Autoionization-detected infrared (ADIR) spectroscopy of molecular cations

A newly developed technique for infrared spectroscopy of cold molecular cat ions in the gas phase is reviewed. Very high Rydberg stares converging to t he first ionization threshold are prepared by two-color double resonance ex citation. Vibrational excitation of the ion core of the Rydberg states indu ces autoionization, leading to the molecular ion. An infrared spectrum is o btained by monitoring the ion current due to the autoionization as a functi on of the vibrational excitation laser frequency. The observed spectrum pra ctically provides the vibrational frequency of the bare ion. This new techn ique is called autoionization-detected infrared (ADIR) spectroscopy, and it is very advantageous to observe high frequency vibrations of molecular cat ions, such as OH and CH stretches. The following applications of ADIR spect roscopy to the phenol derivative cations are reviewed in this paper; (1) OH stretching vibrations of typical intramolecular hydrogen-bonded cations. ( 2) Unconventional intramolecular hydrogen bonds between the hydroxyl and al kyl groups. (3) Aromatic and alkyl CH stretching vibrations in aromatic cat ions. (C) 2000 Elsevier Science B.V. All rights reserved.