Diode laser spectroscopy of gaseous HCl

Concentrations of gaseous HCl less than 1 ppm were measured with a strained-layer quantum-well distributed-feedback laser emitting the wavelength 1.74 μm at room temperature. Second-harmonic wavelength-modulation spectroscopy was used to reach a detection limit of 0.1 ppm m length with a signal-averaging time of 4 s. The measurements were carried out on gaseous HCl mixed with nitrogen (N₂) at 25°C when the pressure was 1 bar.