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 CH3Cl+ 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 CH3+ dissociated from CH3Cl+(A2A1 and B2E) ions were recorded. CH3+ was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH2Cl+ fragment was very low. For dissociation of CH3Cl+(A2A1) ions, a series of homocentric rings was clearly observed in the CH3+ image, which was assigned as the excitation of umbrella vibration of CH3+ ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH3+(11A′) provided a direct experimental evidence of a shallow potential well along the C–Cl bond rupture. For CH3Cl+(B2E) ions, total kinetic energy released distribution for CH3+ fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the...