A simple model for high rotational excitations of molecules in a superfluid

Abstract

Recently it became possible to study highly excited rotational states of molecules in superfluid helium through nonadiabatic alignment experiments [Cherepanov \et al., Phys. Rev. A 104, L061303 (2021)]. This calls for theoretical approaches that go beyond explaining renormalized values of molecular spectroscopic constants, which suffices when only the lowest few rotational states are involved. As the first step in this direction, here we present a basic quantum mechanical model describing highly excited rotational states of molecules in superfluid helium nanodroplets. We show that a linear molecule immersed in a superfluid can be seen as an effective symmetric top, similar to the rotational structure of radicals, such as OH or NO, but with the angular momentum of the superfluid playing the role of the electronic angular momentum in free molecules. The simple theory sheds light onto what happens when the rotational angular momentum of the molecule increases beyond the lowest excited states accessible by infrared spectroscopy. In addition, the model allows to estimate the effective rotational and centrifugal distortion constants for a broad range of species and to explain the crossover between light and heavy molecules in superfluid $^4$He in terms of the many-body wavefunction structure. Some of the above mentioned insights can be acquired by analyzing a simple 2x2 matrix.

Focus on the cold and ultracold chemistry of atoms, ions and molecules

“…Owing to the system-bath interaction, the observed rotational spectra in a helium nanodroplet may deviate significantly from the gas phase. Cherepanov et al present a detailed quantum-mechanical model for the description of highly excited rotational states of linear molecules embedded in a superfluid helium nanodroplet [2]. The results of this model can be used to describe experimental data and it provides a more intuitive means for understanding several properties of molecules in superfluids.…”