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

Abstract

Thermal and photochemistry of methyl iodide (<TEX>$CH_3I)\;adsorbed\;on\;D_2O$</TEX> ice film on Cu(111) at 100 K were studied using temperature-programmed desorption (TPD) time-of-flight mass spectrometry (TOF-MS), X-ray and ultraviolet photoelectron spectroscopies. On the basis of TPD, multilayer and monolayer <TEX>$CH_3I$</TEX> molecules desorb from <TEX>$D_2O$</TEX> ice layer at 120 and 130 K, respectively. Photo-irradiation at 100 K exhibits dramatic changes in the TPD and I <TEX>$3d_{5/2}\;XPS\;of\;CH_3I$</TEX> on ice film, due to a dramatic dissociation of <TEX>$CH_3I$</TEX>. The dissociation is likely activated by solvated electrons transferred from the metal substrate during photo-irradiation. No other photo-initiated reaction products were found within our instrumental detection limit. During photo-irradiation, the <TEX>$CH_3I$</TEX>, <TEX>$CH_3$</TEX> and I could be trapped (or solvated) in ice film by rearrangement (and self-diffusion) of water molecules. A newly appeared parent molecular desorption peak at 145 K is attributed to trapped <TEX>$CH_3I$</TEX>. In addition, the <TEX>$CH_3$</TEX> and I may diffuse through ice and chemisorb on Cu(111), indicated by TPD and I <TEX>$d_{5/2}$</TEX> XPS taken with photo-irradiation time, respectively. No molecular ejection was found during photo-irradiation at 100 K. The work functions for <TEX>$CH_3I/Cu(111),\;D_2O/Cu(111)\;and\;CH_3I/D_2$</TEX>O/Cu(111) were all measured to be about 3.9 eV, 1.0 eV downward shift from that of clean Cu(111).