Photoinduced changes in the structure of a van der Waals complex. Three-dimensional inertial axis reorientation in the S₁←S₀ electronic spectrum of indole-Ar

Rotationally resolved S₁←S₀ electronic spectra of the van der Waals complexes indole-Ar and N-deuterated-indole-Ar in the gas phase are described. Both spectra exhibit inertial axis reorientation. A comparison of the parameters derived from fits of the two spectra with those obtained from a previous study of the bare molecule [Berden et al., J. Chem. Phys. 103, 9596 (1995)] shows that the Ar atom in indole-Ar is attached above the indole plane, displaced toward the N atom in the five-membered ring, in both electronic states. However, the measured center-of-mass coordinates of the Ar atom in the principal axis frames of indole in the two states are different, leading to the observed axis reorientation in the high resolution spectra. The S₁ state in indole itself is shown to be the ¹L_b state, which has a different electronic distribution from that of the ground state. Thus, the Ar atom "moves" when the photon is absorbed because there are significant differences in the intermolecular potential energy surfaces of the two states.