title: Density effect on infrared spectrum for supercritical water in the low- and medium-density region studied by molecular dynamics simulation.
abstract: The origin of the line shape of the O-H stretch vibrational spectrum is analyzed for supercritical water in the low- and medium-density region by using classical molecular dynamics simulation for the flexible point-charge model, SPC/Fw. The spectrum calculated for the water model is in good agreement with the experimental one in the low-density region. The spectral origins in the low-density region of 0.01-0.04 g cm(-3) are assigned to a sharp peak due to the bond oscillation along the O-H vector and two broad bands due to the rotational coupling, by taking an isolated single molecule as a reference in the low-density limit. The bands due to the rotational coupling reduce in intensity with increasing density as the rotations are more hindered by the hydrogen-bonding interactions, and their intensities increase with increasing temperature due to the accelerated rotational motion. The O-H stretch oscillation in the time correlation function attenuates in a timescale comparable with the lifetime of the hydrogen bonds, and the spectra conditioned by the number of hydrogen bonds are dominantly controlled by the local solvation structure.
authors.affiliation: Department of Chemical Science and Technology, Faculty of Engineering, University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima 770-8506, Japan. yoshida@chem.tokushima-u.ac.jp
authors.forename: Ken
authors.initials: K
authors.lastname: Yoshida
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Matubayasi
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Uosaki
authors.forename: Masaru
authors.initials: M
authors.lastname: Nakahara