15min:

CHEOLHWA KANG AND DAVID W. PRATT, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260.

The rotationally resolved S₁ S₀ fluorescence excitation spectra of para-difluorobenzene (pDFB) and its N₂ complex have been observed. The high resolution spectrum of pDFB is pure b-type and spans an unusually wide energy region of over 20 cm^-1. The N₂ complex of pDFB exhibits two absorption bands shifted to the red of the bare molecule origin, at -27 and -25cm^-1. Each of these bands is a superposition of the two pure c-type spectra that result from the internal rotation of the N₂ molecule in the complex. Analyses of these bands show that the N₂ molecule lies 3.59Å above the center of the ring plane in the S₀ state; this distance decreases to 3.47Å in the S₁ state. The barriers to internal rotation in the two electronic states are also different. A full discussion of the factors responsible for the differences will be given.