Ultrafast two-dimensional infrared (2DIR) spectroscopy in dense gases and supercritical fluids: Rotational and vibrational energy relaxation, critical slowing, and the onset of liquid character

Ultrafast two-dimensional infrared (2DIR) spectroscopy is used to learn about rotational and vibrational energy relaxation in dense gas and supercritical fluid solutions, special solvation effects near the critical point, and the evolution of liquid phase dynamics as a function of fluid density from that of a gas to the near liquid regime. The echo character of 2DIR results in unique spectral signatures that identify free quantum rotors and solvated liquid-like environments within the same dense fluid sample, and allows evaluation of the adequacy of isolated binary collision (IBC) relaxation models at these high densities. Analysis of the 2DIR spectra of the asymmetric stretching band of N₂O in SF₆ and Xe reveal efficient rotational energy relaxation rates (1.5 – 3 collisions) but much slower vibrational energy relaxation rates. A critical slowing effect is found on the rate of J relaxation and liquid-like solvation is evident in dense gas solutions at gas state points far from the critical region. This methodology provides new molecular-level insights into the nature of the supercritical region and our understanding of small molecule relaxation in dense fluid environments.