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We present results of extensive computer simulations and theoretical analysisof vibrational phase relaxation of a nitrogen molecule along the criticalisochore and also along the gas-liquid coexistence. The simulation includes allthe different contributions [atom-atom (AA), vibration-rotation (VR) andresonant transfer] and their cross-correlations. Following Everitt and Skinner,we have included the vibrational coordinate ($q$) dependence of the interatomicpotential. It is found that the latter makes an important contribution. Theprincipal important results are: (a) a crossover from a Lorentzian-type to aGaussian line shape is observed as the critical point is approached along theisochore (from above), (b) the root mean square frequency fluctuation showsnonmonotonic dependence on the temperature along critical isochore, (c) alongthe coexistence line and the critical isochore the temperature dependentlinewidth shows a divergence-like $\lambda$-shape behavior, and (d) the valueof the critical exponents along the coexistence and along the isochore areobtained by fitting. The origin of the anomalous temperature dependence oflinewidth can be traced to simultaneous occurrence of several factors, (i) theenhancement 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 probedon a femtosecond time scale. A mode coupling theory (MCT) analysis shows theslow decay of the enhanced density fluctuations near critical point. The MCTanalysis demonstrates that the large enhancement of VR coupling near CP arisesfrom the non-Gaussian behavior of density fluctuation and this enters through anonzero value of the triplet direct correlation function.Comment: 35 pages, 15 figures, revtex4 (preprint form

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