Quantum mechanics direct photodissociation

The classical theory is a valuable complement of the quantum mechanical approaches. It is best suited for fast and direct photodissociation. Quantum mechanical effects, however, such as resonances or interferences inherently cannot be described by classical mechanics. The obvious extension is a semiclassical theory (Miller 1974, 1975) which incorporates the quantum mechanical superposition principle without the complexity of full quantum mechanical calculations. All ingredients are derived solely from classical trajectories. For an application in photodissociation see Gray and Child (1984). 

The general theory for the absorption of light and its extension to photodissociation is outlined in Chapter 2. Chapters 3-5 summarize the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical theories as well as the classical trajectory picture of photodissociation. The two fundamental types of photofragmentation — direct and indirect photodissociation — will be elucidated in Chapters 6 and 7, and in Chapter 8 I will focus attention on some intermediate cases, which are neither truly direct nor indirect. Chapters 9-11 consider in detail the internal quantum state distributions of the fragment molecules which contain a wealth of information on the dissociation dynamics. Some related and more advanced topics such as the dissociation of van der Waals molecules, dissociation of vibrationally excited molecules, emission during dissociation, and nonadiabatic effects are discussed in Chapters 12-15. Finally, we consider briefly in Chapter 16 the most recent class of experiments, i.e., the photodissociation with laser pulses in the femtosecond range, which allows the study of the evolution of the molecular system in real time. 

Over the past few years it has often been observed that the photochemical behaviour of adsorbed molecules is distinctly different to that of their gas phase counterparts. Even direct dissociations of molecules physisorbed on insulator substrates were found to have different dynamics to the analagous gas phase reaction, and exhibited a dependence on the coverage. This needs to be understood. For adsorbed molecules a new kind of "dissocation" is possible, namely desorption, Photolytic (non thermal) desorption has been reported from all kinds of substrate. On metal surfaces it is often found that the quantum yield for a direct photodissociation reaction is much lower than in the isolated molecule. This must be accounted for. Finally, the observation which has stimulated a great deal of research in surface photochemistry, photolysis is observable at energies where the gas phase molecules are transparent. It turns out that all of these interrelated effects can be interpreted by a delicate interplay of excitation mechanism and transient quenching. The fine details of course depend on particular adsorbate-substrate systems, which are described in section 4. 

The field of applications of molecular quantum dynamics covers broad areas of science not only in chemistry but also in physics and biology. Historically, due to the fact that the full quantum-mechanical simulation of molecular processes is limited to small systems, molecular quantum dynamics has given rise mainly to important applications of astrophysical and atmospheric relevance. In the interstellar medium or the Earth atmosphere, molecules are generally in the gas phase. Since many accurate spectroscopic data are available, these media have provided various prototype systems to study quantum effects in molecules and to calibrate the theoretical methods used to simulate these effects. In this context, it is not surprising that much theoretical effort is still directed toward modeling the full quantum-mechanical treatment of small molecules. Among others, one can cite the studies of the spectroscopy of water [159-161], and of the spectroscopy, photodissociation. 

Laser-initiated Radical Production. Although there are different physical mechanisms involved in laser chemistry, we are concerned here with the photodissociation, i.e., the breaking of molecular bonds directly by UV photons. The laser emission is used to produce electronically excited molecules which split into reactive radicals, with the highest possible quantum yield. Since the substrate usually behaves as a poor photoinitiator, an additional molecule must be introduced in order to enhance the radical production, much in the same way as in conventional photoinitiated reactions. In this work,... 

In an alternative approach, exact (numerical) time-dependent quantum wave-packet methods have been employed since the early eighties of the last century to explore the d3mamics of ob-initio-haseA models of conical intersections, see Refs. 6-8 for reviews. It has been found by these calculations that the fundamental dissipative processes of population and phase relaxation at femtosecond time scales are clearly expressed already in fewmode systems, if a directly accessible conical intersection of the PE surfaces is involved. The results strongly support the idea that conical intersections provide the microscopic mechanism for ultrafast relaxation processes in polyatomic molecules. " More recently, these calculations have been extended to describe photodissociation and photoisomerization processes associated with conical intersections. The latter are particularly relevant for our understanding of the elementary mechanisms of photochemistry. 

In the chapters of Part II of this book it has been demonstrated for a variety of examples that conical intersections can provide the mechanism for extremely fast chemical processes, e.g. photodissociation, photoisomerization and internal conversion to the electronic ground state. Time-dependent quantum wave-packet calculations have established that radiationless transitions between electronic states can take place on a time scale of the order of 10 fs, if a conical intersection is directly accessible after preparation of the wave packet in the excited state, see, e.g. Chapters 8-11 and 14 15. In view of these findings and the omnipresence of conical intersections in polyatomic molecules (cf. Chapter 6), it is now widely accepted that conical intersections are of fundamental importance for the understanding of the reaction mechanisms in photochemistry and photobiology. 

---