Translational and internal energy distributions of methyl and hydroxyl radicals produced by 157 nm photodissociation of amorphous solid methanol

Methanol is typically observed within water-rich interstellar ices and is a source of interstellar organic species. Following the 157 nm photoexcitation of solid methanol at 90 K, desorbed CH₃(v=0) and OH(v=0,1) radicals have been observed in situ, near the solid surface, using resonance-enhanced multiphoton ionization (REMPI) detection methods. Time-of-flight and rotationally resolved REMPI spectra of the desorbed species were measured, and the respective fragment internal energy and kinetic energy distributions were obtained. Photoproduction mechanisms for CH₃ and OH radicals from solid methanol are discussed. The formation of O (¹D and ³P) atoms and H₂O was investigated, but the yield of these species was found to be negligible. CH₃ products arising following the photoexcitation of water-methanol mixed ice showed similar kinetic and internal energy distributions to those from neat methanol ice.