Effect of Surface Roughness on Surface Photochemistry
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However, it was also realized that quenching (2) may also be enhanced on a rough surface or metal particle. 1-3 A*
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4 Since those predictions and some earlier experimental studies on surface photochemistry, -6 there have been numerous investigations of metal surface photochemistry on well-defined single crystal surfaces. Two important results found from the single crystal studies are the following. 7 -9 First, both substrate mediated and direct adsorbate excitations are important in surface photochemistry. Second, the two mechanisms are wavelength dependent. We felt that with the information gleaned from studies on single crystal metal surfaces, it would be worthwhile to reinvestigate the effect of surface morphology on surface photochemistry. Here, we discuss some recent results obtained in our laboratory concerning the effect of surface roughness on the surface photochemistry of 3-chloropyridine and chlorobenzene. These molecules were chosen for study because chlorinated hydrocarbons are known to photodissociate on metal surfaces.7 The smooth surface, an atomically flat Ag(1 11) surface, is prepared by standard polishing techniques while the rough surface is prepared by 2 keV Ar-ion sputtering the same surface. Although the rough surfaces are not well characterized with this method of sample preparation, it does allow for the direct comparison of the effect of surface roughness on the same sample.
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Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
EXPERIMENTAL SECTION The ultra-high vacuum (UHV) chamber has been described previously. 10 Briefly, the chamber is equipped with a cylindrical mirror analyzer, quadrupole mass spectrometer (QMS), ion sputter gun and three dosers. Five mass signals can be monitored simultaneously in TPD. The Ag crystal is mounted on a tantalum cup that can be cooled with liquid nitrogen to 110 K. The tantalum cup is resistively heated and in direct contact with the crystal. Heating rates of 1.5 K/s are typically used in temperature programmed desorption (TPD) experiments. The Ag sample is cleaned by 2 keV Ar+ bombardment to remove sulfur and oxygen impurities from the surface. A smooth surface is prepared by annealing the sample to 900 K. for several hours. A rough surface was prepared by 2 keV Ar+ sputtering at room temperature 4 This procedure has been shown to give a Ag surface that exhibits SERS. C6 H 5 C1 and 3-C 5 H4 C1N were purchased from Aldrich Chemical Company Inc. and freezepump-thawed several times prior to use. Gases are introduced by backfilling the chamber and gas exposures are given in units of Langmuir (L), where 1L = 1 x 10-6 torr-s. The third harmonic of a 10 Hz Nd:YAG laser (Continuum YG661-10) and the frequencydoubled light from a Nd:YAG pumped dye laser (Continuum TDL-60) are used to photolyze the sample at various wavelengths. The laser beam irradiated the sample at an angle of 300 with respect to the surface normal. Photolysis times of two hours were typical and the laser fluence was kept below 10 mj/cm2 per pulse in
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