Spin-torsion dominated hyperfine splittings in the first excited torsional state (v = 1) of methanol

Doublet, triplet and quartet hyperfine splitting patterns have been observed in the E-species component of the first excited (vt = 1) torsional state of CH3OH. Four series of lines dominate the available data: (i) a K = 6 ← 7, Q branch series of quartets, with 7 ≤ J ≤ 15; (ii) a K = 3 ← 2, Q branch series, with 3 ≤ J ≤ 18, which starts as quartets, changes to doublets at J = 7, and then finally to singlets at J = 17; (iii) a K = −2 ← −3, P branch series of doublets, with 8 ≤ J ≤ 13; and (iv) a K = 8 ← 7, Q branch series, with 8 ≤ J ≤ 24, which starts as triplets and becomes doublets at J = 12. There are also a few isolated doublets and quartets, which do not form long spectroscopic branches. We have modeled the hyperfine quartet and doublet splittings with empirically chosen symmetry-allowed spin-torsion, spin-rotation, and spin–spin interaction terms appropriate for the two I = ½ spin systems arising from the OH proton and from the CH3 protons, respectively, and have achieved a least-squares fit of 144 hyperfine components (88 hyperfine intervals) to six hyperfine parameters with a standard deviation (0.75 kHz) near experimental measurement accuracy. The three spin-rotation parameters associated with one I = ½ spin system in the fit agree well with ab initio predictions for IOH parameters in the literature. The two spin-torsion parameters agree to some extent with other fitted values in the literature. The physical effects included in the present Hamiltonian differ from those included in the Hamiltonian for our earlier work on hyperfine splittings in vt = 0 E states, because the experimentally observed vt = 1 splittings are found to decrease approximately as 1/J, whereas the vt = 0 splittings increase approximately as J. The position and relative intensities of the weak central features of the triplets are explained as three-level Λ-type crossover resonances.