Spectroscopy and ion thermometry of C$_{2}^{-}$ using laser-cooling transitions

TL;DR

This study develops a method to precisely measure transition frequencies in C$_{2}^{-}$ ions for laser cooling, enabling better control of their internal states and advancing molecular ion cooling techniques.

Contribution

The paper introduces a versatile approach using pump and photodetachment beams to determine transition frequencies with high accuracy for molecular anions.

Findings

Measured transition frequencies with 20 MHz accuracy.

Resolved spin-rotation splitting and determined splitting constants.

Characterized ion temperatures, revealing collisional heating effects.

Abstract

A prerequisite for laser cooling a molecular anion, which has not been achieved so far, is the precise knowledge of the relevant transition frequencies in the cooling scheme. To determine these frequencies we present a versatile method that uses one pump and one photodetachment light beam. We apply this approach to C$_{2}^{-}$ and study the laser cooling transitions between the electronic ground state and the second electronic excited state in their respective vibrational ground levels, $B ^{2} \Sigma _{u} ^{+}(v=0) \leftarrow X ^{2} \Sigma _{g} ^{+}(v=0) $. Measurements of the R(0), R(2), and P(2) transitions are presented, which determine the transition frequencies with a wavemeter-based accuracy of $0.7\times10^{-3}$ cm$^{-1}$ or 20 MHz. The spin-rotation splitting is resolved, which allows for a more precise determination of the splitting constants to $\gamma ' = 7.15(19)\times10^{-3}$ cm$^{-1}$ and $\gamma '' = 4.10(27)\times10^{-3}$ cm$^{-1}$. These results are used to characterize the ions in the cryogenic 16-pole wire trap employed in this experiment. The translational and rotational temperature of the ions cooled by helium buffer gas are derived from the Doppler widths and the amplitude ratios of the measured transitions. The results support the common observation that the translational temperature is higher than the buffer gas temperature due to collisional heating under micromotion, in particular at low temperatures. Additionally, a rotational temperature significantly lower than the translational is measured, which agrees with the notion that the mass weighted collision temperature of the C$_{2}^{-}$-He system defines the internal rotational state population.