Electronic spectral shifts of large van der Waals molecules

Abstract Experimental values for the red spectral shifts in the fluorescence excitation spectra of large van der Waals complexes of aromatic molecules and rare gas atoms have been used, in conjunction with calculated equilibrium distances between the aromatic and rare gas atom, to estimate differences between the dispersion forces in the ground and excited states. Theoretical red-shifts calculated by a formalism given by Longuet-Higgins and Pople are an order of magnitude too large and this discrepancy is traced to incorrect assumptions concerning values chosen for average excitation energies. Modifications of the theory for a van der Waals complex are explored. A simpler and more easily used formula to correlate the magnitudes of the spectral shifts is derived and tested.