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 N 2 O ν 3 asymmetric stretch in SF 6 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 N 2 O is highly efficient, occurring in ∼1.5 to ∼2 collisions with SF 6 at all non-liquid densities. In contrast, N 2 O ν 3 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 N 2 O ν 3 in liquid SF 6 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 SF 6 densities ≥ ρ* = 0.3, and increases with SF 6 density. 2DIR spectral analysis offers direct time domain evidence of critical slowing for SF 6 solutions closest to the critical point density. Applications of 2DIR to other high density and supercritical solution dynamics and descriptions are discussed.