Resonance enhanced multiphoton ionization/time-of-flight measurements of the velocity and internal energy content of thermal and photochemical methyl radical sources

A single-stage time-of-flight mass spectrometer used in conjunction with resonance enhanced multiphoton ionization has been employed to study the dynamics of surface photodissociation processes as well as methyl radicals produced from a continuous source. By utilizing ion rather than neutral flight times, species that have an impressed velocity along the detection axis can be readily distinguished from species that exhibit an isotropic velocity distribution. This allows for experimental discrimination between photofragments produced from adsorbate photolysis and those produced as a result of probe laser photolysis of gas-phase species photodesorbed from the surface. For species generated in continuous sources, such as methyl radicals produced from azomethane pyrolysis, the same approach permits an unambiguous determination of the total-energy content, despite the presence of additional radicals within the ionizing volume that have scattered from the chamber walls. (C) 1997 American Institute of Physics.