Double imaging photoelectron photoion coincidence sheds new light on the dissociation of energy-selected CH3Cl+ ions

Abstract

The vacuum ultraviolet (VUV) photoionization and dissociative photoionization of CH₃Cl in the energy range of 11–17 eV have been investigated in detail by combining synchrotron radiation and double imaging photoelectron photoion coincidences (i²PEPICO). Three low-lying electronic states of the CH₃Cl⁺ molecular ion, X²E, A²A₁ and B²E, were prepared and analyzed. The appearance energies of the energetically accessible fragment ions, CH₂Cl⁺, CHCl⁺, CH₃⁺ and CH₂⁺, have been obtained from their respective mass-selected threshold photoelectron spectra (TPES) or photoionization efficiency (PIE) curves. The dissociation mechanisms of energy-selected CH₃Cl⁺ ions, prepared in the A²A₁ and the B²E electronic states, as well as outside the Franck–Condon region, have been revealed to be state-specific via ion/electron kinetic energy correlation diagrams. In particular, the umbrella mode vibrational progression of the CH₃⁺ fragment ion in the direct dissociation of the A²A₁ electronic state was identified and assigned indicating that this state correlates to the CH₃⁺(1¹A₁′) + Cl(²P_1/2) dissociation limit, in agreement with the theoretical calculations performed in this work.