Energetics of the vibrational manifold of the aqueous azide ion constructed from experimental infrared and Raman spectroscopic data. According to Morita and Kato (Ref. 4), the vibrational eigenstates, νi, can be constructed from a harmonic oscillator basis, |ijk〉, under consideration of anharmonic coupling in the point group D∞h. The vibrational state initially prepared by the femtosecond pump pulse has predominant asymmetric stretching character, |001〉, with some additional symmetric stretching character from |101〉, which is mixed in via intramolecular anharmonicity. Energetically nearby states that might act as intermediate state during VER via pathways b and c are dictated by Fermi resonances between |100〉 and |020〉 or between |110〉 and |030〉. In contrast, the bending fundamental, ν2, corresponds to a pure state, |010〉. See Ref. 4 for further details.
Temperature-density phase diagram of liquid water covering the gas, the liquid, and the supercritical phase. The open circles indicate the thermodynamic state points at which femtosecond infrared pump-probe experiments were carried out. In addition, FTIR spectra were also recorded along the 500 bars isobar.
(a) FTIR spectra of aqueous azide solutions for different thermodynamic conditions. (b) Pure solute FTIR difference spectrum obtain by subtracting the independently measured pure solvent contribution. (c) Normalized solute spectra. The arrow indicates the direction of increasing temperature.
Temperature (a) and density (b) dependence of the peak frequency (circles) and the first moment (squares) of the ν3 band of the aqueous azide ion. The horizontal bars represent the two spectral positions at which the ν3 band has decayed to its half maximum. They can therefore be used to read off the full width at half maximum as well as the intrinsic asymmetry of the absorption band as a function of temperature and density.
Pump-induced spectra of the ν3 band of the aqueous azide ion for a few representative thermodynamic conditions (open circles: 333 K and 55.77 mol/l, filled circles: 393 K and 53.68 mol/l, open squares: 483 K and 49.27 mol/l, filled squares: 573 K and 43.14 mol/l, open diamonds: 633 K and 37.42 mol/l, and filled diamonds: 663 K and 33.60 mol/l). The inset emphasizes the temperature dependence of the anharmonic shift between ground-state bleach/stimulated emission and the excited-state absorption.
(a) Pump-induced transient spectra of the ν3 band of the aqueous azide ion at 423 K and 52.37 mol/l for a few representative time delays. (b) Kinetic traces recorded at probe frequencies located near the maximum bleach/emission (2036 cm−1), near the maximal induced absorption (2000 cm−1), and in between absorption and bleach/emission (2014, 2021, and 2028 cm−1). The experimental traces (gray) at 2036 and 2000 cm−1 have been fitted by a double-exponential decay including a convolution with the instrument response (black).
Semi-logarithmic plot of the pump-induced anharmonically shifted absorption as a function of the pump-probe time delay. The experimental data (circles) have been fitted with a biexponential decay including a convolution with the instrument response (solid curves). For clarity, the data at different temperatures have been shifted vertically.
Radial distribution functions for the four different binary solute-solvent interactions in the azide/water system obtained from molecular dynamics simulations at two different temperatures.
Experimental vibrational relaxation rates obtained from the transient absorption decay of the aqueous azide ion as a function of temperature (a) and density (b). The solid and dashed curves correspond to the isolated binary collision model using the terminal nitrogen-oxygen and the terminal nitrogen-hydrogen local density correction.
Experimental vibrational relaxation rates obtained from the transient absorption decay of the aqueous azide ion as a function of temperature (a) and density (b). The solid and dashed curves correspond to the isolated binary collision model using the central nitrogen-oxygen and the central nitrogen-hydrogen local density correction.
Experimental vibrational relaxation rate constants as a function of temperature and density at a common pressure of 500 bars. The peak values of the four radial distribution functions, i.e., the maximal value of g(r) for the four different solute-solvent contacts are also listed.
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