Spectroscopy and ion thermometry of C${}_2^{-}$ using laser-cooling transitions

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 $0.7\times 10^{-3}$ cm${}^{-1}$ or 20 MHz. The spin-rotation splitting is resolved, which allows for a more precise determination of the splitting constants to ${\gamma }^{\prime }=7.15(19)\times 10^{-3}$ cm${}^{-1}$ and ${\gamma }^{\prime \prime }=4.10(27)\times 10^{-3}$ 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.

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Cited by in corpus (4)

• Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C${}_2^{-}$
• Energy Levels and Transition Rates of Laser-Cooling Candidate Th^-
• Molecular laser cooling using serrodynes: Implementation, characterization and prospects
• Ion-kinetic-energy sampling in a 22-pole trap using ring-electrode evaporation