Ultrafast photochemical processes

Table 2. List of ultrafast photochemical processes whose excited-state path is characterized by a small or negligible barrier.

We expect the effective-mode models described here to be versatile tools that can predict general trends, and that can be used in conjunction with microscopic information provided from other sources, i.e., spectral densities, energy gap correlation functions, and possibly cross-correlation functions. Further, model parametrizations could be provided by QM/MM type simulations, and the model-based dynamics could be employed to analyse the wealth of microscopic information provided by such simulations. Such complementary strategies would bridge the gap between system-bath theory approaches and explicit multi-dimensional simulations for ultrafast photochemical processes in various types of environments. 

Nese C, Unterreiner AN (2010) Photochemical processes in ionic hquids on ultrafast timescales. Phys Chem Chem Phys 12 1698-1708... 

In Scheme 8, we show the structures of the path for a standard photochemical reaction [Scheme 8(a)] and for an ultrafast photochemical reaction [Scheme 8(b)]. The fact that these processes occur on an ultrafast time... 

It should be stressed that the wave-packet picture of photophysical relaxation and photochemical reaction dynamics described in this chapter is substantially different from the traditional concepts in this area. In contrast to the established picture of radiationless transitions in terms of interacting tiers of zero-order molecular eigenstates, the dynamics is rationalized in terms of local properties of PE surfaces such as slopes, barriers and surface intersections, a view which now becomes widely accepted in photochemistry. This picture is firmly based on ah initio electronic-structure theory, and the molecular relaxation d3mamics is described on the basis of quantum mechanics, replacing previously prevaUing kinetic models of electronic decay processes. Such a more detailed and rigorous description of elementary photochemical processes appears timely in view of the rich and specific information on ultrafast chemical processes which is provided by modern time-resolved spectroscopy. " ... 

In this chapter, we will focus on the discussion of experiments that directly monitor the time-evolution of the electronic excited-state dynamics. In particular, we shall consider transient transmittance, time-resolved fluorescence, and time-resolved ionization spectroscopy. This is because these techniques have the potential to directly observe the ultrafast photochemical excited-state processes triggered by conical intersections. 

Very short pulses. Pulses as short as 10 fs can be generated relatively easily, enabling ultrafast photochemical reactions and processes to be studied. Over the last decade, attosecond (10 s) laser pulses have been reported. However, their applications in photochemistry are limited to a few specific systems since Heisenberg s uncertainty principle dictates that these short pulses have relatively broad spectral bandwidths. 

In summary, time-resolved photoelectron imaging spectroscopy with the very high time-resolution of 22 fs using two-colour deep UV pulses and ab initio nonadiabatic dynamics simulations have for the first time revealed the ultrafast deactivation processes from S2 to So state in furan. Joint theoretical and experimental results represent a general approach for investigation of ultrafast photochemical reactions, allowing to identify the fingerprints of the character of electronic states with an unprecedented precision. 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

Efficient and selective excitation of electronic target states in atoms and molecules lies at the heart of photochemical applications (see corresponding references in Section 6.1) as well as quantum information processing [102, 103]. Here we demonstrate the potential of SPODS, introduced in the previous sections, for ultrafast electronic switching in a multistate model system. In the previous... 

We have already mentioned in the Introduction (Section 3.7.1) the importance of conical intersections (CIs) in connection with excited electronic state dynamics of a photoexcited chromophore. Briefly, CIs act as photochemical funnels in the passage from the first excited S, state to the ground electronic state S0, allowing often ultrafast transition dynamics for this process. (They can also be involved in transitions between excited electronic states, not discussed here.) While most theoretical studies have focused on CIs for a chromophore in the gas phase (for a representative selection, see refs [16, 83-89], here our focus is on the influence of a condensed phase environment [4-9], In particular, as discussed below, there are important nonequilibrium solvation effects due to the lack of solvent polarization equilibration to the evolving charge distribution of the chromophore. 

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