Ultrafast infrared spectroscopy of nitrous oxide as a probe in salt water and supercritical sulfur hexafluoride

Abstract

Different forms of ultrafast spectroscopy, namely two-dimensional infrared spectroscopy (2DIR) and pump probe spectroscopy, are used to determine spectral diffusion, vibrational energy relaxation (VER), and the dynamics of the solvent environments around probe molecules in various solutions. Supercritical fluids are valuable as potential green solvents, so it is important to explore and understand these dynamical processes in these solvents. Ultrafast pump-probe spectroscopy was used to investigate the VER of nitrous oxide (N2O) dissolved in water with different concentrations of ionic salts. Chloride salts of different cations were used to determine changes in VER and reorientation anisotropy decay (R(t)) of N2O as a function of cation. These rates were found to be consistent with the Hoffmeister series trend of structure makers and structure breakers and showed evidence of cation effects on the water hydrogen bonding network beyond the first solvation shell. Ultrafast two-dimensional infrared (2DIR) spectroscopy of N2O’s asymmetric stretching (√3) mode in sulfur hexafluoride (SF6) gas resulted in rovibrational spectral lineshapes which show complicated behavior compared to 2DIR lineshapes of traditional condensed phase samples. There is an additional antidiagonal features which appears in the spectra of dense state points. These observed spectra indicate that there is a quasi-free rotor population, which is captured by a model including all 36 possible rovibrational density matrix pathways originating from a specific J-level. The decays of these 2DIR spectral features can be attributed to the N2O and SF6 collisions, roughly decaying on the scale of one to two collisions. Further 2DIR spectra were taken of the system in the supercritical regime of SF6. Even at these supercritical state points, the frequency correlation decay is still on the order of one to two collisions and confirm that an independent binary collision model is sufficient for describing rotational relaxation in these SCF solutions. Pump-probe spectroscopic measurements offer N2O √3 vibrational relaxation timescales ~10 times slower than spectral diffusion dynamics attributable to rotational relaxation. The growth of a “Q”-type 1 ← 0 √3 transition feature in the null region of the mid-IR spectrum is observed with increasing fluid density. Analysis based on these 2DIR results indicates this feature is a sign of liquid-like character in these solutions and that both hindered and quasi-free rotors exist in the same solvent environment in the dense and supercritical N2O – SF6 solution. This experimental methodology enables future lineshape studies on the effects of solvent interactions on vibrational and rotational relaxation rates, dynamics from non-equilibrium states at high temperature and pressures, and critical point solvation dynamics