Citation:
 | pub47.pdf | 1.2 MB |
Abstract:
The Fourier transform spectrum of the CH317OH isotopologue of methanol has been recorded in the 120–350 cm 1 far-infrared region at a resolution of 0.00096 cm 1 using synchrotron source radiation at the CanadianLight Source. The study, motivated by astrophysical applications, is aimed at generating a sufficiently accurateset of energy level term values for the ground vibrational state to allow prediction of the centres of thequadrupole hyperfine multiplets for astronomically observable sub-millimetre transitions to within anuncertainty of a few MHz. To expedite transition identification, a new function was added to the Ritz programin which predicted spectral line positions were generated by an adjustable interpolation between the knownassignments for the CH316OH and CH318OH isotopologues. By displaying the predictions along with theexperimental spectrum on the computer monitor and adjusting the predictions to match observed features, rapidassignment of numerous CH317OH sub-bands was possible. The least squares function of the Ritz program wasthen used to generate term values for the identified levels. For each torsion-K-rotation substate, the term valueswere fitted to a Taylor-series expansion in powers of J(Jþ1) to determine the substate origin energy and effectiveB-value. In this first phase of the study we did not attempt a full global fit to the assigned transitions, but insteadfitted the sub-band J-independent origins to a restricted Hamiltonian containing the principal torsional andK-dependent terms. These included structural and torsional potential parameters plus quartic distortional andtorsion–rotation interaction terms.