Collisional cooling of the NO-He system The pressure broadening cross sections between 4·3 and 1·8 K

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Abstract
We report the results of a very low temperature pressure broadening experiment on the NO-He system. A newly developed collisional cooling technique was used to provide an environment for the study of gas phase collisions below 4K. In this temperature regime typical rotational energies are greater than available thermal energies. The helium pressure broadening cross section for the J = 1/2-3/2 transition of NO in the 2Π1/2 electronic state was found to drop by ∼35 per cent from 26 Å2 at 4·2K to 17Å2 at 1·8K. The 300K cross section is 22Å2. The results of this experiment are compared with results from an earlier experiment on the CO-He system in which the cross section rose by ∼30 per cent over this temperature range. The spectroscopic attributes of the collisional cooling technique are also discussed, some of which are: narrow line widths, large absorption coefficients, and the simplification of otherwise complex spectra. In addition, because, hvr > kT, the number of available collisional channels is vastly reduced. This makes exact theoretical calculations possible and allows a more direct comparison between experimental observables and fundamental molecular parameters.