Absolute ν2 Line Intensities of HOCl by Simultaneous Measurements in the Infrared with a Tunable Diode Laser and Far-Infrared Region Using a Fourier Transform Spectrometer

Abstract

We have measured absolute line intensities in the ν2 fundamental band at 1238 cm−1 of both isotopomers of hypochlorous acid, HOCl. To obtain the partial pressure of the species in the sample mixture, unavailable through direct measurement since HOCl exists only in equilibrium with H2O and Cl2O and may decay by secondary reactions, we relied on known absolute line intensities in the pure rotational far-infrared (FIR) spectrum determined from Stark effect measurements. We have thus recorded simultaneously the FIR pure rotation spectrum of HOCl using a Bruker IFS120HR interferometer and the spectrum of a few vibration–rotation lines in the infrared (IR) ν2 band using a tunable diode laser spectrometer. The absolute intensities of these IR lines thus determined allowed us to “calibrate” the intensities of vibration–rotation lines in the whole ν2 band, measured previously using Fourier transform spectroscopy. The treatment of the data took into account the blackbody emission contribution in the FIR and the evolution of the HOCl amount during the recording of the spectra. The latter was found to be almost constant over hours after conditioning of the cell. The square of the ν2 band vibrational transition dipole moment was determined to be 0.013947(23) D2 and 0.013870(51) D2 for HO35Cl and HO37Cl, respectively, that is, 29 to 73% lower than previous measurements. A linear Herman–Wallis factor was also determined for both isotopomers. Finally, the line intensities were least-squares fitted using a model that takes into account a weak resonance between the (010) and (002) levels.