Delayed fragmentation of acetonitrile upon short-pulse UV irradiation: Possible role of long-lived superexcited intermediates

We have studied fragment emission in the spectral range 200–900 nm, following short-pulse UV excitation (25 ps, 266 nm, 0.2–10 mJ) of acetonitrile. The dominant components of emission are the CN B 2Σ+→X 2Σ+ violet system and the C2 d 3Πg→a 3Πu Swan system. Emissive CH and NH are also observed. Each of these diatomics is produced with a pressure-dependent rate. An instrumental risetime of 0.6 ns enabled study of the rise and fall of violet and of Swan emission over the pressure range 0.27–10 Torr. The formation kinetics suggest that there are long-lived intermediates to the violet and Swan emissive populations. These precursors are not observed in emission. They have zero-pressure lifetimes of 20 to 30 ns; in collisions with the parent they are removed with large bimolecular rate constants (≳10−9 cm3 s−1). With the use of isotopically labeled acetonitrile, we have established that major fractions of CN B 2Σ+ and of C2 d 3Πg are formed intramolecularly. Further, two channels of intramolecular CN B 2Σ+ production are recognized. They share a common risetime, which may indicate a common precursor. The preferred channel retains the CN association of the parent. In the other channel, the carbon atom originates in the methyl group of the parent, suggesting rearrangement within an intermediate. We believe that the precursors are superexcited or highly excited species C2HnN** (0≤n≤3).