ZEKE (zero-kinetic-energy) photoelectron spectra of p-difluorobenzene (p-DFB) were obtained by two-color 1+1' photoionization via the S1 0(0), 3(1), 6(1), 9(2), 17(1), 27(1), and 30(1) vibrational states as intermediate resonances. The ZEKE spectra show substantial activity in transitions to combination bands with totally symmetric modes and a number of transitions which are symmetry forbidden under D2h SYMMetry. New fundamentals and combination bands of the electronic ground state of the cation are assigned, and improved vibrational frequencies are derived. Ab initio SCF calculations on the S0 and S1 states of p-DFB and the ground ionic state are presented and discussed in comparison to the experimental results. It is concluded that the geometries of the ground ionic state and the S0 state are of D2h symmetry and that a lowering of symmetry in the S1 state, together with a breakdown of the Born-Oppenheimer approximation, is the most likely cause for the appearance of the 'symmetry-forbidden'' bands.
