Photodissociation dynamics computational methods

Considerable work has already been carried out using ab initio calculations to predict the photodissociation dynamics of gas-phase metal carbonyls (45). This is a fertile area for computational work, given the extensive experimental results available, which include the use of ultrafast methods to characterize the short time behavior in photoexcited states. There is considerable evidence that surface crossings, especially of a spin-forbidden nature, play a considerable part in the dynamics. Much of the theoretical work so far has focused on reduced-dimensionality models of the PESs, which have been used in quantum mechanical smdies of the nonadiabatic nuclear dynamics, in which spin-forbidden transitions are frequently observed (45). Here, too, the potential benefits to be derived from a proper understanding of the spin-state chemistry are considerable, due to the importance of light-induced processes in organometallic and bioinorganic systems. 

In gas-phase dynamics, the discussion is focused on the TD quantum wave packet treatment for tetraatomic systems. This is further divided into two different but closed related areas molecular photofragmentation or half-collision dynamics and bimolecular reactive collision dynamics. Specific methods and examples for treating the dynamics of direct photodissociation of tetraatomic molecules and of vibrational predissociation of weakly bound dimers are given based on different dynamical characters of these two processes. TD methods such as the direct projection method for direct photodissociation, TD golden rule method and the flux method for predissociation are presented. For bimolecular reactive scattering, the use of nondirect product basis and the computation of the initial state-selected total reaction probabilities by flux calculation are discussed. The descriptions of these methods are supported by concrete numerical examples and results of their applications. 

Since the first experiments on the I-CN [21] bond cleavage and the wavepacket oscillations between the ionic and covalent potentials in the photodissociation of Nal [22, 23], pump-probe techniques have been applied to a wide range of important photochemical processes. However, the data obtained Ifom such experiments are often difficult to interpret and theoretical modeling is needed to get further insight into the excited state dynamics of the systems of interest at the atomistic level. In this context, the development of efficient and accurate computational methods for the description of ground and excited electronic states of mid-size molecular systems in a balanced way [24, 25], has greatly facilitated the theoretical study of photochemical processes. 

Another aspect of dynamics was evaluated by Niv, Bargheer, and Gerber who studied the photodissociation dynamics of F2 in an At54 cluser. Modeling condensed-phase reaction dynamics is difficult due to many-body interactions. However, a model with an inert medium is a convenient way to examine the role that a medium can play in the condensed phase. Because the system is large and a full quantum mechanical description of the dynamics is computationally prohibitive, the authors used the semiclassical surface hopping method to carry out the dynamics simulations. The states of F2, corresponding to a gas-phase equivalent, included in the condensed-phase study are... 

Figure 1.3 HD rotational and vibrational state distributions measured for the H + D2 reaction at a collision energy of 1.3 eV. The energy is determined by the recoil energy of the H atom in the photodissociation of HI at a wavelength where it dissociates primarily to ground state I atoms. The experimental results shown [adapted from D. P. Gerrlty and J. J. Valentini, J. Chem. Phys. 81, 1298, (1984) and Valentin and Phillips (1989)] used CARS spectroscopy to determine the state of HD. E. E. Marinero, C. T. Rettner, and R. N. Zare, J. Chem. Phys. 80,4142 (1984) used resonance enhanced multiphoton ionization, REMPI, for this purpose. The figure also shows curves. Those on the left are the so-called linear surprisal representation, see Section 6.4. The plot on the right shows the same experimental data on a logarithmic scale. The curves [adapted from N. C. Blais and D. G. Truhlar, J. Chem. Phys. 83, 2201 (1985)] are a dynamical computation by the method of classical trajectories. Section 5.2.

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... 

Most recently, the photodissociation in the state has been investigated by quantum scattering calculations on the potential surface computed using the effective Hamiltonian method of Freed and co-workers [68], In this study, the dynamics is assumed to be electronically adiabatic in nature. Theoretical absorption spectra as well as the branching ratios and photofragment translation energy distributions corresponding to various... 

Our group has already studied all the fluoro-, chloro-and bromo-carbenes [11-16]. We used the complete active space self-consistent field (CASSCF) method as well as complete active space second-order perturbation theory (CASPT2) and multi-reference configuration interaction (MRCI) approaches to compute the geometries, force constants, and vibrational frequencies of the (singlet) X and A states as well as the (triplet) a states. Our theoretical studies of most of these carbenes were carried out specifically to complement LIF studies that were pursued in our laboratories by Kable et al. [6]. In addition to the determination of spectroscopic constants, the spectroscopic and theoretical studies considered dynamics on the A surfaces, i.e. whether photodissociation or internal conversion to the ground state would occur. 

Parallel to the developments achieved in methodology and hardware, the conventional methods and some of the new approaches have been employed to study several types of photoinduced processes which are relevant mainly in biology and nanotechnology. In particular, important contributions have been made related to the topics of photodissociations, photostability, photodimerizations, photoisomerizations, proton/hydrogen transfer, photodecarboxylations, charge transport, bioexcimers, chemiluminescence and bioluminescence. In contrast to earlier studies in the field of computational photochemistry, recent works include in many cases analyses in solution or in the natural environment (protein or DNA) of the mechanisms found in the isolated chromophores. In addition, semi-classical non-adiabatic molecular dynamics simulations have been performed in some studies to obtain dynamical attributes of the photoreactions. These latter calculations are however still not able to provide quantitative accuracy, since either the level of theory is too low or too few trajectories are generated. Within this context, theory and hardware developments aimed to decrease the time for accurate calculations of the PESs will certainly guide future achievements in the field of photodynamics. 

This review summarises and discusses the advances of computational photochemistry in 2012 and 2013 in both methodology and applications fields. The methodological developments of models and tools used to study and simulate non-adiabatic processes are highlighted. These developments can be summarised as assessment studies, new methods to locate conical intersections, tools for representation, interpretation and visualisation, new computational approaches and studies introducing simpler models to rationalise the quantum dynamics near and in the conical intersection. The applied works on the topics of photodissociation, photostability, photoisomerisations, proton/charge transfer, chemiluminescence and bioluminescence are summarised, and some illustrative examples of studies are analysed in more detail, particularly with reference to photostability and chemi/ bioluminescence. In addition, theoretical studies analysing solvent effects are also considered. We finish this review with conclusions and an outlook on the future. 

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