Internal state thermometry of cold trapped molecular anions

Abstract

− 2 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 H 3 O − 2 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.

Incomplete rotational cooling in a 22-pole ion trap

“…al. [20] and extend them to higher temperatures. The lowest rotational temperatures measured in the present work are slightly below the lowest temperature measured there.…”

“…[4,20] and only a brief account is given below. The hydoxyl molecular anions are produced in a pulsed plasma discharge source from a helium-water mixture.…”

“…A photon whose energy exceeds this electron binding energy can detach the electron. The total photodetachment cross section near threshold can be written as [20]:…”

“…Powernormalised photodetachment loss rates near threshold are obtained by repeating the measurements for different exposure times, ranging between few hundred milliseconds and several seconds, and for different laser frequencies. These rates are fitted to the detachment cross section described by equation 1, which yields the rotational temperature of the trapped ions [20].…”

“…This work is based on the rotational temperature diagnostics by near-threshold photodetachment that our group has demonstrated previously [20]. Precise internal state thermometry measurements of OH − in He and OD − in HD have been carried out over a temperature range of more than 100 K. Furthermore, the rotational temperature was measured for different settings of the trapping potential.…”