Photodissociation of
nitromethane has been investigated for decades both theoretically and experimentally; however, as a whole picture, the dissociation dynamics for
nitromethane are still not clear, although many different mechanisms have been proposed. To make a complete interpretation of these different mechanisms, photolysis of
nitromethane at 226 and 271 nm under both collisional and collisionless conditions is investigated at nanosecond and femtosecond time scales. These two
laser wavelengths correspond to the pi* <-- pi and pi* <-- n excitations of
nitromethane, respectively. In nanosecond 226 nm (pi* <-- pi) photolysis experiments, CH(3) and NO radicals are observed as major products employing resonance enhanced multiphoton ionization techniques and time-of-flight mass spectrometry. Additionally,
OH and CH(3)O radicals are weakly observed as dissociation products employing
laser induced fluorescence spectroscopy; the CH(3)O product is only observed under collisional conditions. In femtosecond 226 nm experiments, CH(3), NO(2), and NO products are observed. These results confirm that
rupture of C-N bond should be the main primary process for the photolysis of
nitromethane after the pi* <-- pi excitation at 226 nm, and the NO(2) molecule should be the precursor of the observed NO product. Formation of the CH(3)O radical after the recombination of CH(3) and NO(2) species under collisional conditions rules out a nitro-
nitrite isomerization mechanism for the generation of CH(3)O and NO from pi pi* CH(3)NO(2). The
OH radical formation for pi pi* CH(3)NO(2) should be a minor dissociation channel because of the weak
OH signal in both nanosecond and femtosecond (nonobservable) experiments. Single color femtosecond pump-probe experiments at 226 nm are also employed to monitor the dynamics of the dissociation of
nitromethane after the pi* <-- pi excitation. Because of the ultrafast dynamics of product formation at 226 nm, the pump-probe transients for the three dissociation products are measured as an autocorrelation of the
laser pulse, indicating the dissociation of
nitromethane in the pi pi* excited state is faster than the
laser pulse duration (180 fs). In nanosecond 271 nm (pi* <-- n) photolysis experiments, pump-probe experiments are performed to detect potential dissociation products, such as CH(3), NO(2), CH(3)O, and
OH; however, none of them is observed. In femtosecond 271 nm
laser experiments, the
nitromethane parent ion is observed with major intensity, together with CH(3), NO(2), and NO fragment
ions with only minor intensities. Pump-probe transients for both
nitromethane parent and fragment
ions at 271 nm excitation and 406.5 nm ionization display a fast exponential decay with a constant time of 36 fs, which we suggest to be the lifetime of the excited n pi* state of
nitromethane. Combined with the 271 nm nanosecond pump-probe experiments, in which none of the CH(3), NO(2), CH(3)O, or
OH fragment is observed, we suggest that all the fragment
ions generated in 271 nm femtosecond
laser experiments are derived from the parent ion, and dissociation of
nitromethane from the n pi* excited electronic state does not occur in a supersonic molecular beam under collisionless conditions.