Photoexcitation, molecular dynamics

To describe the electronic relaxation dynamics of a photoexcited molecular system, it is instructive to consider the time-dependent population of an electronic state, which can be defined in a diabatic or the adiabatic representation [163]. The population probability of the diabatic electronic state /jt) is defined as the expectation value of the diabatic projector... 

Time-resolved fluorescence from sub-picosecond to the nanosecond time-scale of dye molecules like coumarins has been widely used to study solvation dynamics in liquids [1], As the dye is photoexcited, its dipole moment abruptly changes. Then by monitoring the time-dependent fluorescence energy one can have access to the solvent dynamical response to the charge reorganization in the dye. The microscopic interpretation of these experiments has greatly benefited from Molecular Dynamics (MD) studies [2], Recently, few experimental [3-5] and theoretical [6,7] works have been performed on solvation dynamics in liquid mixtures. A number of new phenomena can arise in mixtures which are not present in pure solvents, like association, mutual diffusion and preferential solvation [6]. We present here a... 

In the present chapter, we will focus on the simulation of the dynamics of photoexcited nucleobases, in particular on the investigation of radiationless decay dynamics and the determination of associated characteristic time constants. We use a nonadiabatic extension of ab initio molecular dynamics (AIMD) [15, 18, 21, 22] which is formulated entirely within the framework of density functional theory. This approach couples the restricted open-shell Kohn-Sham (ROKS) [26-28] first singlet excited state, Su to the Kohn-Sham ground state, S0, by means of the surface hopping method [15, 18, 94-97], The current implementation employs a plane-wave basis set in combination with periodic boundary conditions and is therefore ideally suited to condensed phase applications. Hence, in addition to gas phase reference simulations, we will also present nonadiabatic AIMD (na-AIMD) simulations of nucleobases and base pairs in aqueous solution. 

Laser-induced reaction has been widely used to stimulate gas-surface interaction. Lasers are also used to probe molecular dynamics in heterogeneous systems as well. In the applied area, the laser photochemical techniques are successfully applied to produce well defined microstructures and new materials for microelectronic devices (1). Enhanced adsorption and chemical reaction on surfaces can be achieved by a photoexcitation of gaseous molecules, adsorbed species as well as solid substrates. The modes of the excitation include vibrational and electronic states of the gaseous species and of the adsorbates surface complexes. Both a single and a multiple photon absorption may be involved in the excitation process. 

A systematic study of energy disposal in the photodissociation of the cyanogen halides has utilised the intense fluorescence from CN(fi) as a probe of the molecular dynamics and hence the topography of the photoexcited molecular potential energy surface Photodissociation at a number of wavelengths in thea-continuumgen ... 

Yamato, T., N. Niimura, et al. (1998). Molecular dynamics study of femtosecond events in photoactive yellow protein after photoexcitation of the chromophore. Proteins 32(3) 268-275. 

Excess Heme Kinetic Energy Relaxation Time Constants Following Photoexcitation of Cytochrome c Simulated Using Classical Molecular Dynamics at 300K... 

When 7AI-water clusters are excited into the Si band origin, ESPT occurs slow [16, 17, 32]. For 7AI(H20)3, for instance, while the excited-state Ufetime (an upper bound to the ESPT time) is more than 10 ns at Oq, it is reduced to only 15 ps upon an energy excess of 300 cm [43]. Ab initio molecular dynamics (AIMD) simulations on 7AI(H20)i,2 reported by Kina et al. [44] showed that the excited-state transfer occurs about 50 fs after photoexcitation for 12,000 cm energy excess. 

In the first pump probe experiments with picosecond time resolution the real-time spectra of the Nas E) state (Fig. 4.8) reveal a fast exponential decay caused by ultrafast photo-induced dissociation [306]. This behavior could be well explained with the simple fragmentation model described in Sect. 2.2.2. With femtosecond time resolution it was expected to observe the photodissociation process with more detail in the recorded transients. Especially, the energy dependence of the ultrashort fragmentation process should allow deeper insight into the fragmentation dynamics within a photoexcited molecular beam. 

Nieber, H., 8c Doltsinis, N. L. (2008). Elucidating ultrafast nonradiative decay of photoexcited uracil in aqueous solution byab initio molecular dynamics. Chemical Physics, 347(1-3), 405-412. doi 10.1016/j.chemphys.2007.09.056. 

Recently, an extensive study on the photoexcitation and isomerization of PYP was published. In this study, a combination of molecular dynamics simulation techniques and time-dependent density functional theory calculations were used. Several interesting results were obtained. Out of five separate simulations that were performed of the excited state, two showed a twisted configuration of the chromophore, and in three, the chromophore retained its trans configuration. This could explain the two different excited states observed in the ultrafast absorption measurement (see above). 

Once the mechanisms of dynamic processes are understood, it becomes possible to think about controlling them so that we can make desirable processes to occur more efficiently. Especially when we use a laser field, nonadiabatic transitions are induced among the so-called dressed states and we can control the transitions among them by appropriately designing the laser parameters [33 1]. The dressed states mean molecular potential energy curves shifted up or down by the amount of photon energy. Even the ordinary type of photoexcitation can be... 

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