Simulation and theory of vibrational phase relaxation in the critical and supercritical nitrogen: Origin of observed anomalies

TL;DR

This study combines simulations and theory to analyze vibrational phase relaxation in nitrogen near the critical point, revealing anomalies like line shape crossover, nonmonotonic fluctuations, and divergence-like linewidth behavior.

Contribution

It provides a comprehensive simulation and theoretical analysis of vibrational relaxation in nitrogen near critical conditions, including all contributions and their cross-correlations, highlighting the role of density fluctuations.

Findings

Line shape crossover from Lorentzian to Gaussian near critical point

Nonmonotonic temperature dependence of frequency fluctuations

Divergence-like behavior of linewidth at critical conditions

Abstract

We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions [atom-atom (AA), vibration-rotation (VR) and resonant transfer] and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate ($q$) dependence of the interatomic potential. It is found that the latter makes an important contribution. The principal important results are: (a) a crossover from a Lorentzian-type to a Gaussian line shape is observed as the critical point is approached along the isochore (from above), (b) the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, (c) along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like $\lambda$-shape behavior, and (d) the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, (i) the enhancement of negative cross-correlations between AA and VR contributions and (ii) the large density fluctuations as the critical point (CP) is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. A mode coupling theory (MCT) analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function.