Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging.

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.