Double Imaging Photoelectron Photoion Coincidence Sheds New Light on the Dissociation of State-Selected CH3F+ Ions.

Abstract

The vacuum ultraviolet (VUV) photoionization and dissociative photoionization of methyl fluoride (CH3F) in the 12.2-19.8 eV energy range were investigated by using synchrotron radiation coupled to a double imaging photoelectron photoion coincidence (i2PEPICO) spectrometer. The production of several fragment ions including CH2F+, CHF+, CH3+, and CH2+ as a function of state and internal energy of CH3F+ ions was identified and analyzed, with their individual appearance energies measured through threshold photoelectron spectroscopy. Dynamical information was inferred from electron and ion kinetic energy correlation diagrams measured at chosen fixed photon energies. The detailed mechanisms governing the dissociation of state-selected CH3F+ ions prepared in the X2E, A2A1, and B2E low-lying electronic states as well as outside the Franck-Condon region have been inferred based on the present experimental results and existing theoretical calculations. Both the CH2F+ and CH3+ primary fragment ions have three different channels of production from different electronic states of CH3F+. The spin-orbit splitting states of the F fragment, 2P3/2 and 2P1/2, in the CH3+ + F dissociation channels were assigned and adiabatically correlate to the X2E ground state and the A2A1 electronic state, respectively, with the aid of previous theoretical results. The CH3F+ ions in the high energy part of the X2E ground state are unstable and statistically dissociate to the CH2F+(11A1) and H(2S) fragments along the potential energy curve of the X2E state. The A2A1 electronic state is a repulsive state and exclusively dissociates to the CH3+(11A1') and F(2P1/2) fragments. In addition, the CH2F+, CHF+, CH3+, and CH2+ fragment ions are also produced in the B2E state and in the Franck-Condon gap by indirect processes, such as internal conversion or dissociative autoionization.