Very low temperature spectroscopy: The pressure broadening coefficients for CO–He between 4.3 and 1.7 K

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Abstract
The attributes of very low temperature spectroscopy in the context of a simple collisionally cooled cell are described. Among them are narrow linewidths, large absorption coefficients, and the simplification of complex spectra. Most importantly, the technique makes possible the study of gas phase collisional processes under equilibrium conditions in a variable temperature environment below 4 K. In this regime the rotational energy level spacing is typically larger than kT, thereby both significantly simplifying the most exact theoretical calculations and giving a more direct link between experimental observables and fundamental molecular parameters. The results of a pressure broadening experiment in the CO–He system in the region between 4.24–1.78 K are described. It was found that the pressure broadening parameters varied from 21.3 to 40.0 MHz/Torr over this temperature region. Comparison with earlier theoretical calculations showed excellent agreement at the higher temperatures; however, at the lowest observed temperature the theoretical value was about 20% greater than the measurement.