Anharmonic analysis of the PFI-ZEKE photoelectron spectrum of CH2F2 from the ionization potential to the onset of fragmentation

The complete vibrational fine structure of the pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of difluoromethane is analysed in terms of fully coupled anharmonic calculations. Nine-dimensional ab initio potential functions complete up to five-body terms are constructed directly from ab initio calculations using MOller-Plesset and coupled cluster theory together with up to quadruple- basis sets. The onset of dissociation is accompanied by strongly anharmonic vibrational dynamics involving resonant interactions between both symmetric and non-symmetric modes. The detailed analysis leads to a partial revision of earlier interpretations and provides improved theoretical values for the adiabatic ionization potential IP=12.740 eV and for the lowest dissociation limit [image omitted] both within about 1 kJ mol-1 of experimental estimates.