Development of a HCl laser absorption diagnostic near 3.3 μm for shock-tube chemical kinetics studies

A new hydrogen chloride (HCl) laser absorption diagnostic was developed and combined with a shock tube to obtain HCl time-history profiles behind reflected shock waves. An interband cascade laser was used to access the R(8) transition lines of the two isotopes H³⁵Cl and H³⁷Cl in the fundamental (1 \(\leftarrow\) 0) band at the specific wavelengths of 3045.06 and 3042.74 cm⁻¹ near 3.3 μm, respectively. Spectroscopic parameters were obtained using HCl in 99.5% Ar, focusing on the line strengths and Ar-broadening effects, and were compared with theory from the literature. Experimental calibration of the HCl absorption coefficient and its dependence over a wide range of temperatures and pressures were obtained (i.e. 1261—1759 K, 0.25—0.42 atm, and 2390—3736 K, 1.26—2.00 atm). Measurements of the line strengths, Ar-broadening parameters at 296 K, and temperature-dependence exponents for the R(8) transition lines of H³⁵Cl and H³⁷Cl were validated against these results and can be summarized as follows:

1. 1)

   For H³⁵Cl:\({S}_{12}\left({T}_{0}\right)\) = 2.099 \(\pm\) 0.084 cm⁻²-atm⁻¹, \({\gamma }_{H35Cl-Ar}\left({T}_{0}\right)\) = 0.0110 \(\pm\) 0.0005 cm⁻¹-atm⁻¹, and \({n}_{H35Cl-Ar}\) = 0.4 \(\pm\) 0.01.

2. 2)

   For H³⁷Cl:\({S}_{12}\left({T}_{0}\right)\) = 0.708 \(\pm\) 0.028 cm⁻²-atm⁻¹, \({\gamma }_{H37Cl-Ar}\left({T}_{0}\right)\) = 0.0105 \(\pm\) 0.0005 cm⁻¹-atm⁻¹, and \({n}_{H37Cl-Ar}\) = 0.3 \(\pm\) 0.01.

The new HCl laser probe shows promising results for future measurements to better understand the combustion chemistry of propellants containing chlorine.