Photolysis of adsorbed benzene at 248 nm

Aromatics are widely detected in the interstellar media, although the origin of their photostability is not clear. Gaseous molecules should be decomposed by cosmic UV or VUV radiation and little is known about the photochemistry of adsorbed aromatics. In this study, the channels and mechanisms of KrF laser-induced dissociation of benzene condensed on cooled fused silica have been studied using mass spectrometry. The release of H atoms (along with C6H5), C2H2, and C2H4 from the substrate surface indicates three dissociative pathways, including the last two as the aromatic ring opening. All these photoprocesses occurred to be controlled by interaction with three KrF laser photons (λ = 248 nm). The general mechanism of UV photodissociation of adsorbed benzene includes its two-photon excitation into the vibronic continuum, followed by transitions into repulsive states using stimulated photon emission those occur at strongly elongated molecular bonds. The photochemistry in the benzene adlayer occurred to be completely different from that in the gaseous molecules. This is due to fast vibrational relaxation on the surface.