S2-S0 FLUORESCENCE EXCITATION SPECTROSCOPY OF VAN-DER-WAALS COMPLEXESOF XANTHIONE AND BENZOPYRANTHIONE WITH N2 AND CO

The S2-S0 fluorescence excitation of xanthione (XT) and benzopyranthio ne (BPT) and their 1:1 and 1:2 complexes with N2 and CO have been meas ured in a supersonic expansion. Remarkably long, harmonic progressions in van der Waals modes are observed and are assigned on the basis of the structural and dipole moment changes in the chromophores. The micr oscopic solvation shifts are rationalized in terms of dipolar and disp ersive contributions to the binding energies.