Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions

Two-dimensional infrared spectroscopy from the gas to liquid phase: density dependent J-scrambling, vibrational relaxation, and the onset of liquid character

Ultrafast 2DIR spectra and pump-probe responses of the N₂O ν₃ asymmetric stretch in SF₆ as a function of density from the gas to supercritical phase and liquid are reported. 2DIR spectra unequivocally reveal free rotor character at all densities studied in the gas and supercritical region. Analysis of the 2DIR spectra determines that J-scrambling or rotational relaxation in N₂O is highly efficient, occurring in ∼1.5 to ∼2 collisions with SF₆ at all non-liquid densities. In contrast, N₂O ν₃ vibrational energy relaxation requires ∼15 collisions, and complete vibrational equilibrium occurs on the ∼ns scale at all densities. An independent binary collision model is sufficient to describe these supercritical state point dynamics. The N₂O ν₃ in liquid SF₆ 2DIR spectrum shows no evidence of free rotor character or spectral diffusion. Using these 2DIR results, hindered rotor or liquid-like character is found in gas and all supercritical solutions for SF₆ densities ≥ρ* = 0.3, and increases with SF₆ density. 2DIR spectral analysis offers direct time domain evidence of critical slowing for SF₆ solutions closest to the critical point density. Applications of 2DIR to other high density and supercritical solution dynamics and descriptions are discussed.

Dynamical characterization of rotationally hindered species in liquids

The rotational dynamics of HCl in liquid Ar has been studied by means of molecular-dynamics simulations. We calculate the lifetimes of weakly bound HCl-Ar dimers induced by the anisotropic pair interaction. It is shown that, although lifetimes are small with respect to the reorientational decorrelation, the time interval between the breaking down and formation of the next dimer is negligibly small. Thus, with respect to the rotational dynamics of the probe, the effect is similar to that and eventually would cause a time-stable complex. This provokes a peculiar hindered rotation of the diatomic in the liquid which is macroscopically embodied in the infrared spectrum of the solution as a Q-branch nonexistent otherwise.