A FLUORESCENCE CHARACTERIZATION OF THE P-DIFLUOROBENZENE ARGON VANDERWAALS COMPLEX - ENERGY-LEVELS, GEOMETRIES AND DISSOCIATION-ENERGIES

Fluorescence excitation spectra of p-difluorobenzene (pDFB) seeded in a supersonic argon expansion have been used to identify S1-S0 absorption bands of pDFB-Ar and pDFB-Ar2 van der Waals complexes. Dispersed fluorescence spectra produced by pumping the 0(0) level of each complex and the van der Waals stretching fundamental in pDFB-Ar are presented. Assignments of the spectra and of other dispersed fluorescence spectra not presented establish values of seven ring fundamentals in both the S1 and S0 states of pDFB-Ar: nu-3, nu-5, nu-6, nu-8, nu-27, nu-29, and nu-30. Within a 3 cm-1 uncertainty, in the S0 state none of them if pertubed from the free pDFB value. Only one, the out-of-plane ring distortion nu-8, is perturbed more than 2 cm-1 in the S1 state. Vibrational observations in pDFB-Ar2 involve nu-3", nu-5", nu-6", and nu-8" in the S0 state and nu-5' and nu-6' in the S1 state: none is perturbed. The pDFB-Ar van der Waals stretching fundamental-sigma is 41 +/- 2 cm-1 (S1) and 42 +/- 2 cm-1 (S0). A modest anharmonicity omega-e = 44 cm-1 and omega-e-x(e) = 1.5 cm-1 is seen in fluorescence progressions members of sigma in the S0 state. The observed pDFB-Ar rotational band contours are consistent with Ar centered on the ring at a distance 3.5 +/- 0.5 angstrom in the S0 state and with a contraction of 0.1 +/- 0.04 angstrom in the S1 state. The pDFB-Ar dissociation energy D'0 is estimated from spectroscopic information to be 190 cm-1 less-than-or-similar-to D'0 < 242 cm-1 with D'0-D"0 = 30 cm-1.