ZERO KINETIC-ENERGY PHOTOELECTRON-SPECTROSCOPY OF PARA-DIFLUOROBENZENE

We compare the ZEKE (zero kinetic energy) photoelectron spectrum of p-difluorobenzene, obtained by two-color 1 + 1' photon ionization via the S1 6(1) vibrational state as intermediate resonance, with a recent time-of-flight photoelectron (TOF-PE) spectrum by Sekreta et al. The ZEKE spectrum resolves a large number of fundamental and combination bands of the electronic ground state of the cation, which in the TOF-PE spectrum appear as congested and merged peaks. The assignment of the vibrations observed in the ZEKE spectrum agree well with the assignments obtained for the TOF-PE spectrum. Due to the enhanced resolution of ZEKE spectroscopy (2-3 orders of magnitude compared to photoelectron spectroscopy) improved vibrational frequencies in the cation are derived. The adiabatic ionization energy is determined to 73 872 +/- 3 cm-1.