High-resolution cavity ring-down spectroscopy of the ν1 + ν6 combination band of methanol at 2.0 μm.

Reported here are portions of the infrared absorption cross section for methanol (CH3OH), as measured by frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) at wavelengths near λ = 2.0 µm. High-resolution spectra of two gravimetric mixtures of CH3OH-in-air with nominal mole fractions of 202.2 µmol/mol and 45.89 µmol/mol, respectively, were recorded at pressures between 0.8 kPa and 102 kPa and at a temperature of 298 K. Covering the experimental wavenumber range of 4990 cm-1 to 5010 cm-1 in increments of 0.0067 cm-1 and with an instrument linewidth of 30 kHz, we observed an evolution in the CH3OH spectrum from resolved absorption lines at a low pressure (0.833 kPa) to a pseudocontinuum of absorption at a near-atmospheric pressure (101.575 kPa). An analysis of resolvable features at the lowest recorded pressure yielded a minimum intramolecular vibrational energy redistribution (IVR) lifetime for the OH-stretch (ν1) plus OH-bend (ν6) combination of τIVR ≥ 232 ps-long compared to other methanol overtones and combinations. Consequently, we show that high-resolution FS-CRDS of this relatively weak CH3OH combination band provided an additional avenue by which to study the intramolecular dynamics of this simplest organic molecule with hindered internal rotation.