Photoproducts ejected from liquid surfaces: The importance of photochemical, diffusional, kinetic, and surface structural effects

Abstract

Gas-phase products of liquid surface photochemistry in high vacuum were analyzed by time-of-flight/quadrupole mass spectroscopy. A thin liquid film of a long-chain alkyl iodide, C18H37I, dissolved in squalane (C30H62) was irradiated with nanosecond laser pulses at 275 nm. The photoproducts leaving the liquid after a low-fluence laser pulse (0.5 mJ/cm2) were I, HI, and I2. Since these species may desorb at times delayed by diffusion in the liquid phase, time-of-flight profiles were also recorded using a chopper wheel in front of the surface. This allowed a reconstruction of the time-dependent flux from the surface. The flight time distributions were compared to model calculations which take into account laser photolysis of C18H37I, diffusion and surface evaporation of I, HI, and I2, and the condensed-phase kinetics of radical reactions, and allow for a component of direct photodissociation of surface layer molecules, leading to hyperthermal I atoms. Simulations based on a comprehensive kinetic scheme are in good agreement with our measurements, with no specific surface processes other than evaporation of thermalized species formed at, or diffusing to the surface. However, compared to results previously found for liquid C2H5I, the caging of the geminate pair is much stronger in our system, and the fraction of I atoms promptly reacting to HI is smaller. The absence of prompt hyperthermal I fragments is interpreted by a preferential orientation of the C18H37I molecules in the topmost liquid layer with the I atom pointing into the liquid. The sensitivity of the method is discussed with respect to surface-specific processes, as well as primary and secondary radical reactions occurring in the bulk liquid.