Photodissociation of the acetone cation at 355 nm using the velocity imaging technique

Photodissociation of acetone cations, CH3COCH3+, at 355 nm has been studied by means of the ion velocity imaging technique. Acetone cations are produced via direct photoionization of a supersonic beam of acetone at 118 nm generated by frequency tripling the 355 nm laser. Only the acetyl cation, CH3CO+, could be detected as a dissociation product in the time-of-flight mass spectrometer. The acetyl ion signal depends upon the fifth power of the 355 nm laser energy, while the acetone ion signal depends upon the third power. This suggests that the fragment ion is produced via two-photon absorption of 355 nm photons by the acetone cation. The total translational energy distribution and angular distribution of acetyl cation were derived from the 2D images of CH3CO+ for the reaction CH3COCH3+ + 2h nu(355 nm)--> CH3CO+ + CH3*. The translational energy distribution suggests that methyl radicals are produced in two electronically excited states, the Rydberg 3s 1 (2)A(1)' and the valence 1 (2)A' states. The anisotropy parameter beta shows that the Rydberg state is formed via a perpendicular excitation and the valence state via a parallel transition. (C) 2000 American Institute of Physics. [S0021-9606(00)00233-6].