Photochemical dynamics

School of Chemistry, University of Bristol, Bristol, UKBS8 ITS 

Commentary on Excited Fragments from Excited Molecules Energy Partitioning in the Photodissociation of Alkyl Iodides, S. J. Riley and K. R. Wilson, Discuss. Faraday Soc., 1972, 53, 132. 

A major object of modern chemical physics has been to obtain a detailed understanding of the factors that control chemical reactions. This is a field in which there has been a particularly strong interplay between ongoing experimentation and parallel theoretical developments. Photodissociation is inherently one of the simpler types of reaction in that the products are already in close contact in the reactant so called half collisions . Technical developments in molecular beams and lasers have been particularly instrumental in many recent advances in this field. 

These types of study have been extended to a large number of molecular systems in numerous laboratories around the world. One consequence was to stimulate the need for more detailed theories of vector correlations in the motions of photofragments.  

The more recent development of Rydberg tagging came about following collaboration between my colleague Michael Ashfold and Karl Welge in Bielefeld. One of the photofragments, H-atoms in the 

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We expect the development of the mechanistic aspects of organic photochemistry to continue at the present pace as new methods are developed to probe in increasing detail and shorter time scales the photochemical dynamics of both old and new photoreactions. Since photochemistry is no longer the sole domain of the specialist, it is relatively safe to predict a dramatic increase in the near future of the synthetic and industrial uses of organic photochemistry. 

Chen, X.-B. Fang, W.-H. Phillips, D. L. Theoretical Studies of the Photochemical Dynamics of Acetylacetone Isomerization, Dissociation, and Dehydration Reactions. J. Phys. Chem. A 2006,110, 4434-4441. 

Neutze R, Wouts R, Techert S, Davidsson J, Kocsis M, Kirrander A, Schotte F, Wulff M (2001) Visualizing photochemical dynamics in solution through picoseconds X-ray scattering. Phys Rev Lett 87 195508-1-195508-4... 

Pentanal cluster In order to study the effect of a cluster on the photochemical dynamics, a cluster of five pentanal molecules has been constructed. All the molecules have been aligned such that all carbonyl groups point to the same... 

With all of these new tools, it is no wonder that there has been an explosion of papers on photochemical dynamics, so much so that in this review we shall limit ourselves to those papers that have appeared over the last three years. Earlier reviews cover the work before this time, and the papers that are cited also give references to the earlier work. The papers that are covered are further limited to those that measure and discuss the detailed quantum state distribution of one or more of the photochemical fragments. Those papers that are limited to final product analysis are discussed only if the results bear directly upon the dynamics of the photochemical process. The review is organized so that molecules with similar chromo-phore groups are all discussed at the same time. This emphasizes the similarities and differences between these molecules. The discussion of the molecular systems begins after a brief discussion of some of the newer experimental techniques. In this review any earlier reviews that cover that molecule are cited along with the later papers on the subject. 

The photochemical dynamics of H2S has been studied in its first absorption band between 180 and 260 nm (2) using LIF measurements to determine the quantum state distribution of the SH fragment (169-171), as well as TOF measurements of the velocity distribution of H atom fragment (172). In the former case, the vibrational and rotational distribution of the SH fragment was only measured in the v" = 0 level because fewer radicals with v" > 0 are produced and the LIF technique does not efficiently detect these excited radicals. 

Zimmermann FM, Ho W. State resolved studies of photochemical dynamics at surfaces. Surf Sci Rep. 1995 22 127-247. 

From the agreement between theory and experiment, the sys-tematics of excited-state kinetics can be rationalized. Certainly, recent experiments in which the details of the photochemical dynamics have been studied through rotational-state selection provide a great deal of insight into the excited-state kinetics of "small molecules", just as high-resolution molecular electronic spectroscopy provides detailed information about the structure, and the various forms of interactions, in the excited states. 

Ohta, N. (2002) Electric field effects on photochemical dynamics in solid films. Bull. Chem. Soc.Jpn., 75, 1637-1655. 

Makhlouf, U.B., R.H. Picard, and J.R. Winick, Photochemical-dynamical modehng of the measured response of airglow to gravity waves, 1. Basic model for OH airglow. J Geophys Res 100, 11,289, 1995. 

In the case of a photochemical reaction, the species is instantaneously promoted toward the excited surface by the photon absorption. The species on the excited-state surface (generally or 83) will initially have all the characteristics of the species on the Sq surface. The typical sampling for a photochemical dynamics calculation should therefore be carried out on the Sq surface. When studying a very rare photochemical event (e.g., one associated with a huge activation barrier), there is no guarantee that any of the resulting trajectories will ever follow the desired path. There are several approaches to address this issue. 

Mori T, Glover WJ, Schuurman MS, Martinez TJ (2012) Role of Rydberg States in the photochemical dynamics of ethylene. J Phys Chem A 116 2808... 

In this chapter we briefly review both the pertmbative as well as nonperturbative formalism for the calculation of femtosecond time-resolved spectra, focusing on the detection of the photophysical and photochemical dynamics at conical intersections. To set the stage for a general discussion of the potential of modern time-resolved spectroscopy, we begin with a brief overview of the spectroscopic techniques, which are particularly suitable in the present context. 

We have outlined the theoretical description of femtosecond time-resolved spectroscopy of the photophysical and photochemical dynamics at conical... 

Despite the lack of a comprehensive reaction-path theory for excited- state surfaces, the reaction path concept can fruitfully be applied at a pragmatic level to make progress in the understanding of photochemical dynamics. In the present Chapter... 

Yarasi S, Brost P, Loppnow GR (2(X)7) Initial excited-state structural dynamics of thymine are coincident with the expected photochemical dynamics. J Phys Chem A 111 5130-5135... 

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