MULTIPHOTON IONIZATION AND DISSOCIATION OF NITROMETHANE USING FEMTOSECOND LASER-PULSES AT 375 AND 750 NM

The photochemistry of nitromethane has been studied extensively for ma ny years. Although it is generally agreed that the principal photodiss ociative process is cleavage of the C-N bond to yield the methyl radic al and nitrogen dioxide, there is some evidence of minor competing dis sociation channels. A number of different groups have used lasers of d ifferent wavelengths, but the results of these studies vary considerab ly and no clear picture of the minor dissociative channels has yet eme rged. The use of femtosecond (fs) duration laser pulses for photoioniz ation of molecules is currently an area of considerable interest, sinc e the process can lead to the efficient production of intact molecular ions. It was felt that femtosecond laser mass spectrometry (FLMS) cou ld provide added information on the dissociation pathways of nitrometh ane. Laser pulses of 90 fs time duration at wavelengths of 375 and 750 nm, coupled to a time-of-flight mass spectrometer, have been used in this study, and contrary to photoexcitation using nanosecond (ns) puls es, a large parent ion, 61 (CH3-NO2+), is detected together with stron g peaks at mle 15 (CH3+), 30 (NO+), 46 (NO2+) as well as a number of o ther minor peaks. This fragmentation pattern can be explained by a pre dominantly ID (ionization followed by dissociation) route.