Methane ice photochemistry and kinetic study using laser desorption time-of-flight mass spectrometry at 20 K

Abstract

The ice photochemistry of pure methane (CH₄) is studied at 20 K upon VUV irradiation from a microwave discharge H₂ flow lamp. Laser Desorption Post-Ionization Time-Of-Flight Mass Spectrometry (LDPI TOF-MS) is used for the first time to determine branching ratios of primary reactions leading to CH₃, CH₂, and CH radicals, typically for fluences as expected in space. This study is based on a stable end-products analysis and the mass spectra are interpreted using an appropriate set of coupled reactions and rate constants. This yields clearly different values from previous gas phase studies. The matrix environment as well as the higher efficiency of reverse reactions in the ice clearly favor CH₃ radical formation as the main first generation photoproduct.