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 produc ed via direct photoionization of a supersonic beam of acetone at 118 nm gen erated by frequency tripling the 355 nm laser. Only the acetyl cation, CH3C O+, 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 di stribution 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 a re 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 t he Rydberg state is formed via a perpendicular excitation and the valence s tate via a parallel transition. (C) 2000 American Institute of Physics. [S0 021-9606(00)00233-6].