TAU-CLUSTERING, ANOMALIES AND FORBIDDEN FEATURES IN THE $CH_{3}$-ROCKING AND OH-BENDING BANDS OF $CH_{3}OH$

Author Institution: Centre for Laser Applications and Molecular Science (CLAMS), Department of Physics, University of Brunswick; CLAMS and Department of Physical Sciences, University of New Brunswick; Steacie Institute for Molecular Sciences, National Research Council CanadaThe $CH_{3}$-rocking and OH-bending bands are being investigated in high-resolution Fourier transform spectra of $CH_{3}OH$. For the in-plane $CH_{3}$-rock, subbands have been assigned with origins clustered around $1070 cm^{-1}$ for the $v_{t} = 0$ ground torsional state and around 1100 $cm^{-1}$ for the $v_{t} = 1$ torsionally excited state. The $v_{t} = 1$ subbands are stronger than the $v_{l} = 0$ series, suggesting that intensity borrowing is taking place through vibrational mixing of the $v_{t} = 1$ rocking states with nearby $v_{t} = 0$ OH-bend and $v_{t} = 1$ CO-stretch levels. This idea is supported by observation of forbidden $\Delta K = 0$ subbands from $v_{t} = 0$ ground-state levels to $v_{t} = 1$ rocking levels. For the OH-bending fundamental, the $v_{t} = 0$ subbands are relatively widely distributed over the 1315-1347 $cm^{-1}$ region with the characteristic grouping into two $\tau$-clusters found when the torsional barrier changes significantly. However, the subband origins imply an inverted excited-state energy pattern compared to the usual model, hence the actual torsional barrier height for the OH-bending state is a matter of conjecture. Again, forbidden $v_{t} = 0 \leftarrow 1 \Delta K = 0$ subbands are seen below the main OH-bending series. Interesting anomalous splittings and intensities are also observed for certain of the K-doublets in all of the vibrational bands. As we continue to build our knowledge of the details and the novel features of the torsion-vibration-rotation energy level manifold of the CO-stretch, $CH_{3}$-rock and OH-bend modes of $CH_{3}OH$, more and more interactions are revealed which couple the states and can contirbute to IVR rates in methanol. We are aiming to quantify the degree of mixing and the magnitude of the coupling by examining the relative intensities of allowed and forbidden transitions. We will also look at the relative strengths of parallel and perpendicular transitions in the $v_{t} = 0 CH_{3}$-rocking band to seek further insight into the nature of the $CH_{3}$-rocking normal mode and the origin of the substantial parallel transition moment