Collisional depolarization of rotational angular momentum: what are the observables and how can they be measured?

Abstract

The destruction and transfer of polarization of the rotational angular momentum of small molecules in an isotropic collisional environment is reviewed. Several recent independent treatments are drawn together, including unpublished details from the authors' own work, of the formal kinetics in terms of moments of the density matrix. The final results are shown to be equivalent and directly amenable to comparison with results of exact quantum scattering calculations. In passing, some differences in nomenclature are noted and a self-consistent version is presented that might usefully be adopted. The existing experimental approaches are surveyed, within the common theme of laser-based creation of the initial rotational anisotropy of relatively low rank, K, combined with a spectroscopic probe, also sensitive to restricted K. Those formally defined quantities that may be measured are identified, either individually or in some combination, with each method. In particular, an attempt is made to distinguish between measurements of individual tensor moments of the density matrix, or ‘bulk polarizations’, and alignment moments, which are normalized to the population. Some cases are noted where experimental procedures have compromised the results, or where the analysis has been similarly approximate or carried out on a less rigorous empirical basis.

Keywords:

• rotational angular momentum
• depolarization
• molecular collisions
• energy transfer
• inelastic collisions
• alignment
• orientation
• polarization moment
• density matrix
• tensor moment
• tensor rate constant

Acknowledgements

We are very grateful for extensive helpful correspondence with P.J. Dagdigian and M.H. Alexander, and separately with M. Brouard, during the development of the formalisms described in Section 2. This work was supported partly by research grants from EPSRC and The Leverhulme Trust, by RCUK (Academic Fellowship to MLC) and by the Royal Society (RS Leverhulme Trust Senior Research Fellowship to MGMcK).