A COMPARISON OF THE FEMTO-SECOND, PICO-SECOND 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 ((B) over tilde B-2(2) <-- X (2)A(1)) and 496 nm has been carried out for a num ber of different laser pulsewidths from 15 ns to 300 fs by time-of-fli ght mass spectrometry in a molecular beam, At 248 nm the NO2/NO ion ra tio (NO2+/NO+) varies from about 1% using nanosecond laser pulses to a bout 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 a nd 500 fs pulses. These results are interpreted in terms of the limite d knowledge of the dissociation rates above the NO(X) + O(D-1) fragmen tation threshold at 40126 cm(-1) using a simple rate equation model. T he results can be described using dissociation rates which increase wi th 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 femtoseco nd mass spectrometry are discussed.