Collision-induced light scattering by compressed CF4and CF4-He mixtures

Abstract

We have observed the collision-induced scattering from compressed C${\mathrm{F}}_4$ up to 70 atm and from C${\mathrm{F}}_4$-He mixtures at concentrations of 27%, 47%, and 48% C${\mathrm{F}}_4$, up to a total pressure of 155 atm. To describe the intensity distribution for pure C${\mathrm{F}}_4$, a single exponential function, $I=C$ $\exp (-\frac{\omega }{{\omega }_0})$, was fitted in the frequency range 5-20 ${\mathrm{cm}}^{-1\mathrm{}}$; the density dependence of ${\omega }_0$ follows the scaling law derived by Fleury for rare-gas spectra. An expression involving the sum of two exponentials was fitted over a broader frequency range 5-45 ${\mathrm{cm}}^{-1\mathrm{}}$. It is argued that the expected intensity due to the second-order dipole-induced dipole interaction or to octupole effects is too small to account for the high-frequency tail. The shape of this tail indicates that it may arise from close encounters in which the collision time is determined principally by the intermolecular potential. The values of ${\omega }_0$ determined from the spectra of the mixtures do not differ significantly from those obtained from pure C${\mathrm{F}}_4$. As the total pressure of the mixtures was varied, there were no sudden changes in the scattering analogous to the sharp increase of dielectric loss observed in the microwave absorption spectra and associated with the formation of dimers. In summary, the characteristics of the scattering closely resemble that from the rare gases.