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

Area of Science:

• Spectroscopy
• Laser Technology
• Atmospheric Chemistry

Background:

• Accurate measurement of hydrogen chloride (HCl) is crucial for atmospheric monitoring and industrial process control.
• Previous measurements of HCl line strengths and broadening coefficients have relied on less sensitive techniques.
• The development of tunable diode lasers offers potential for improved spectroscopic analysis.

Purpose of the Study:

• To measure the line strength and air-broadening coefficient of the H(35)Cl P(3) line near 1.2 micrometers.
• To assess the performance of InGaAsP diode lasers for high-resolution near-IR spectroscopy.
• To demonstrate trace concentration measurement of HCl using two-tone frequency modulation (FM) detection.

Main Methods:

• Absorption spectroscopy utilizing an InGaAsP diode laser tuned to the second overtone band of HCl.
• High-frequency two-tone FM detection for enhanced sensitivity.
• Measurement of the H(35)Cl P(3) line strength and air-broadening coefficient.

Main Results:

• The measured line strength of H(35)Cl P(3) was approximately 18% higher than previous grating spectrometer data.
• A sensitivity limit of 3 parts per million (ppm) HCl in air at 50 Torr was achieved with a multimode laser.
• A minimum detectable fractional absorption of 4 x 10(-6) was determined for the multimode laser system.

Conclusions:

• The findings suggest that the commonly used experimental value for the 3ν0 HCl vibrational moment may require revision.
• InGaAsP diode lasers provide a cost-effective and convenient platform for high-resolution spectroscopy and trace gas detection.
• Further optimization of the detection method could enable real-time measurement of HCl concentrations below 0.1 ppm.