Speed dependence of collision parameters in the H2O18 near-IR spectrum: Experimental test of the quadratic approximation

A laser absorption technique, based upon a pair of offset-frequency-locked extended-cavity diode lasers, was employed to perform line-shape measurements for H218O vibration-rotation transitions at 1.38 μm, with an extremely high spectral fidelity. This important feature was mostly ensured by the highly accurate, absolute, and repeatable frequency axis characterizing each acquired spectrum. We present the outcomes of a specific study of the speed dependence of collision parameters influencing the shape of the H218O absorption lines. More particularly, the performance of the quadratic approximation, either used alone or considered in conjunction with the Dicke narrowing effect, was investigated and compared to that of the confluent hypergeometric dependence. Hence, experimental evidences of the partial failure of the quadratic approximation are given and discussed. Finally, we investigate the possible influence of the choice of the speed-dependent line-shape model on the retrieved collisional parameters.