Very-low-temperature infrared laser absorption spectroscopy of n₂o, no, and no₂

We have constructed an absorption cell which is cooled to close to liquid helium temperature by an open cycle refrigerator. Infrared spectra of gaseous N₂O, NO, and NO₂ cooled by collisions with helium buffer gas nearly to the cell wall temperature of approximately 5 K at pressures near 50 mTorr have been recorded by diode laser absorption. In the present design, the probe laser beam makes four passes through the cold sample. The enhancement of absorption line intensities is consistent with our expectations, as is the decrease in Doppler-broadened linewidths. The sample rotational temperatures were estimated by measuring relative intensities of low-J transitions. The increased resolution resulting from the linewidth reduction allows routine measurement of fine and hyperfine splittings masked at the Doppler width of an ambient temperature sample.