Doppler-limited CW infrared cavity ringdown spectroscopy of the <f>ν1+ν3</f> OH + CH stretch combination band of jet-cooled methanol

A high resolution cavity ringdown spectrometer (CRDS) has been constructed using a 1.5 μm continuous-wave external-cavity tunable diode laser, a mode-matched near-confocal ringdown cavity, and 2 cm pulsed slit jet. Without signal averaging, the RMS noise in the absorption signal is 1.7 × 10-9 cm-1. The rotationally resolved overtone spectrum of the OH(<f>ν1</f>) + CH(<f>ν3</f>) stretch combination band of methanol between 6510 and 6550 cm-1 has been observed for <f>J′=0–8</f> and <f>K′=0–3</f> at sub-Doppler resolution. In total, 418 lines are assigned and global fits yield molecular torsion–rotation parameters for the upper state. Four <f>K′</f>-localized perturbations are analyzed and the pattern of residuals is discussed. [Copyright &y& Elsevier]