Photodissociation Dynamics of Nitromethane at 226 and 271 nm at Both Nanosecond and Femtosecond Time Scales

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, CH3 and NO radicals are observed as major products employing resonance enhanced multiphoton ionization techniques and time-of-flight mass spectrometry. Additionally, OH and CH3O radicals are weakly observed as dissociation products employing laser induced fluorescence spectroscopy; the CH3O product is only observed under collisional conditions. In femtosecond 226 nm experiments, CH3, NO2, 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 NO2 molecule should be the precursor of the observed NO product. Formation of the CH3O radical after the recombination of CH3 and NO2 species under collisional conditions rules out a nitro-nitrite isomerization mechanism for the generation of CH3O and NO from pi pi* CH3NO2. The OH radical formation for pi pi* CH3NO2 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 CH3, NO2, CH3O, and OH; however, none of them is observed. In ferntosecond 271 nm laser experiments, the nitromethane parent ion is observed with major intensity, together with CH3, NO2, 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 CH3, NO2, CH3O, 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.