Knowledge Bank

The rotationally resolved overtone spectrum of the OH ($\ni 1$) + CH ($\nu 3$) stretch combination band of methanol between 6510 and $6550cm-1$ has been recorded at sub-Doppler resolution by continuous-wave cavity ringdwon spectroscopy (CW-CRDS). Of 572 recorded lines, 358 lines have been assigned by ground state combination differences, including the 21 subbands with $J\prime =0-8$ and $K\prime =0-3$. The perturbations of $E K\prime =-1,-2$, and -3 levels have been observed and deperturbations have been carried out. The torsion-rotational constants in the upper state, obtained by fitting the observed spectrum to the Herbst Hamiltonian, are in a reasonable agreement with the predicted values based on the constants of the fundamental bands, $\nu 1$ and $\nu 3$. This combination band has fewer perturbations than the fundamental $\nu 1$, and the coupling matrix elements are smaller, 0.02 to $0.66cm-1$ as compared to 0.35 to $1.6cm-1$.