Anomalous Behavior of Isotropic Raman Line Shapes near Gas-Liquid Critical Points

Temperature dependencies of Raman linewidths $\mathrm{FWHM}\left(T\right)$ of the two polarized modes in ${\mathrm{N}}_2\mathrm{O}$ have been determined along the coexistence line with mK resolution. Comparing the neat fluid with diluted states of ${\mathrm{N}}_2\mathrm{O}$ in Xe (and ${\mathrm{CO}}_2$), quite opposite behaviors of $\mathrm{FWHM}\left(T\right)$ have been observed when the liquids approach their respective critical temperatures $T_c$: While the mixture $({\mathrm{N}}_2\mathrm{O}{)}_{0.05}{-\mathrm{X}\mathrm{e}}_{0.95}$ shows the well known $\lambda$-shaped effect of critical broadening, a unique and sharp density-correlated line narrowing has been observed in the neat state. The gradual switchoff of intermolecular resonance couplings with decreasing density is responsible for this narrowing phenomenon.