aCl(001), giving the most probable C-Br bond directions. The IND chann el was comprised of CH3(g) which escaped from the adlayer after suffer ing an inelastic collision (including sometimes reaction) with neighbo ring CH3Br(ad) molecules. The DIR channel dominated at all CH3Br(ad) c overages on LiF(001) but the IND channel dominated at all coverages on NaCl(001), consistent with a model in which CH3Br(ad) on LiF(001) pre dominantly ''stands up'' and CH3Br(ad) on NaCl(001) predominantly ''li es down.'' Polarized IR spectroscopy performed in this laboratory supp orts these geometries. There was a peak in the LIF(001) IND energy dis tribution centered at 0.9 eV and there were two peaks in the NaCl(001) IND energy distribution, IND(1) and IND(2), centered at 1.1 and 0.5 e V, respectively. These substantial but specific energy losses (compare d with the energy of the directly photorecoiling CH3) are indicative o f a strongly inelastic encounter with constrained dynamics, believed t o be the exchange reaction CH3+BrCH3'(ad) --> CH3Br+CH3' with retentio n of direction in the CH3'. The LiF(001) and NaCl(001) IND angular dis tributions, P(Theta'), were generally broad and of the form cos(n) The ta' centered on the surface normal but there was evidence for retentio n of direction of methyls in the IND channel at multilayer coverages o n LiF(001), characteristic of exchange reaction. (C) 1997 American Ins titute of Physics.