High-resolution cavity ring-down spectroscopy of the $\nu_1 + \nu_6$ combination band of methanol at 2.0 $\mu$m

Reported here are portions of the infrared absorption cross-section for methanol (CH$_3$OH) as measured by frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) at wavelengths near $\lambda$ = 2.0 $\mu$m. High-resolution spectra of two gravimetric mixtures of CH$_3$OH-in-air with nominal mole fractions of 202.2 $\mu$mol/mol and 45.89 $\mu$mol/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 CH$_3$OH spectrum from resolved absorption lines at a low pressure (0.833 kPa) to a pseudo-continuum 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 ($
u_1$) plus OH-bend ($
u_6$) combination of $\tau_{IVR} \geq$ 232 ps - long compared to other methanol overtones and combinations. Consequently, we show that high-resolution FS-CRDS of this relatively weak CH$_3$OH combination band provided an additional avenue by which to study the intramolecular dynamics of this simplest organic molecule with hindered internal rotation.