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

摘要

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH _3Cl 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 _3Cl(A ~2A _1 and B ~2E) ions were recorded. CH 3was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH _2Cl fragment was very low. For dissociation of CH _3Cl(A ~2A _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 ~1A′) provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH _3Cl(B ~2E) 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 ~2E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH _3Cl, CH _3 formation from CH _3Cl(A ~2A _1) ions was a rapid direct fragmentation, while CH _3Cl(B ~2E) ions statistically dissociated to CH _3 Cl via internal conversion to the high vibrational states of X ~2E.