Determination of Low Concentration HD through Laser Thermal Lensing Spectroscopy

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Laser-induced thermal lensing spectroscopy was studied to analyze hydrogen gas which contained natural abundance of HD, 300ppm. An infrared laser system of 1.382 μm InGaAs P type was used to excite HD molecule with a precise temperature control and frequency modulation of 1.4 to 110 kHz. At first, infrared spectrum of HD was measured with a single-path cell which contained HD gas of 100 to 800 torr and 0.8 mm long. The R(1) branch of the 2-0 band was observed with a half width of 0.02 cmィイD1-1ィエD1 at 7241.82 cmィイD1-1ィエD1, which was the same wavelength observed by Herzberg in 1950. The detection limit of HD with a normal adsorption spectroscopy was 20 torr, which was equipment to a concentration of 100 times higher than that of the natural abundance of HD. We applied a multi-path cell of 32 m in the effective path length to detect HD at the low pressure of less than 20 torr, but background noise brought about by the multi-reflection was too high detect HD absorption. In the thermal lensing spectroscopy, a reference cell which was evacuated less than 0.001 torr and 0.8 m long was placed in a parallel geometry of the sample cell contained the HD gas of 0.2 to 2 torr. Each laser beam was passed through a pinhole of 0.1 mm in diameter and detected. A difference signal was detected for a sample of 0.2 torr HD with 100kHz modulation, and it shows that we can determine HD molecule of the natural abundance with thermal lensing spectroscopy.