Trans- to gauche-transitions in the far infrared spectrum of methanol-D2 (CHD2OH) in the torsional ground state

In this work detailed transition assignments and wavenumbers for interstate transitions from the lowest lying trans-state to higher lying gauche-states (o1 and e1) for doubly deuterated asymmetric methanol (CHD2OH) in the high resolution Fourier transform far infrared (FIR) spectrum are presented. The assignment methods and the structure of the spectra are described. The observations are in agreement with previous predictions of the term values and the transition matrix elements. The matrix elements of the transition dipole moment suggest that the pure symmetry properties of the internal rotational states are broken. In other words, the asymmetry induced in the threefold internal rotational potential barrier mixes the symmetry species heavily so that the transitions between levels within a symmetry species could be as strong as interspecies transitions. Therefore the symmetry is not a good quantum number. The identified transitions are analyzed in terms of the Q-branch origins and state dependent parameters. The use of “Loomis Wood” type spectral arrangements made the assignment easier with the help of a priori knowledge of the term values. In all, a catalogue of some 700 new such transitions have been prepared. A refinement of the rotational parameters were possible but confident improvement of the torsional parameters need assignments connecting excited torsional vibrational states, which is in progress. The present work should be useful for interstellar detection in the star forming regions in “Radio Astronomy” and discovery of new spontaneously amplified light sources in the far infrared and sub-millimeter wave (SMMW) regions. Lastly the accurately measured transition lines in the atlas could be used as a secondary standard in the far infrared regions.