A diatomic molecule in a superfluid helium droplet: Is the anomalous Zeeman splitting a signature of a microscopic Einstein - de Hass effect

The first beam of helium droplets was reported in the 1961 paper Strahlen aus kondensiertem Helium im Hochvakuum by Von E. W. Becker and co-workers [1]. However, molecular spectroscopy of helium-solvated dopants wasn't realized until 30 years later in the laboratories of Scoles and Toennies [2,3].
It has now been over two decades since this early, seminal work on doped helium droplets, yet the field of helium droplet spectroscopy is still fresh with vast potential. Analogous in many ways to cryogenic matrix isolation spectroscopy, the helium droplet is an ideal environment to spectroscopically probe difficult to prepare molecular species, such as radicals, carbenes and ions. The quantum nature of helium at 0.35 K often results in molecular spectra that are sufficiently resolved to evoke an analysis of line shapes and fine-structure that demands rigorous “effective Hamiltonian” treatments, revealing the microscopic details associated with the interaction between the molecular dopant and the superfluid solvent environment. This lecture will focus on our successful attempts to dope the hydroxyl radical (OH) and hydroxyl-containing molecular complexes into helium droplets [4,5,6]. The properties of these systems have been probed with infrared laser spectroscopy. Anomalous effects such as an unusually large Zeeman splitting are interpreted with a model that accounts for angular momentum transfer between dopant and solvent, which is akin to a microscopic Einstein – de Hass effect.

[1]  E. W. Becker, R. Klingelhöfer, P. Lohse, Z. Naturforsch. A 16A, 1259 (1961).
[2]  S. Goyal, D. L. Schutt, G. Scoles, Phys. Rev. Lett. 69, 933 (1992).
[3]  M. Hartmann, R. E. Miller, J. P. Toennies, A. F. Vilesov, Phys. Rev. Lett. 75, 1566 (1995).
[4]  Raston, P.L.; Liang, T.; Douberly, G.E., “Anomalous -doubling in the infrared spectrum of the hydroxyl radical in helium nanodroplets” Journal of Physical Chemistry A 117, 8103-8110 (2013).
[5]  Brice, J.T.; Liang, T.; Raston, P.L.; McCoy, A.B.; Douberly, G.E. “Infrared Stark and Zeeman spectroscopy of OH-CO: The entrance channel complex along the OH + CO →  trans-HOCO reaction pathway” Journal of Chemical Physics 145, 124310 (2016).
[6]  Raston, P.L; Obi, E.I.; Douberly, G.E. “Infrared Spectroscopy of the Entrance Channel Complex Formed Between the Hydroxyl Radical and Methane in Helium Nanodroplets” Journal of Physical Chemistry A 121, 7597-7602 (2017).