FIR SYNCHROTRON SPECTROSCOPY OF HIGH TORSIONAL LEVELS OF CD3OH: THE TAU OF METHANOL

Sub-bands involving high torsional levels of the CD$_3$OH isotopologue of methanol have been analyzed in Fourier transform spectra recorded at the Far-Infrared beamline of the Canadian Light Source synchrotron in Saskatoon. Energy term values for $A$ and $E$ torsional species of the third excited torsional state, v$_t$ = 3, are now almost complete up to rotational levels $K$ = 15, and thirteen substates have so far been identified for v$_t$ = 4. The spectra show interesting close groupings of high-v$_t$ sub-bands related by Dennison�s torsional symmetry label $tau$, rather than $A$ and $E$, that can be understood in terms of a simple and universal free-rotor �spectral predictor� chart. Transitions between states on the same free rotor curve have torsional overlap matrix elements close to unity, so give rise to strong sub-bands providing radiative routes for rapid population transfer through the high torsional manifold. Where the energy curves for the v$_t$ = 3 and 4 ground-state torsional levels pass through the excited vibrational states, strong resonances can occur and a number of anharmonic and Coriolis interactions have been detected through perturbations to the spectra and appearance of forbidden transitions due to strong mixing and intensity borrowing