Vibrational relaxation of azide ions in liquid-to-supercritical water

Olschewski, Martin; Knop, Stephan; Lindner, Jörg; Vöhringer, Peter

doi:doi:10.1063/1.3598108

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The role of attractive forces in viscous liquids

We present evidence from computer simulation that the slowdown of relaxation of a standard Lennard-Jones glass-forming liquid and that of its reduction to a model with truncated pair potentials without attractive tails are quantitatively and qualitatively different in the viscous regime. The pair structure of the two models is however very similar. This finding, which appears to contradict the common view that the physics of dense liquids is dominated by the steep repulsive forces between atoms,...

Singlet state relaxation via intermolecular dipolar coupling

The intermolecular contribution to the relaxation of singlet states has been derived on the basis of a translational-rotational diffusion model that describes molecules as impenetrable spheres which translate and rotate in an isotropic low-viscosity medium. The equations for the relaxation rate constants obtained are discussed and the dependence on physical parameters is exploited. Theoretical predictions are compared with experiments when the intermolecular relaxation is due to both protons and...

The dynamics of vibrational energy relaxation (VER) of the aqueous azide anion was studied over a wide temperature (300 K ≤ T ≤ 663 K) and density (0.6 g cm⁻³ ≤ ρ ≤ 1.0 g cm⁻³) range thereby covering the liquid and the supercritical phase of the water solvent. Femtosecond mid-infrared spectroscopy on the ν₃ band associated with the asymmetric stretching vibration of the azide anion was used to monitor the relaxation dynamics in a time-resolved fashion. The variation of the vibrational relaxation rate constant with temperature and density was found to be rather small. Surprisingly, the simple isolated binary collision model is able to fully reproduce the experimentally observed temperature and density dependence of the relaxation rate provided a local density correction around the vibrationally excited solute based on classical molecular dynamics simulations is used. The simulations further suggest that head-on collisions of the solvent with the terminal nitrogen atoms rather than side-on collisions with the central nitrogen atom of the azide govern the vibrational energy relaxation of this system. Finally, the importance of hydrogen bonding for the VER dynamics in this system is briefly discussed.

Article Outline

INTRODUCTION
EXPERIMENTAL AND COMPUTATIONAL DETAILS
RESULTS AND DISCUSSION

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KEYWORDS and PACS

Keywords

high-speed optical techniques, hydrogen bonds, infrared spectra, molecular dynamics method, organic compounds, time resolved spectra, vibrational modes

PACS

78.47.D-

Time resolved spectroscopy (>1 psec)
78.30.Jw

Organic compounds, polymers

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3598108

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0021-9606 (print)

1089-7690 (online)

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American Institute of Physics

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