Simulating rotationally inelastic collisions using a direct simulation Monte Carlo method

Abstract

A new approach to simulating rotational cooling using a direct simulation Monte Carlo (DSMC) method is described and applied to the rotational cooling of ammonia seeded into a helium supersonic jet. The method makes use of ab initio rotational state changing cross sections calculated as a function of collision energy. Each particle in the DSMC simulations is labelled with a vector of rotational populations that evolves with time. Transfer of energy into translation is calculated from the mean energy transfer for this population at the specified collision energy. The simulations are compared with a continuum model for the on-axis density, temperature and velocity; rotational temperature as a function of distance from the nozzle is in accord with expectations from experimental measurements. The method could be applied to other types of gas mixture dynamics under non-uniform conditions, such as buffer gas cooling of NH₃ by He.

Keywords:

• Direct simulation Monte Carlo
• inelastic scattering
• rotational population
• molecular beam

Supplemental data

The underlying research materials for this article can be accessed at the Oxford Research Archive, DOI 10.5287/bodleian:ht24wj516.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors are grateful to the Wiener Anspach Foundation for its financial support of this work.