			15min:

THE CAVITY RING--DOWN ABSORPTION SPECTRUM OF THE S₀-> T₁ AND S₀-> S₁ TRANSITION OF JET--COOLED AROMATIC THIONE COMPOUNDS .

A. A. RUTH, Institut für Atomare und Analytische Physik, TU--Berlin, Hardenbergstr. 36, 10623 Berlin, Germany; T. FERNHOLZ, Physikalisch Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany.

The cavity ring--down method has been applied to the measurement of the absorption of 4--H--1--benzopyrane--4--thione (BPT) and 4--H--pyrane--4--thione (PT) in a supersonic jet in the wavelength region of the weak S₀-> T₁ and S₀-> S₁ transition.

In the case of jet--cooled BPT the absorption energies of vibronic states between 15800 and 16650~ cm^-1 corroborate previous assignments in the S₀-> T₁ phosphorescence excitation and resonance--enhanced multi--photon excitation spectra. The symmetry forbidden 0,0 transition, \hbox S_0,0-> S_1,0 at 16522~ cm^-1, was found to be 7.5 times weaker than the absorption transition to the triplet origin, \hbox S_0,0-> T_1z,0. Relative phosphorescence quantum yields were estimated.

For the more volatile PT the CRD absorption spectrum of jet--cooled PT was compared to PT in a static cell. Through the hot band analysis ground state vibrations were assigned in comparison to ab initio force field calculations