Nonmonotonic composition dependence of vibrational phase relaxation rate in binary mixtures

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time $\tau_c$ at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation $[ \langle \Delta \omega^2(0) \rangle ]$ with $\tau_c$ indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition \chi A dependence of the dephasing time $\tau_v$ is found to be primarily due to the nonmonotonic \chi dependence of $\tau_c$, rather than due to a similar dependence in the amplitude of $\langle \Delta \omega^2(0) \rangle$ . The probability distribution of \Delta \omega shows a markedly non-Gaussian behavior at intermediate composition ( \chi A \simeq 0.5). We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity $\eta^*$ with that of $\tau_v$ and $\tau_c$. It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture