Surface science investigations of photoprocesses in model interstellar ices

The kinetic energy of benzene and water molecules photodesorbed from astrophysically relevant ices on a sapphire substrate under irradiation by a UV laser tuned to the S 1 ← S 0 π → π * transition of benzene has been measured using time-of-flight mass spectrometry. Three distinct photodesorption mechanisms have been identified—a direct adsorbate-mediated desorption of benzene, an indirect adsorbate-mediated desorption of water, and a substrate-mediated desorption of both benzene and water. The translational temperature of each desorbing population was well in excess of the ambient temperature of the ice matrix.