Departures from the soft collision model for Dicke narrowing: Raman measurements in the Q branch of D2

Abstract

With high quality spectral data, we have observed departures from the soft collision model for translational motion, in the transition region from Doppler broadening to Dicke narrowing. The departures are in agreement with theoretical calculations based on the Boltzmann equation. The implications of the results concerning the dynamics of fluids are discussed. In addition we show that the mass diffusion constant describes the translational diffusion of the optical coherence and we give precise measurements of the broadening coefficients of the Q(O) to Q( 6) lines.

Testing the validity of the optical diffusion coefficient: Line-shape measurements of CO perturbed byN2

“…Our conclusions are unaltered, which is not surprising since the two models for y(k, t) are indistinguishable on our time scales. On the other hand, the nonlinear pressure or density dependence of the narrowing parameter is not seen in the recent Raman work of Forsman et al [4] in Dz. Thus there are conAicting experimental results concerning the need to introduce the concept of an optical diffusion constant.…”

“…Like Forsman et al [4] and many others, we have used as the reference or "standard" model for the spectral time correlation function, C (t), a form given by C (t)=y"(k, t)@ (t), where y"(k, t) is the self-part of the intermediate scattering function for the soft-collision model [5]. Using the notation of Varghese and Hanson…”

“…One can think of two extreme cases: If the decay of C&(t) is slow relative to the decay of g(k, t), it forms a window in time over which the temporal evolution of y(k, t) may be examined. Such an examination has recently been carried out by Forsman et al [4] using the Raman spectrum of D2. Small but predicted departures from the soft-collision model for y(k, t) [5] were measured.…”

Line mixing and state-to-state rotational relaxation rates inD2determined from the RamanQbranch

“…From fits to experimental profiles we should be able, in principle, to extract information about broadening, shifting, line mixing, and the translational motion or Dicke narrowing [27,28]. Except for the very lowest densities, Dicke narrowing makes a negligible contribution to the width of the lines and need not concern us in this paper; we corrected for it using the soft-collision model [28].…”

Line Broadening and the Temperature Exponent of the Fundamental Band in CO–N2 Mixtures

“…Both the hard-and soft-collision models have the correct high-and low-density limit. Furthermore, it is known from numerical solutions to the Boltzmann equation, how well these models describe translational motion for realistic intermolecular interactions (see (38) and references therein).…”