Photoproduct Characterization and Dynamics in the 248 nm Photolysis of CH3I Thin Films on Ag(111)

Abstract

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.

Electron stimulated reactions of methyl iodide coadsorbed with amorphous solid water

“…5͒. 23 In codeposition experiments illustrated in Fig. 4͔͒, neither products should be retained in RAIRS experiments where the film's temperature is Ͼ100 K. The fact that methane is observed in Fig.…”

“…8,[14][15][16][17][18][19][20][21][22][23] In addition to serving as a model adsorbate, CH 3 I adsorbed even transiently on the surface of water droplets or water ͑ice͒ particles in the upper atmosphere has the potential to interact with solar photons and other energetic sub-atomic particles and generate environmentally important species. This wealth of information on gas phase MeI photochemistry has enabled researchers to assess the effects of the condensed phase on the reaction dynamics, including a determination of the relative importance of direct photodissociation and DEA initiated by "hot" electrons photoexcited from the substrate.…”

Photodissociation of methyl iodide adsorbed on low-temperature amorphous ice surfaces

“…14 There have been extensive experimental and theoretical studies on the photodissociation of CH 3 I in the gas phase [15][16][17][18][19][20][21][22][23][24] or adsorbed on different solid substrates. [25][26][27][28][29][30][31][32][33][34][35] Along with this wealth of information, the recent discovery of iodinecontaining organic compounds as potential sources of reactive halogen species during ozone depletion processes 36 has a) Author to whom correspondence should be addressed. Electronic mail:…”

Thermal and Photochemistry of Methyl Iodide on Ice Film Grown on Cu(111)

“…Assisted by temperature programmed desorption, X-ray and ultra-violet photoelectron spectroscopies, we have studied CH3I which has widely been a Deceased August 31,2007 employed for understanding (heterogeneous) catalytic photochemical and thermal reaction processes of halocarbons adsorbed on a metal substrate. [12][13][14][15][16][17] Because the initial thermal or photo-dissociation channel is a breakage of C-I bond, and no reformation of C-I bond occurs it is possible to separately elucidate the behaviors of CH3 and I.…”