Thermometry of Guided Molecular Beams from a Cryogenic Buffer-Gas Cell

TL;DR

This paper characterizes cold molecular beams from a cryogenic buffer-gas cell, using electrostatic guiding and spectroscopy to measure their temperature and state distributions, advancing understanding of buffer-gas cooling physics.

Contribution

It introduces a comprehensive thermometry method combining trajectory simulations and spectroscopy to determine rotational and translational temperatures of molecules from buffer-gas sources.

Findings

Rotational and translational temperatures can be accurately measured.

Faster rotational thermalization observed for CH3F-He at low He density.

Different rotational states exhibit varying relaxation rates.

Abstract

We present a comprehensive characterization of cold molecular beams from a cryogenic buffer-gas cell, providing an insight into the physics of buffer-gas cooling. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. Molecules' rotational-state distribution is probed via radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, we are able to determine both the rotational and the translational temperature of the molecules at the output of the buffer-gas cell. This thermometry method is demonstrated for various regimes of buffer-gas cooling and beam formation as well as for molecular species of different sizes, $\rm{CH_3F}$ and $\rm{CF_3CCH}$. Comparison between the rotational and translational temperatures provides evidence of faster rotational thermalization for the $\rm{CH_3F-He}$ system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.