Quantification of a radical beam source for methyl radicals

A radical beam source for methyl radicals (CH3) was characterized applying ionization-threshold mass spectrometry. The beam source is based on thermal dissociation of methane (CH4) or azomethane (N-2(CH3)(2)) in a heated tung sten capillary. A flux of (3+/-1)X10(13) cm(-2)s(-1) CH3 radicals is produc ed using methane as precursor gas and a capillary temperature of 1650 K. Al ternatively, a flux of (3+/-1)X10(14) cm(-2) s(-1) CH3 is produced using az omethane as precursor gas and a capillary temperature of 1150 K. The domina nt production of methyl from the precursor methane occurs due to reaction 2 CH4+M-->2 CH3+H-2+M at the hot tungsten surface. The dominant production o f methyl from azomethane occurs due to the reaction N-2(CH3)(2)-->2 CH3+N-2 . Besides methyl radicals, only stable molecules contribute to the emitted flux; within the detection limit, no atomic hydrogen is observed. From the comparison of ionization-threshold mass spectrometry and standard mass spec trometry, it is concluded that the cracking pattern of methane varies with the methane gas temperature. This is explained by the vibrational assisted dissociation of methane. (C) 2001 American Vacuum Society. [DOI: 10.1116/1. 1326939].