The 248 nm photochemistry of methyl iodide thin films was studied using reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TF'D), and time-of-flight quadrupole mass spectrometry (TOF-QMS). The formation of predominantly CH212 and C h and some C2H6, CH3CH21, CHI3, and I2 photoproducts retained in the film was characterized by RAIRS and TPD. The integrated areas of the IR absorption bands for the two major photoproducts, CH212 and C h , increase to a maximum and then decrease as photolysis of the film proceeds. A cross section for the loss of CH31 by 248 nm photolysis of the film was measured to be (1.6 f 0.1) x cm2, approximately 1 order of magnitude lower than the gasphase cross section. At all laser fluences used in this study, CH3, I, and CH31 were ejected into the gas phase. The CH3 TOF distribution showed the signature of gas-phase CH31 photodissociation dynamics-two sharp peaks corresponding to the production of iodine atoms in the I(2P3/2) and I*(2P112) states. The TOF distributions of I and CH31 were fit by Maxwell-Boltzmann distributions corresponding to temperatures of 1400 and 1170 K, respectively. Three other species-Ch, I2 and CH&-were observed in TOF-QMS, but only at higher laser fluences. It was determined that the I2 and CH212 species are most likely fragments of a larger molecule, perhaps a cluster species, that photodesorbs as the film becomes enriched with photoproducts.The mechanism for C h photoejection appears to be of a different nature. The photochemistry of methyl iodide thin films can be understood in terms of a combination of photoprocesses occurring in the film and at the film surface.
We have investigated the thermal and photon-induced chemistry of several aromatics adsorbed on smooth and roughened Ag( 11 1) surfaces at low temperatures (1 10 K). Pyrazine, pyridine, 3-chloropyridine, and chlorobenzene quantitatively desorb from Ag( 11 1) near 200 K. After roughening the surface with 2 kV Ar ion bombardment, the breadth of the molecular desorption curves increases, and there is a high-temperature tail due to desorption from defect sites produced by the roughening process. Upon UV photolysis of 1 monolayer, chlorobenzene and 3-chloropyridine photodissociate on smooth and rough Ag surfaces, whereas no detectable photoreactions were observed for pyrazine or pyridine adsorbed on either surface. For the molecules that did undergo photodissociation, a shift to lower energy in the photodissociation threshold was observed on the rough surface relative to the smooth: 3.3 eV versus 3.5 eV for chlorobenzene and 3.5 eV versus 3.9 eV for 3-chloropyridine. We postulate that the decrease in the photodissociation threshold is due to defect sites produced by surface roughening, through either excitation of the surface plasmon resonance, which is allowed on the rough but not the smooth surface, or a more localized excitation near defect sites. Excitation in the molecular absorption band shows that the photodissociation yield is enhanced for 3-chloropyridine and quenched for chlorobenzene on the rough surface relative to the smooth. Theoretical calculations suggest that there is a decrease in the decay rate for 3-chloropyridine and an increase in the decay rate for chlorobenzene on the rough surface compared to the smooth. Differences between the quenching rates for 3-chloropyridine and chlorobenzene may be related to the different molecular orientations of these two molecules on the surface.
We have studied the surface chemistry of trifluoromethyl iodide adsorbed on Ni( 100) under ultrahigh-vacuum conditions. Temperature-programmed desorption and reflection absorption infrared spectroscopy were used to detennine gas-phase products and species formed at the surface. Several reaction products were found from trifluoromethyl iodide dissociation on Ni(100). Iodine atoms and nickel fluoride, NiF2, desorb at high temperatures near 1000 and 800 K, respectively. At high coverages, carbon-containing species desorb from the surface as well. CF31 and CF3 desorb at 168/136 K (mono/multilayer) and 316 K, respectively. In the presence of background hydrogen, HF and C H~F Z are also detected in temperature-programmed desorption at 178 and 235 K, respectively. The infrared spectra of multilayer, monolayer, and submonolayer coverages of CF31 on the surface and adsorbed CF, groups have been measured. The temperature-programmed desorption and infrared data show that both carbon-iodine and carbon-fluorine bonds are readily activated on nickel at low temperatures. It is estimated that approximately 90% of adsorbed CF31 decomposes on Ni(100).
We have investigated the photodissociation of chlorobenzene and 3-chloropyridine adsorbed on smooth and rough Ag surfaces. Photolysis of adsorbed chlorobenzene and 3-chloropyridine with UV radiation results in C-Cl bond dissociation. Biphenyl and bipyridyl desorb from the surface near 400 K, and AgCl desorbs near 800 K in postirradiation temperature-programmed desorption. Compared to a smooth surface, there is a red shift in the photodissociation threshold for both molecules when adsorbed on a rough surface. Possiblemechanisms for the red shift are discussed. This study demonstrates the importance of substrate morphology on surface photochemistry.
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