Knowledge Bank

We have constructed an absorption cell from high purity coper which is cooled to close to liquid helium temperature by an open cycle refrigerator. The molecule to be studied is introduced into a low pressure bath of helium gas in equilibrium with the cell walls via a heated nozzle injector. The sample molecules suffer collisions with the cold helium atoms as they gradually diffuse to the walls where they are lost by condensation. During the diffusion process, the translational and internal degrees of freedom of the sample are cooled down and a steady state concentration of cold gaseous sample may be maintained in the viewing region. In proof of principle experiments, we have studied infrared spectra of gaseous NO and $N2O$ at cell temperatures of approximately 5K. The encouraging results are that the sensitivity of absorption due to the cold sample is at least as good as for a similar density at ambient temperature and the expected decrease in Doppler broadened line width is observed. Future plans include investigations of spectra of heavier molecules whose infrared spectra are badly congested and an attempt to study chemically reactive species following photolysis of a suitable precursor molecule in the low temperature environment. Currently, we are studying infrared spectra of $NO2$ using a redesigned cell with Brewster windows combined with multiple pass optics. This will increase the absorption sensitivity.