The photodissociation of nitrobenzene and the nitrotoluene isomers at 375 nm, induced by a 90 femtosecond laser, is analyzed and compared with the fragmentation by a 10 nanosecond laser at the same wavelength. The molecular ionization is attributed to a nonresonant multiphoton process, and the observed fragmentation can be explained predominantly by an above ionization mechanism (ladder climbing). The mass spectra of the three nitrotoluene isomers show differences which can be used for analytical purposes. The molecular rearrangement taking place prior to the dissociation is also discussed. For nitrobenzene, it is suggested that most of the dissociation occurs from the nitrobenzene structure rather than that of phenyl nitrite. In the case of o-nitrotoluene, it seems that the hydrogen transfer from the -CH 3 to the NO 2 group (ortho effect) is favored in ionic states, while the rearrangement to a nitrite structure is possible in the excited electronic states.

In the present Letter we report that a number of polyatomic molecules (M) when irradiated with short pulse lasers <90 fs at 750−790 nm and intensities up to 1015 W cm-2 produce multiply charged parent ions and do not fragment to any great degree. This surprising observation is found in both linear and ring structured molecules and is very similar to the behavior of inert atoms such as xenon under the same irradiation conditions. This is a very different behavior from irradiating with nanosecond pulses at 109 W cm-2 where low-mass fragments dominate the spectrum. For the hydrocarbon molecules presented in this work, there exists an envelope of 2+ ionized peaks, which corresponds to the parent and a number of (M − nH) satellites. This feature is characteristic of these molecules in the intensity region 1014-15 W cm-2 and is interpreted as evidence for tunneling or barrier suppression. Coulomb explosion leading to multiply charged atoms, which is evident for CS2, does not seem to be operating for the larger hydrocarbon molecules.

The photochemistry of nitromethane has been studied extensively for many years. Although it is generally agreed that the principal photodissociative process is cleavage of the C−N bond to yield the methyl radical and nitrogen dioxide, there is some evidence of minor competing dissociation channels. A number of different groups have used lasers of different wavelengths, but the results of these studies vary considerably and no clear picture of the minor dissociative channels has yet emerged. The use of femtosecond (fs) duration laser pulses for photoionization of molecules is currently an area of considerable interest, since the process can lead to the efficient production of intact molecular ions. It was felt that femtosecond laser mass spectrometry (FLMS) could provide added information on the dissociation pathways of nitromethane. 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) pulses, a large parent ion, 61 (CH3NO2 +), is detected together with strong peaks at m/e = 15 (CH3 +), 30 (NO+), 46 (NO2 +) as well as a number of other minor peaks. This fragmentation pattern can be explained by a predominantly ID (ionization followed by dissociation) route.