RESONANCE-ENHANCED MULTIPHOTON IONIZATION TIME-OF-FLIGHT MEASUREMENTSOF THE VELOCITY AND INTERNAL ENERGY CONTENT OF THERMAL AND PHOTOCHEMICAL METHYL RADICAL SOURCES/

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