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Excited‐state dynamics

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Excited‐state dynamics is the field of theoretical and physical chemistry devoted to simulating molecular processes induced upon UV‐visible light absorption. This involves nuclear dynamics methods to determine the time evolution of the molecular geometry used in concert with electronic structure methods capable of computing electronic excited‐state potential energy surfaces. Applications concern photochemistry (see Chapter CMS‐030: Computational photochemistry) and electronic spectroscopy. Most of the work in this field looks at unsaturated organic molecules as these provide widely used chromophores with a straightforward photochemistry that can be described by a small number (usually two) of electronic states. The electronic ground state of closed‐shell organic molecules is a singlet (electronic spin zero) termed S0. Molecules are promoted to their electronic excited states through absorption of UV‐visible light (200–700 nm), usually to the first or second singlet, S1 or S2. Typical examples are well represented as a one‐electron transition from the π or n highest occupied molecular orbital to a π* or σ* low‐lying unoccupied molecular orbital. The photo‐excited system will deactivate and return to the electronic ground state over a timescale that can be as short as about 100 fs for ultrafast mechanisms. For example, the initial event of vision is a photo‐isomerization of the retinal chromophore in the rhodopsine protein that occurs in ca. 200 fs.1,2 The goal of a computational approach to the simulation of photo‐induced processes is the complete description of what happens at the molecular level from the promotion to the excited electronic state to the formation of products or regeneration of reactants back in the electronic ground state. © John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 460‐475 DOI: 10.1002/wcms.26

Figure 1.

Jablonski diagram (A: absorption; F: fluorescence; P: phosphorescence; IVR: intramolecular vibrational redistribution; IC: internal conversion; ISC: intersystem crossing). The numbers from 0 to 10 indicate the quantum number of the photoactive vibrational mode in each of the three electronic states involved.

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Figure 2.

One‐dimensional cartoon of nonadiabatic and adiabatic reaction paths involving two potential energy surfaces (R: reactant; TS: transition state; P: product; CoIn: conical intersection).

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Figure 3.

Three representations of a photochemical mechanism: the upper‐state and lower‐state branches of the minimum energy path (white lines), a beam of surface‐hopping trajectories (red and blue lines), and the propagation of a wavepacket.

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Figure 4.

Three successive snapshots of a wavepacket expanded in a basis set of three Gaussian functions. The black arrowed curves represent the quantum trajectories followed by the centers of the three basis functions in the space of the nuclear coordinates.

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Figure 5.

Pyrazine: calculated absorption spectrum (solid line) compared to the experimental spectrum (from Ref 54).

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Figure 6.

Formaldehyde: nonadiabatic reaction channel for the molecular photodissociation (from Refs 120, 136).

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Figure 7.

Difference between peaked and sloped conical intersections. The former favors reactant regeneration (photostability); the latter, product formation (photochemistry).

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Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics

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