VUV laser photofragmentations of an acrylonitrile molecular beam: One-, two-, and three-photon processes

A laser induced photofragmentation study at 193 nm has been made by detecting the fragments directly through a mass spectrometer and by observing the high resolution emission spectra of fragments from acrylonitrile. Time-of-flight (TOF) signals are observed at mass numbers of 27, 26, 25, 24, and 13. They are assigned to primary photofragments produced via the dissociation C2H3CN+hν→C2H3+CN, and via the molecular elimination C2H3CN+hν→C2H2+HCN, and to the products of secondary fragmentation of the primary photofragments produced in the ionizer. The TOF spectra are analyzed as superpositions of two thermalized components at the characteristic temperatures of 2500 and 6000 K for the dissociation and the molecular elimination processes, respectively. The angular distribution of fragments with respect to the polarization direction of the laser beam is found to be isotropic. None of the TOF signals show a higher order dependence of intensities on the photon flux. Two-photon excitation produces emission due to the CN(A 2Πi→X 2Σ+) transition with a lifetime (τ) of 7.1±0.6 μs and due to the CN(B 2Σ+→X 2Σ+) transition with τ = 65±2 ns. The vibrational distribution in the A 2Πi state is found to be highly nonstatistical with maximum population around the v = 9 level which is close to the origin of the B 2Σ+ state. The vibrational and rotational populations of CN(B 2Σ+) are almost thermalized at characteristic temperatures of 5860±420 and 2830±25 K, respectively. Three-photon excitation produces emission of C2(d3Πg →a3Πu) with a lifetime of 111±5 ns. The vibrational temperature is observed to be 7150±160 K and the rotational one to be 4960±150 K. The third photon also pumps the CN(A 2Πi) radical in its highly excited vibrational states generated by two-photon excitation to the E 2Σ+ and the F 2Δr states. Possible multiphoton fragmentation processes are presented based on the observed internal energies of the fragments.