Photodissociation dynamics coordinates

Study the dissociation dynamics of such a system, the development of simple models can best be accomplished using semiclassical or classical techniques. In Section IV C a curve-hopping model is developed, based on a collisional reorientation of the electronic angular momentum. It assumes that a bath atom collides with just one of the diatoms and reorients its electronic angular momentum on a time scale that is short compared to the relative motion of the diatoms. The model is applied to iodine photodissociation dynamics in Section IV D. The dissociation dynamics of polyatomic systems with their internal degrees of freedom is more complex than for diatomics. If these degrees of freedom are not thermally equilibrated and are coupled to the dissociation coordinate, then their dynamics cannot simply be projected out, but rather they can act as an indirect source of excitation of the dissociation coordinate. 

The effect of conical intersections on the state-specific and state-to-state reactive and nonreactive scattering attributes was demonstrated with the aid of an extended two coordinate quasi Jahn-Teller (JT) model. In recent years, the photodissociation dynamics of triatomic molecules, for example O3 and H2S, have been studied by calculating the diabatic electronic states and their couplings employing an ah initio approach. The reactive scattering dynamics of insertion reactions, for example, C - - H2, ... 

Figure 3, Wavepacket dynamics of the photodissociation of NOCl, shown as snapshots of the density (wavepacket amplitude squared) at various times, The coordinates, in au, are described in Figure b, and the wavepacket is initially the ground-state vibronic wave function vertically excited onto the 5i state. Increasing corresponds to chlorine dissociation. The density has been integrated over the angular coordinate. The 5i PES is ploted for the geometry, 9 = 127, the ground-state equilibrium value,...

The surface hopping study was rather expensive in terms of CPU time, and consequently large numbers of trajectories could not be run. This is important to obtain statistically converged dynamical properties. The main goal of the surface hopping study was thus not to obtain such information but to provide mechanistic insight into the photodissociation and subsequent relaxation processes. The semi-classical work in the full space of nuclear coordinates provides the important vibrational degrees of freedom that one needs to include in any quantum model of the nuclear motion. This will now be described. 

Although, relevant information about ferrous hemeproteins kinetics, dynamics and ligand photodissociation pathways has been obtain, less is known about ferric hemeproteins photophysic processes. Recent studies performed with Hbl-CN and Mb-CN at ultrafast time scale, have suggested that some of the transients intermediaries observed after ferrous complexes ligand photodissociation are observed in ferric Mb and Hbl [7], However, time-resolved infrared data shows that the complex remained six coordinated after photoexcitation. In this work we present ultrafast data on ferric Hbl-NO, HM-N3, HM-H2S and metHbl complexes that suggest a mechanism for the photoinduced reduction of Hbl species. 

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Absorption and photodissociation cross sections are calculated within the classical approach by running swarms of individual trajectories on the excited-state PES. Each trajectory contributes to the cross section with a particular weight PM (to) which represents the distribution of all coordinates and all momenta before the vertical transition from the ground to the excited electronic state. P (to) should be a state-specific, quantum mechanical distribution function which reflects, as closely as possible, the initial quantum state (indicated by the superscript i) of the parent molecule before the electronic excitation. The theory pursued in this chapter is actually a hybrid of quantum and classical mechanics the parent molecule in the electronic ground state is treated quantum mechanically while the dynamics in the dissociative state is described by classical mechanics. 

The photodissociation of methyl nitrite in the first absorption band, CH30N0(Si) — CH3O + NO(n, j), exemplifies indirect photodissociation (Hennig et al. 1987). Figure 1.11 shows the two-dimensional potential energy surface (PES) of the S electronic state as a function of the two O-N bonds. All other coordinates are frozen at the equilibrium values in the electronic ground state. Although these two modes suffice to illustrate the overall dissociation dynamics, a more realistic picture... 

Fig. 10.7. The bond coordinates used to describe the photodissociation of H2O2.

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