Indole-H2O in the gas phase.: Structures, barriers to internal motion, and S1←S0 transition moment orientation.: Solvent reorganization in the electronically excited state

Rotationally resolved S-1 <-- S-0 fluorescence excitation experiments have been performed on a solute-solvent complex of indole and water in a molecular beam. The results show that the complex whose S-1 <-- S-0 origin is shifted by 132 cm(-1) below the bare molecule origin is a 1:1 complex, with the water molecule linked to the indole frame via a quasi-linear N-Hz ... OH2 sigma hydrogen bond. The results also show that both the position and the orientation of the attached water molecule change when the photon is absorbed. The properties of the intermolecular potential-energy surfaces that govern these motions in both electronic states are derived from an analysis of the high-resolution spectrum.