A comparison of the femto-, pico- and nano-second multiphoton ionization and dissociation processes of NO2 at 248 and 496 nm

The multiphoton ionization and dissociation of NO2 at 248 nm (B̃2B2←X2A1) and 496 nm has been carried out for a number of different laser pulsewidths from 15 ns to 300 fs by time-of-flight mass spectrometry in a molecular beam. At 248 nm the NO2/NO ion ratio (NO2+/NO+) varies from about 1% using nanosecond laser pulses to about 60% for the shortest pulses used (300 fs). In contrast, at 496 nm the NO2+/NO+ ratio remains very small for excitation with both 5 ps and 500 fs pulses. These results are interpreted in terms of the limited knowledge of the dissociation rates above the NO(X)+O(1D) fragmentation threshold at 40126 cm−1 using a simple rate equation model. The results can be described using dissociation rates which increase with decreasing pulsewidth, an idea which conforms with the distinction that has been drawn between time and frequency domain experiments. The implications of the results for the analytical potential of femtosecond mass spectrometry are discussed.