Radiative Rotational Lifetimes and State-Resolved Relative Detachment Cross Sections from Photodetachment Thermometry of Molecular Anions in a Cryogenic Storage Ring

Photodetachment thermometry on a beam of ${\mathrm{OH}}^{-}$ in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20 min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the $J=1$ natural lifetime (about 193 s) and determine the ${\mathrm{OH}}^{-}$ rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.

Figure 1

Near-threshold photodetachment cross section of ${\mathrm{OH}}^{-}$ modeled by ${\sigma }_T(\tilde{\nu };T)=\sum _J{p}_J(T){\sigma }_J(\tilde{\nu })$ with ${\sigma }_J(\tilde{\nu })$ for $a=0.20$, $b=-2.8$, and for $T=10\mathrm{K}$ and 300 K as labeled. The reference cross section is ${\sigma }_r={\sigma }_{J=0}({\tilde{\nu }}_r)$ at the HeNe laser wave number (full vertical mark, see the text). Probing wave numbers ${\tilde{\nu }}_k$ are indicated by dashed marks.

Figure 2

Photodetachment signals $S_k(t)=N({\tilde{\nu }}_k,t)/N({\tilde{\nu }}_r,t)$ as functions of the ion storage time for two sample runs with 1151 (a) and 38 (b) injections. Ranges of $J$ levels contributing to the signals are marked. Data points represent the measured count ratios and their $1\sigma$ uncertainties. The full lines show a fit to the signals of all runs using the modeled photodetachment cross sections and level populations. In (a) the shaded areas indicate the model variation with $T_0$ at the short times when the data were excluded from the fit (see text).

Figure 3

Near-threshold photodetachment cross section ratios ${\sigma }_J(\tilde{\nu })/{\sigma }_r$ of ${\mathrm{OH}}^{-}$ for the probing lines ($\tilde{\nu }={\tilde{\nu }}_k$) determined from the fit (filled symbols with overall $1\sigma$, statistical, and systematic uncertainties) and the model with $a=0.20$, $b=-2.8$ (full lines). Open diamonds mark theoretical values of ${\sigma }_J({\tilde{\nu }}_k)/{\sigma }_r$ for the channels where separate features cannot be identified in the data. The model value of ${\sigma }_{J=0}({\tilde{\nu }}_3)/{\sigma }_r$ serving as a reference (see text) is marked by the large diamond.