Internal state thermometry of cold trapped molecular anions

TL;DR

This study uses photodetachment spectroscopy in a cryogenic trap to measure the internal states of cold molecular anions, demonstrating cooling effects and vibrational ground state population, with implications for understanding molecular ion behavior at low temperatures.

Contribution

It presents a novel application of bound-free spectroscopy to analyze rotational and vibrational cooling of molecular anions in a cryogenic trap, including modeling of the detachment threshold.

Findings

Rotational temperature of OH- deviates from buffer gas temperature below 50 K

Vibrational ground state population increases for H3O2-

Photodetachment threshold modeled with Franck-Condon analysis

Abstract

Photodetachment spectroscopy of OH- and H3O2- anions has been performed in a cryogenic 22-pole radiofrequency multipole trap. Measurements of the detachment cross section as a function of laser frequency near threshold have been analysed. Using this bound-free spectroscopy approach we could demonstrate rotational and vibrational cooling of the trapped anions by the buffer gas in the multipole trap. Below 50 K the OH- rotational temperature shows deviations from the buffer gas temperature, and possible causes for this are discussed. For H3O2- vibrational cooling of the lowest vibrational quantum states into the vibrational ground state is observed. Its photodetachment cross section near threshold is modelled with a Franck-Condon model, with a detachment threshold that is lower, but still in agreement with the expected threshold for this system.