Abstract
The second vibrational overtone transition, ${\ensuremath{\nu}}^{\ensuremath{'}},{J}^{\ensuremath{'}}=3,1\ensuremath{\leftarrow}\ensuremath{\nu},J=0,0$, in ${\text{OH}}^{\ensuremath{-}}$ confined in a 22-pole radio frequency ion trap has been measured using a laser induced reaction with molecular hydrogen. The resultant spectral line is found at 10 150.94(2) ${\mathrm{cm}}^{\ensuremath{-}1}$ and the excitation rate of this overtone excitation is obtained, which sensitively probes the molecular potential surface. The Doppler broadened line profile is used to determine the translational temperature of the ions in buffer gas temperatures ranging from 9(1) to 52(1) K. The translational temperatures closely follow those of the buffer gas over this range, including an offset caused by radio frequency heating. In comparison with previous measurements of the rotational temperatures of trapped ions, we show that translational and rotational temperatures do not equilibrate at the low buffer gas temperatures.
Highlights
Precision spectroscopy of trapped molecular ions is a burgeoning field with applications in tests of fundamental physical constants, quantum coherence studies, ultracold chemistry, and astrochemistry
To find the overtone transition, the excitation laser was scanned over a broad frequency range
For this search the ion trapping and exposure time was set to 100 s to minimize the influence of ion number fluctuations in the trap
Summary
Precision spectroscopy of trapped molecular ions is a burgeoning field with applications in tests of fundamental physical constants, quantum coherence studies, ultracold chemistry, and astrochemistry. The application of a neutral, inert buffer gas to a trapped ion setup allows for the cooling of both the translational and internal degrees of freedom [8,10] At low temperatures, this allows for full quantum state preparation of rotational levels. Using a laser-induced reaction to probe the absorption, we have found the third harmonic of the OH stretch vibration and determined its frequency and absorption strength This allows us to benchmark high-level quantum. In the present work we are able to resolve the Doppler profile at different buffer gas temperatures in the ion trap This has allowed us to determine the ion translational temperature that can be compared with the previously measured rotational temperatures. We compare the measured temperatures to numerical simulations This enables an improved understanding of the buffer gas thermalization process in radio frequency multipole ion traps. The reaction rate with H2 is much faster than the radiative lifetime of the excited vibrational levels, which provides essentially unit detection efficiency for every overtone transition
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