Abstract
A direct simulation Monte Carlo (DSMC) method is applied to model collisions between He buffer gas atoms and ammonia molecules within a buffer gas cell. State-to-state cross sections, calculated as a function of the collision energy, enable the inelastic collisions between He and NH3 to be considered explicitly. The inclusion of rotational-state-changing collisions affects the translational temperature of the beam, indicating that elastic and inelastic processes should not be considered in isolation. The properties of the cold molecular beam exiting the cell are examined as a function of the cell parameters and operating conditions; the rotational and translational energy distributions are in accord with experimental measurements. The DSMC calculations show that thermalisation occurs well within the typical 10-20 mm length of many buffer gas cells, suggesting that shorter cells could be employed in many instances-yielding a higher flux of cold molecules.
Highlights
In contrast to techniques such as laser cooling, cold buffer gas atoms can be employed to translationally cool atomic and molecular species almost independently of the internal energy structure of the species
It is worth noting that the equilibration of rotational and translational temperatures occurs at roughly similar rates, implying that the energy exchange per collision is approximately equal for translation and for rotation
direct simulation Monte Carlo (DSMC) simulations performed both including and excluding inelastic collisions confirm that the final equilibrated translational temperature of the molecules is influenced by inelastic collisions—with the translational temperature thermalising to a value some 10% higher when rotational-state-changing collisions are included in the simulations
Summary
In contrast to techniques such as laser cooling, cold buffer gas atoms can be employed to translationally cool atomic and molecular species almost independently of the internal energy structure of the species. As macroscopic properties represent averages of microscopic quantities (provided there are sufficient numbers of molecules in the system), both approaches should yield the same result. This is often not the case, as there are limitations with both treatments. If taking the microscopic approach, the calculation of pair-wise interactions scales quadratically with the number of particles and becomes unfeasible for all but the smallest of systems. We simulate the collisions between helium buffer gas atoms and ammonia molecules within a buffer gas cell, yielding a rotationally and translationally cold molecular beam. The simulated rotational and translational temperatures are in good agreement with experimental measurements, verifying the accuracy of the DSMC approach
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