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.
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.
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.
Using TPD, ΔΦ, UPS, and XPS, the decomposition promoted by potassium of methyl bromide (CH3Br) on Ag(111) was studied. While adsorption of CH3Br on K-free Ag(111) lowers the work function, on K/Ag(111) it increases the work function. In contrast to the K-free surface, on which CH3Br adsorbs and desorbs with no detectable decomposition, it adsorbs both molecularly and dissociatively on both submonolayer and multilayer K covered Ag(111) at 100 K. A new adsorption state of CH3Br on K/Ag(111) desorbs at 160–165 K, higher than for desorption of CH3Br from the K-free surface (142–145 K). Most of CH3 radicals desorb upon dissociation. The small fraction retained reacts to form CH4 and C2H4 above 250 K. All Br atoms are retained and react with K to form KBr, which desorbs at 650 K. The dissociation, attributed to electron harpooning from K to CH3Br to form temporary CH3Br− ions which then dissociate, is not thermally activated. Mechanistic comparison is made with photon induced dissociation of CH3Br on K-free Ag(111).
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