Resonance coupling in the fourth OH-stretching overtone spectrum of formic acid.

Abstract

The room-temperature vibrational overtone spectra of the formic acid isotopomers HCOOH and DCOOH have been recorded in the third and fourth OH-stretching overtone regions with intracavity laser photoacoustic spectroscopy. Resonance coupling between the OH- and CH-stretching vibrations in HCOOH is clearly identified in the fourth overtone region. This is an example of strong coupling across bonds. In the third overtone region, no resonance is observed. Vibrational energies and intensities of the OH- and CH-stretching overtones and combination bands have been calculated with an anharmonic oscillator local mode model. The pure OH-stretching bright state carries almost all the intensity prior to resonance coupling.