Photoinduced changes in the structure of a van der Waals complex. Three-dimensional inertial axis reorientation in the S-1 <- S-0 electronic spectrumof indole-Ar

Rotationally resolved S-1<-- S-0 electronic spectra of the van der Waals co mplexes indole-Ar and N-deuterated-indole-Ar in the gas phase are described . Both spectra exhibit inertial axis reorientation. A comparison of the par ameters derived from fits of the two spectra with those obtained from a pre vious study of the bare molecule [Berden , J. Chem. Phys. 103, 9596 (1995)] shows that the Ar atom in indole-Ar is attached above the indole plane, di splaced toward the N atom in the five-membered ring, in both electronic sta tes. 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 t o the observed axis reorientation in the high resolution spectra. The S-1 s tate in indole itself is shown to be the L-1(b) state, which has a differen t electronic distribution from that of the ground state. Thus, the Ar atom "moves" when the photon is absorbed because there are significant differenc es in the intermolecular potential energy surfaces of the two states. (C) 1 999 American Institute of Physics. [S0021-9606(99)01033-8].