Charge-transfer dynamics, nonadiabatic

In this section, we switch gears slightly to address another contemporary topic, solvation dynamics coupled into the ESPT reaction. One relevant, important issue of current interest is the ESPT coupled excited-state charge transfer (ESCT) reaction. Seminal theoretical approaches applied by Hynes and coworkers revealed the key features, with descriptions of dynamics and electronic structures of non-adiabatic [119, 120] and adiabatic [121-123] proton transfer reactions. The most recent theoretical advancement has incorporated both solvent reorganization and proton tunneling and made the framework similar to electron transfer reaction, [119-126] such that the proton transfer rate kpt can be categorized into two regimes (a) For nonadiabatic limit [120] ... 

The dynamical processes can be investigated in two different ways the adiabatic processes, where the system remains in the electronic ground state and the nonadiabatic processes where electronic excitation, ionization or charge transfer occur. Only the ways to study adiabatic phenoma will be described here. 

Similar works were performed for the description of the photo-physics of formamide in an Ar matrix [855], the nonadiabatic deactivation of azomethane in gas phase, water and -hexane [856], the cis-trans isomerization of iV-methyl-acetamide in water [516] and the ultrafast nonadiabatic dynamics of Nat in a water cluster [857]. By comparing to an older work of Koch et al. [857] the latter study allows an insight into the importance of polarizable force fields for the description of charge-transfer (CT) states. Solvent effects on the vertical spectra of small carbonyl compounds were computed by Malaspina et al. [858], Nielsen et al. [859] and Lin and Gao [860]. Using CASSCF approaches in combination with the solvent model based on the polarizable NEMO force field [861], Hermida-Ramon et al. studied the influence of water as a solvent on the balance between zwitterionic and biradical valence structures of methylene peroxide [862]. 

Although the dynamical problem of the reacting molecular system is of the same complexity as that encountered in gas-phase reaction dynamics, the presence of a surface adds additional processes and phenomena. Such phenomena are, for instance, not only the importance of the structure, including corrugation, steps, and surface anomalities, but also the interaction with the possible excitation processes in the solid, such as phonon (surface vibrations) and electronic excitations. Also for charge transfer and other nonadiabatic electronic processes in the gas phase, the importance of the surface temperature adds additional features to the problem. Aside from this, the various processes of interest occur on different time scales, from fast reactive chemisorption processes on the sub-pico second time scale to the relatively slow diffusion and desorption processes. Thus different theoretical tools are needed in order to describe the variety of processes and the large time span one needs to cover. Also the many-body problem of the solid combined with the few-body gas-phase problem makes it necessary to introduce different methods for treating the dynamics, from classical trajectories and... 

Figure 6.10 Ultrafast efficient switching in the five-state system via SPODS based on multipulse sequences from sinusoidal phase modulation (PL). The shaped laser pulse shown in (a) results from complete forward design of the control field. Frame (b) shows die induced bare state population dynamics. After preparation of the resonant subsystem in a state of maximum electronic coherence by the pre-pulse, the optical phase jump of = —7r/2 shifts die main pulse in-phase with the induced charge oscillation. Therefore, the interaction energy is minimized, resulting in the selective population of the lower dressed state /), as seen in the dressed state population dynamics in (d) around t = —50 fs. Due to the efficient energy splitting of the dressed states, induced in the resonant subsystem by the main pulse, the lower dressed state is shifted into resonance widi die lower target state 3) (see frame (c) around t = 0). As a result, 100% of the population is transferred nonadiabatically to this particular target state, which is selectively populated by the end of the pulse.

This chapter describes a general theoretical formulation for PCET and summarizes the results of applications to a wide range of different types of PCET reactions in solution and enzymes. This theoretical formulation treats the active electrons and transferring proton(s) quantum mechanically and includes the interactions among the electrons, proton(s), and solvent or protein environment. Moreover, this formulation allows the inclusion of the proton donor-acceptor motion, explicit molecular solvent and protein, and dynamical effects. The theory described here is directly applicable to nonadiabatic PCET reactions accompanied by substantial solute charge redistribution and solvent reorganization. 

---