Collisional-broadened and dicke-narrowed lineshapes of H216O and H218O transitions at 1.39 μm

We present results of a study on the self-broadening and broadening by nitrogen and oxygen of (H2O)-O-16 and (H2O)-O-18 lines in the 1.39-mum wavelength region using a distributed feedback semiconductor diode laser. To estimate the broadening coefficients, the absorption lineshapes were analyzed using the ordinary Voigt profile which provided good fits for most of the investigated lines. The broadening coefficients were found to be larger for nitrogen used as a perturber than for oxygen. This agrees with the fact that the quadrupole moment of N-2 is larger than that of O-2. Nevertheless, for lines involving high-rotational quantum number J, relatively smaller broadening coefficients were found and deviations of the measured profiles from the standard Voigt profile were observed. These deviations were ascribed as caused by Dicke-narrowing effect. Corresponding to this effect, collisional-broadening and narrowing coefficients were determined using the Nelkin-Ghatak profile which is suitable for the "hard"-collision model. The optical diffusion coefficients of the water in both nitrogen and oxygen gases were determined from the measurements of the collisional-narrowing coefficients. (C) 2001 Academic Press.