CO2 laser assisted vibrational overtone spectroscopy

Abstract

We report the implementation of a new spectroscopic scheme for measuring vibrational overtone spectra of molecules in collisionless environments. This approach is based upon selective infrared multiphoton dissociation (IRMPD) of molecules that have undergone a light atom stretch vibrational overtone transition. Overtone excitation is ultimately monitored by laser induced fluorescence (LIF) detection of the dissociation products. Application of this technique to measure the 5←0 OH stretch transition methanol at 50 m Torr produces an excitation spectrum which is virtually identical to a photoacoustic spectrum recorded at 40 Torr, indicating that the former faithfully represents the vibrational overtone absorption. When applied to methanol cooled in a supersonic expansion, this method results in considerable simplification of the 5νOH vibrational overtone spectrum and reveals the presence of a strong Fermi resonance which divides the 5νOH oscillator strength equally between two vibrational levels. This new spectroscopic technique should permit the application of supersonic expansion and double resonance techniques to vibrational overtone spectroscopy of a wide variety of molecules.