Computer Simulation Study of the Density and Temperature Dependence of Fundamental and Overtone Vibrational Dephasing in Nitrogen: Interplay between Different Mechanisms of Dephasing

Abstract

Vibrational phase relaxation of the fundamental and the overtones of the N-N stretch of nitrogen in pure nitrogen has been studied by extensive molecular dynamics simulations. The thermodynamic state points simulated include states near the melting point, the boiling point, and the critical point, a normal liquid away from the transition, and also supercritical nitrogen. The simulations provide the following results. (1) The well-known result of Clouter and Kiefte (J. Chem. Phys. 1977, 66, 1736) on the pronounced insensitivity of vibrational phase relaxation of the fundamental to the change in thermodynamic conditions from the triple point (TP) to beyond the boiling point (BP) is found to originate from a competition between density relaxation and resonant energy transfer terms. The latter becomes increasingly important as the melting point is approached. (2) It is found that the experimentally observed sharp rise in the relaxation rate near the gas-liquid critical point (CP) can be attributed at least partly to the sharp rise in the contribution of vibration-rotation coupling. (3) The sharp rise in the vibration-rotation coupling in turn leads, in an unusual lately, to a substantial subquadratic quantum number dependence of the overtone dephasing rate near the critical point and in supercritical fluids. The quantum number dependence, however, is found to be quadratic in the condensed phase. (4) The quantum number dependence of overtone dephasing is found to be critically dependent on the separation of time scales between the relaxations of the frequency modulation time correlation function and the normal coordinate time correlation function. The two decays must overlap to observe a significant subquadratic quantum number dependence. The latter is found to occur in the supercritical fluid state. (5) In the absence of a clear separation of time scales between the frequency modulation and the normal coordinate time correlation functions and because of the pronounced Gaussian decay of the latter, the Kubo-Oxtoby method of correlating the motionally narrowing limit with a homogeneous line shape becomes ambiguous. (6) An interesting crossover behavior across the liquid-gas phase transition region is observed in the density-temperature dependencies of the overtone dephasing rate. This crossover is most pronounced for higher overtones. (7) Although the simulation results reproduce the experimental data semiquantitatively (within 40% in most cases) and get the trends correct, quantitative agreement has not been achieved. We attribute this to the large number of terms (correlations and cross-correlations) which contribute to dephasing. In addition, the change of pair intermolecular potential with the thermodynamic state may itself play an increasingly important role, particularly in supercritical fluids.