A velocity-memory model for the spectral lineshape from the Doppler to thecollision regime

The radiator speed-memory has been recently shown to have important consequ ences for spectral lineshapes in the collision regime. This speed-memory co ntrols the speed changing collisional mechanism, which becomes crucial when inhomogeneities arise from the speed dependence of the line broadening and /or shifting parameters. The extension of such a memory approach to lower d ensities requires the introduction of a pertinent velocity-memory function (instead of that based on speed) in order to also account for the confineme nt narrowing, A simple velocity-memory model which describes separately the correlation processes for the velocity-modulus (i.e, the speed) and for th e velocity-orientation is proposed, The resulting analytical line profile i s derived within the impact kinetic equation. The evolution of the resultin g lineshape vs, the density is studied from the Doppler to the collision re gime for the H-2-Ar prototype system, The specific roles of the velocity-or ientation memory and speed memory processes are analyzed in a wide range of density, for which both confinement narrowing and speed dependence of rela xation parameters influence the spectral lineshape. (C) 2000 Elsevier Scien ce B.V. All rights reserved.