Measurements in the HCl 3 ? 0 band using a near-IR InGaAsP diode laser

Hydrogen chloride gas is measured by absorption in the second overtone band near 1.2microm using an InGaAsP diode laser. The strength and air-broadening coefficient of the H(35)Cl P(3) line are measured. The line strength is ~18% higher than the previous grating spectrometer measurement and suggests that the commonly used experimental value for the 3 ?0 HCl vibrational moment may be low. High-frequency two-tone FM detection is also used in this study to measure trace concentrations of HCl. Using a multimode laser, the sensitivity limit is 3-parts per million HCl in air at 50 Torr, corresponding to a minimum detectable fractional absorption (multimode) of 4 x 10(-6). Optimization of the detection method should permit real-time measurement of concentrations below 0.1 ppm. The low cost and convenient operating characteristics of InGaAsP diode lasers make them attractive for many applications in high-resolution near-IR spectroscopy and trace species detection.