15min:

OSKAR ASVANY, EDOUARD HUGO, Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands; STEPHAN SCHLEMMER, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany.

Laser Induced Reaction (LIR) is a novel technique for the spectroscopy of molecular ions as well as the study of state-specific rate coefficients and lifetimes of excited states. It is based on trapping mass-selected ions in the cold environment of a reactant gas and probing the laser-induced excitation of the ion by the outcome of the corresponding endothermic ion-molecule reaction. The feasibility of this method has been demonstrated in the past for the

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% \captionMeasured and calculated spectra of CH₅⁺

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electronic spectroscopy of N₂⁺ in the charge transfer reaction with Ar, and the spectroscopy of the IR-active modes of the acetylene cation in the abstraction reaction of C₂H₂⁺ with hydrogen.

Recently, LIR has been applied to record the first far infrared spectra of bare CH₅⁺ and some of its deuterated versions combining a low temperature 22--pole ion trap with the large tuning range of the free electron laser FELIX.

To obtain the LIR spectrum, the product ions of the endothermic proton transfer reaction CH₅⁺ + CO₂ CH₄ + OCOH⁺ have been counted as a function of the FELIX wavelength. In addition to the earlier observed C--H-stretching feature at 3000~cm^-1, low-frequency extensions down to 2200~cm^-1 become discernible, as well as a H--C--H bending feature at 1250~cm^-1. Comparison of the measured data to computed ones shows that CH₅⁺ is fluxional at the temperature of the experiment, T=110K.