Vibrational action spectroscopy of the C-H and C-D stretches in partially deuterated formic acid dimer.

Vibrational action spectroscopy of jet-cooled formic acid dimer measures the frequency of the C-H(D) stretching vibration and its coupling to nearby states. The action spectrum of (DCOOH)2 reveals a specific Fermi resonance between the C-D stretch and two antisymmetric combination states formed from the C-O stretch and DCO bend. A three-state deperturbation analysis shows that there is a relatively strong coupling between the fundamental vibration and each of the combination vibrations (mid R:13 cm(-1)mid R:) as well as between the combination states themselves (mid R:7 cm(-1)mid R:). This situation contrasts with that for the action spectrum of (HCOOD)2, where the C-H oscillator is isolated and not strongly coupled to other states.