Internal state thermometry of cold trapped molecular anions

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.