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WIREs Comput Mol Sci
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State‐to‐state photodissociation dynamics of the water molecule

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Photodissociation provides an ideal proving ground for an in‐depth understanding of the microscopic mechanism and dynamics of bond breaking processes at a state‐to‐state level. After a brief outline of the requisite theory, we review the latest developments on the state‐to‐state photodissociation dynamics of the water molecule via the lowest two excited states, focusing on the absorption spectrum and product state distributions. A detailed discussion is given on the competition between different adiabatic and nonadiabatic pathways of the dissociation. Quantum mechanical studies of this prototypical system on accurate coupled potential energy surfaces not only offer an interpretation of the existing experimental results but also provide a clear and comprehensive dynamical picture of water photodissociation.

The measured absorption spectra in room temperature for H2O and the one‐dimensional schematic potential energy curves as a function of the dissociative OH bond with the other OH bond and the HOH angle kept at their equilibrium values of the ground state.
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Comparison of experimental and calculated differential cross sections at the five peak energies. Adapted from Ref with permission.
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Rotational state distributions of the OH (X˜, v = 0) fragment at the five peak energies. The populations from the two nonadiabatic pathways via the CI and via the RT coupling are given in gray and dark cyan bars. Theoretical distributions with RT coupling (red‐dashed lines), without RT coupling (blue‐dotted lines) and the experimental distributions (black solid lines) are included. Adapted from Ref with permission. Copyright (2013) American Chemical Society.
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Calculated total photodissociation cross sections from the 000 (red‐dashed line) and 101 (green‐dashed line) states, compared with the results of 000 state without the RT coupling (blue‐dotted line), the calculated results based on DK PESs (pink dash‐dotted line), and the experimental spectrum (black solid line). Adapted from Ref with permission. Copyright (2013) American Chemical Society.
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Population ratios of the two spin–orbit components (Π3/2+ and Π1/2+) of the OH product in its low‐lying vibrational channels upon the photodissociation of H2O(X˜, (0, 0, 0)) obtained from several theoretical and experimental studies. Adapted from Ref with permission.
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The relative vibrational state distributions of the OH(X˜) product obtained from several theoretical and experimental studies. Adapted from Ref with permission.
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Diabatic PESs for the diagonal ( V Σ, V Π) and off‐diagonal ( V ΣΠ) terms, as well as the 1A PES, as a function of ROH1 and the HOH angle, with the other OH bond fixed at 1.8a0. The contour intervals are 0.5 eV for all panels. Adapted from Ref with permission.
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The X˜1A, A˜1A, and B˜1A adiabatic PESs for H2O plotted in one O─H bond and H─O─H angle, with the other O─H bond fixed at the equilibrium distance. There are two CIs between the X˜ and B˜ states, one with HOH and another with HHO linear configurations. The three states form degenerate RT pairs at linearity. Adapted from Ref with permission. Copyright (2013) American Chemical Society.
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