Impulsive IR-multiphoton dissociation of acrolein: observation of non-statistical product vibrational excitation in CO ( v=1–12) by time resolved IR fluorescence spectroscopy

Abstract

On IR-multiphoton excitation, vibrationally highly excited acrolein molecules undergo concerted dissociation generating CO and ethylene. The vibrationally excited products, CO and ethylene, are detected immediately following the CO 2 laser pulse by observing IR fluorescence at 4.7 and 3.2 μm, respectively. The nascent CO is formed with significant vibrational excitation, with a Boltzmann population distribution for v=1–12 levels corresponding to T v=12 950±50 K. The average vibrational energy in the product CO is found to be 26 kcal mol −1, in contrast to its statistical share of 5 kcal mol −1, available from the product energy distribution. The nascent vibrationally excited ethylene either dissociates by absorbing further infrared laser photons from the tail of the CO 2 laser pulse or relaxes by collisional deactivation. Ethylene IR-fluorescence excitation spectrum showed a structure in the quasi-continuum, with a facile resonance at 10.53 μm corresponding to the 10P(14) CO 2 laser line, which explains the higher acetylene yield observed at a higher pressure. A hydrogen atom transfer mechanism followed by C–C impulsive break in the acrolein transition state may be responsible for such non-statistical product energy distribution.