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

A comprehensive characterisation of cold molecular beams from a cryogenic buffer-gas cell, providing insight into the physics of buffer-gas cooling, is presented. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. The rotational-state distribution of the molecules is probed by radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, it is possible 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 (CH₃ F and CF₃ CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH₃ F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.