Vibrational spectra of p-, m-, and o-difluorobenzene cations in the ground electronic states have been measured by vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. From the MATI spectra, ionization energies of p-, m-, and o-difluorobenzenes to the ionic ground states have been determined to be 73 861+/-5, 75 332+/-5, and 75 003+/-5 cm(-1), respectively. A number of vibrational modes, including fundamentals, their combinations, and overtones were distinctly observed in the MATI spectra and could be assigned successfully by comparing with the results from density functional theory calculation at the B3LYP/6-311++G (2df,2pd) level. Appearance of the overtone progression of some totally symmetric vibrations could be explained also in terms of the overlap between the geometrical changes upon ionization and normal mode eigenvectors. Excellent correlation between the experimental and calculated results suggests that p-, m-, and o-difluorobenzene cations in the ground states retain D-2h, C-2v, and C-2v symmetries, respectively, of the corresponding neutrals. (C) 2003 American Institute of Physics.