A possible molecular structural indicator of the liquid–glass transition

In studying the VH depolarized Rayleigh light-scattering spectrum of orthoterphenyl we have separated an ‘‘intermediate’’ line with almost temperature-independent halfwidth of 50 GHz from both the narrower rotational line with strongly temperature-dependent halfwidth and the broader base line associated with short-range overlap interactions. The integrated intensity IVH of the intermediate line decreases by a factor of 25 as the temperature decreases over a range for which the viscosity increases by 11 orders of magnitude. It appears that the extrapolated value of (IVH/ρ2)1/2 for this line vanishes as T→T0, where ρ is the density and T0 is the ideal liquid–glass transition temperature established by fitting a Vogel–Fulcher relation to the viscosity. We associate this intermediate line with dipole–induced-dipole interactions; its intensity is then given by an equal-time correlation function involving two-, three-, and four-body interactions. Because dipole interactions are quite long range, this correlation function may be a good probe of local molecular order, and IVH may thus be a good molecular structural (thermodynamic) indicator (order parameter) of the liquid–glass transition, the first such indicator identified.