Wavelength-modulation laser hygrometer for ultrasensitive detection of water vapor in semiconductor gases.

Water vapor is measured by use of a near-infrared diode laser and wavelength-modulation absorption spectroscopy. Humidity levels as low as 5 nmol/mol [1 nmol/mol = 1 ppb (1 ppb equals 1 part in 10(9))] of water vapor in air are measured with a sensitivity of better than 0.2 nmol/mol (3varsigma). The sensitivity, linearity, and stability of the technique are determined in experiments conducted at the National Institute of Standards and Technology, Gaithersburg, Maryland, by use of the low frost-point humidity generator over the range from 5 nmol/mol to 2.5 mumol/mol of water vapor in air. The pressure-broadening coefficients for water broadened by helium [0.0199(6) cm(-1) atm(-1) HWHM] and by hydrogen chloride [0.268(6) cm(-1) atm(-1) HWHM] are reported for the water line at 1392.5 nm.