Collisional effects on spectral line shape from the Doppler to the collisional regime: A rigorous test of a unified model

High-precision cavity-enhanced spectroscopy for studying the H2-Ar collisions and interactions

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H₂-Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H₂-Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ₀, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra.

Collisional-Broadened and Dicke-Narrowed Lineshapes of H(2)(16)O and H(2)(18)O Transitions at 1.39 µm

We present results of a study on the self-broadening and broadening by nitrogen and oxygen of H(2)(16)O and H(2)(18)O lines in the 1.39-µm 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. Copyright 2001 Academic Press.

Relaxation and Lineshape of the 500.4-GHz Line of Ozone Perturbed by N(2) and O(2)

N(2)- and O(2)-broadened linewidths of the J = 34(2,32) <-- 34(1,33) transition of ozone located at 500.4 GHz have been measured at three temperatures in the 247-295 K range by using a video-type spectrometer. Besides the Voigt profile commonly used as theoretical model for millimeter-wave investigations, different other theoretical lineshapes, the speed-dependent Voigt profile, the Galatry profile, and the speed-dependent Galatry profile, have been considered to analyze the experimental spectra, to retrieve the pressure-broadening parameters, and to give an account of the pointed out line-narrowing effect. The results for O(3) perturbed by N(2) and O(2) are finally compared to calculations based on the complex semiclassical theory of Robert and Bonamy, and with previous reported values involving the same kind of transitions. Finally, it is shown that observed line narrowings result nearly exclusively from the dependence of relaxation on molecular speeds. Copyright 2000 Academic Press.