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  • Letter

Excited rotational states of molecules in a superfluid

Igor N. Cherepanov, Giacomo Bighin, Constant A. Schouder, Adam S. Chatterley, Simon H. Albrechtsen, Alberto Viñas Muñoz, Lars Christiansen, Henrik Stapelfeldt, and Mikhail Lemeshko
Phys. Rev. A 104, L061303 – Published 30 December 2021
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    Abstract

    We combine experimental and theoretical approaches to explore excited rotational states of molecules embedded in helium nanodroplets using CS2 and I2 as examples. Laser-induced nonadiabatic molecular alignment is employed to measure spectral lines for rotational states extending beyond those initially populated at the 0.37 K droplet temperature. We construct a simple quantum-mechanical model, based on a linear rotor coupled to a single-mode bosonic bath, to determine the rotational energy structure in its entirety. The calculated and measured spectral lines are in good agreement. We show that the effect of the surrounding superfluid on molecular rotation can be rationalized by a single quantity, the angular momentum, transferred from the molecule to the droplet.

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    • Received 29 June 2021
    • Accepted 13 December 2021

    DOI:https://doi.org/10.1103/PhysRevA.104.L061303

    ©2021 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Atomic & molecular clustersMolecular spectraQuasiparticles & collective excitationsStrong electromagnetic field effectsSuperfluidityUltrafast phenomena
    1. Physical Systems
    Bose gasesBose-Einstein condensatesQuantum fluids & solids
    Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

    Authors & Affiliations

    Igor N. Cherepanov1, Giacomo Bighin1, Constant A. Schouder2, Adam S. Chatterley2, Simon H. Albrechtsen2, Alberto Viñas Muñoz2, Lars Christiansen2, Henrik Stapelfeldt2,*, and Mikhail Lemeshko1,†

    • 1Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria
    • 2Department of Chemistry, Aarhus University, 8000 Aarhus C, Denmark

    • *Corresponding author: henriks@chem.au.dk
    • †Corresponding author: mikhail.lemeshko@ist.ac.at

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    Issue

    Vol. 104, Iss. 6 — December 2021

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    • Figure 1
      Figure 1

      The left column shows 〈cos2θ2D〉(t) for CS2 molecules at 12 different fluences of the alignment pulse given in each panel and the right column shows the power spectra of the corresponding 〈cos2θ2D〉 traces.

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    • Figure 2
      Figure 2

      (a) and (b) Frequency differences (EL+2−EL)/h as a function of L for (a) CS2 and (b) I2. The black squares are the experimental results obtained from the central positions of the peaks in the spectra (cf. Fig. 1), the red circles are the results from the theoretical model, and the green triangles are the values from Eq. (1) using the data obtained in [23]. (c) The three contributions to the rotational energy of I2 molecules in He droplets. (d) and (e) First derivative of the He solvent angular momentum squared d〈Λ2〉/dL for (d) CS2 and (e) I2.

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    • Figure 3
      Figure 3

      The gas-phase molecular rotational states (blue open circles) are perturbed by a band of excited states (red closed circles; only single solvent excitations are shown), resulting in the rotational states of the molecule in the presence of the solvent (green open squares; a calculation for the I2 molecule including multiple solvent excitations is shown).

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    • Figure 4
      Figure 4

      Anisotropic component of the helium density in the molecular frame for (a) L=0 and (b) L=40. (c) The average distance of He atoms from the molecular z axis grows with the angular momentum L. All plots are for the I2 molecule.

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