Speed-dependent effects in the near-infrared spectrum of self-colliding H218O molecules

The absorption line shape of a given (H2O)-O-18 vibration-rotation line at 1.38 mu m was deeply investigated using a spectroscopic approach based upon the use of a pair of offset-frequency locked extended-cavity diode lasers. This dual-laser apparatus ensures extreme levels of accuracy in controlling and measuring any variation of the laser frequency around a given absolute reference. As a result, high levels of precision and accuracy were reached in the observation of a molecular absorption line shape in the near infrared. A variety of semiclassical models, accounting for Dicke narrowing and speed-dependent effects, were implemented and tested in order to describe the physical situation of self-colliding (H2O)-O-18 molecules. Our study demonstrates that the molecular confinement alone is unable to explain entirely the departures from the Voigt profile and that the speed dependence of pressure-induced broadening and shift cannot be ignored, even in the case of pure water samples at relatively small pressures. The absorption spectrum was successfully interpolated using the uncorrelated version of the speed-dependent Galatry profile, with a hypergeometric dependence on the absorber speed for both pressure broadening and pressure shift parameters, thus reaching an agreement between theory and experiment at the level of 5 x 10(-5).