HIGH-RESOLUTION UV SPECTROSCOPY OF VIBRONIC BANDS IN P-DIFLUOROBENZENE AND P-DIFLUOROBENZENE-AR - THE ROLE OF HERZBERG-TELLER COUPLING

Rotationally resolved spectra of two bands in the S-1 <-- S-0 transiti on of the p-difluorobenzene molecule and its van der Waals complex wit h Ar have been measured by mass-selective resonance-enhanced two-photo n ionization. The rotational structure of the 0(0)(0) and the 27(0)(1) bands in the monomer as well as in the complex differ from each other . They can be theoretically reproduced assuming a transition moment or iented along the short in-plane axis of the molecule in the case of th e 0(0)(0) transition and the long in-plane axis in the case of the 27( 0)(1) transition. Since the magnitude of the moments of inertia is cha nged in the complex by adding an Ar atom, complexation leads to a chan ge of rotational structures of the same band. The analysis of the rota tional structure points to Herzberg-Teller coupling by vibronic intera ction with the S-2 (B-1(1u)) state as the mechanism responsible for th e appearance of the 27(0)(1) band. The rotational constants determined from a fit of the spectra yield an effective van der Waals distance o f 3.55 (2) Angstrom (1 Angstrom = 10(-10) m) of the Ar atom from the p -difluorobenzene plane, which decreases by 0.06 Angstrom on electronic excitation to the S-1 state.