Ultraviolet/infrared-double resonance spectroscopy and ab initio calculations on the indole+ and indole(H2O)1+ cations

In this paper we report on the application of infrared/photoinduced Rydberg ionization (IR/PIRI) and IR-photodissociation spectroscopy to investigate the CH, NH or OH stretching vibrations of indole(+) and the indole(H2O)(1)(+) cluster cation. All vibrational frequencies of indole(+) and indole(H2O)(1)(+) are compared with the values obtained from ab initio calculations. In the case of the indole(+) cation the NH vibration is observed. This is the first observation of a NH vibration in a bare cation. For indole(H2O)(1)(+) a hydrogen-bonded structure with a nearly linear hydrogen bond can be derived both from ab initio calculations and the IR-spectra. By applying the state selective IR/PIRI spectroscopy to indole(H2O)(1)(+), no vibrational couplings between the intermolecular O-H . . .N stretching vibration and the intramolecular OH stretching modes of the water moiety are observed. In the IR-photodissociation spectra of indole(H2O)(1)(+) the NH, OH, and CH stretching vibrations as well as overtones of bending modes are observed. In agreement with the prediction of ab initio calculations the frequency of the NH stretching vibration of the cation is shifted by 300 cm(-1) to lower frequencies compared to the neutral cluster. By analyzing the fragmentation of indole(H2O)(1)(+) as a function of excess energies within the ion it can be concluded that only one IR photon is absorbed if OH stretching vibrations of the ion are excited, whereas at least two photons can be absorbed if the NH stretching mode is excited. Furthermore, comparison of IR/PIRI and IR-photodissociation spectroscopy indicates that the frequency of the OH stretching vibration decreases very slightly with increasing excess energy within the ion. (C) 2000 American Institute of Physics. [S0021-9606(00)01242-3].