Absolute nu(2) line intensities of HOCl by simultaneous measurements in the infrared with a tunable diode laser and far-infrared region using a Fourier transform spectrometer

We have measured absolute line intensities in the nu (2) fundamental band a t 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 int ensities in the pure rotational far-infrared (FIR) spectrum determined from Stark effect measurements. We have thus recorded simultaneously the FIR pu re rotation spectrum of HOCl using a Bruker IFS120HR interferometer and the spectrum of a few vibration-rotation lines in the infrared (IR) nu (2) ban d using a tunable diode laser spectrometer. The absolute intensities of the se IR lines thus determined allowed us to "calibrate" the intensities of vi bration-rotation lines in the whole nu (2) 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 HO Cl amount during the recording of the spectra. The latter was found to be a lmost constant over hours after conditioning of the cell. The square of the nu (2) band vibrational transition dipole moment was determined to be 0.01 3947(23) D-2 and 0.013870(51) D-2 for (HOCl)-Cl-35 and (HOCl)-Cl-37, respec tively, that is, 29 to 73% lower than previous measurements. A linear Herma n-Wallis factor was also determined for both isotopomers. Finally, the line intensities were least-squares fitted using a model that takes into accoun t a weak resonance between the (010) and (002) levels. (C) 2000 Academic Pr ess.