The photochemistry at a gas-liquid interface was investigated by time-of-flight/quadrupole mass spectroscopy (TOF/QMS). A thin liquid film of 4-iodobenzoic acid (IBA), dissolved in glycerol, was irradiated with nanosecond laser pulses at 275 nm. Atomic and molecular iodine were the only photoproducts leaving the liquid after a low-fluence laser pulse (<5 mJ/cm(2)). The amount of released I atoms was 2 orders of magnitude larger than the amount of desorbed IZ Model calculations were carried out that take into account laser photolysis of IBA, diffusion, and surface evaporation of I and It, and the condensed-phase kinetics of radical reactions. Ejection of hyperthermal I atoms by direct photodissociation of surface layer molecules is also considered. The quantitative analysis is restricted to low laser fluence conditions at which laser-induced heating of the illuminated liquid is negligible. The results of the model calculations were compared to previously obtained TOF data of an alkyl iodide (C18H37I) dissolved in the apolar liquid squalane (C30H62) The velocity distribution of the iodine atoms from the alkyl iodide solution corresponds to the temperature of the liquid (278 K). The contribution of I atoms from depths greater than I nm is large (>99%). In contrast, T atoms desorbing from IBA/glycerol are hyperthermal (T-trans = 480 K) and originate almost exclusively from the topmost molecular layer (1 nm). TOF measurements with a fast chopper wheel in front of the surface provide the time-dependent desorption flux from the surface and confirm that the contribution from deeper layers in the alkyl iodide solution is much larger than in the aryl iodide solution. Model calculations predict the behavior of the two solutions correctly if differences in caging of the geminate pair, diffusion coefficients of the free radicals, and the set of bulk radical reactions in the two solutions are taken into account. The hyperthermal energies of the ejected I atoms from the IBA solution are discussed in terms of the surface orientation of excited IBA molecules. The dependence of the TOF spectra on laser power and IBA concentration is interpreted by a surface excess of IBA. The result is compared to temperature-dependent surface tension measurements of IBA solutions in glycerol and water. The response of the surface tension to an accumulation of IBA at the surface is very weak.