# Fingerprints of angulon instabilities in the spectra of matrix-isolated molecules

## TL;DR

This paper demonstrates that angulon instabilities can explain the spectral line broadening of molecules in superfluid helium, revealing a microscopic angular momentum transfer mechanism distinct from vortex formation.

## Contribution

It introduces the application of angulon theory to interpret spectroscopic anomalies in superfluid helium, linking microscopic impurity-superfluid interactions to observable spectral features.

## Key findings

- Angulon instabilities cause spectral line broadening.
- Spectroscopic signatures match angulon theory predictions.
- Provides experimental fingerprint of angulon formation.

## Abstract

The formation of vortices is usually considered to be the main mechanism of angular momentum disposal in superfluids. Recently, it was predicted that a superfluid can acquire angular momentum via an alternative, microscopic route -- namely, through interaction with rotating impurities, forming so-called `angulon quasiparticles' [Phys. Rev. Lett. 114, 203001 (2015)]. The angulon instabilities correspond to transfer of a small number of angular momentum quanta from the impurity to the superfluid, as opposed to vortex instabilities, where angular momentum is quantized in units of $\hbar$ per atom. Furthermore, since conventional impurities (such as molecules) represent three-dimensional (3D) rotors, the angular momentum transferred is intrinsically 3D as well, as opposed to a merely planar rotation which is inherent to vortices. Herein we show that the angulon theory can explain the anomalous broadening of the spectroscopic lines observed for CH$_3$ and NH$_3$ molecules in superfluid helium nanodroplets, thereby providing a fingerprint of the emerging angulon instabilities in experiment.