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Torsionally mediated spin-rotation hyperfine splittings at moderate to high J values in methanol



Sergey P. Belov, Alexandr V. Lapinov, Arthur A. Mescheryakov, Jon T. Hougen, Li-Hong Xu


This paper presents an explanation based on torsionally mediated proton-spinoverall-rotation interaction for the observation of doublet hyperfine splittings in some Lamb-dip sub-millimeter-wave transitions between ground-state torsion-rotation states of E symmetry in methanol. These unexpected doublet splittings, some as large as 70 kHz, were observed for rotational quantum numbers in the range of J = 13 to 34, and K = -2 to +3. Because they increase nearly linearly with J for a given branch, we confined our search for an explanation to hyperfine operators containing one nuclear-spin angular momentum factor I and one overall-rotation angular momentum factor J (i.e., to spin-rotation operators), and ignored both spin-spin and spin-torsion operators, since they contain no rotational angular momentum operator. Furthermore, since traditional spin-rotation operators did not seem capable of explaining the observed splittings, we constructed totally symmetric "torsionally mediated spin-rotation operators" by combining the E-species spin-rotation operator with an E-species torsional-coordinate factor. The resulting operator is capable of connecting the two components of a degenerate torsion-rotation E state. This has the effect of turning the hyperfine splitting pattern upside down for some nuclear-spin states, which leads to bottom-to-top and top-to-bottom hyperfine selection rules for some transitions, and thus to an explanation for the unexpectedly large observed hyperfine splittings. The constructed operator has no nonzero matrix elements within the set of torsion-rotation A1 and A2 states, and thus cannot contribute to hyperfine splittings in the A manifold. The theory developed here fits the observed large doublet splittings to an overall residual of less than 1 kHz, and predicts much smaller splittings for a number of transitions in which no doublet splitting was detected.
Journal of Chemical Physics


degenerate torsion-rotation E states, doublet splittings, hyperfine structure, Lamb-dip measurements, methanol, nuclear-spin overall-rotation interaction


Belov, S. , Lapinov, A. , Mescheryakov, A. , Hougen, J. and Xu, L. (2016), Torsionally mediated spin-rotation hyperfine splittings at moderate to high J values in methanol, Journal of Chemical Physics, [online],, (Accessed May 21, 2024)


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Created July 12, 2016, Updated October 12, 2021