Experimental techniques photodissociation dynamics

The first reaction hlmed by X-rays was the recombination of photodissociated iodine in a CCLt solution [18, 19, 49]. As this reaction is considered a prototype chemical reaction, a considerable effort was made to study it. Experimental techniques such as linear [50-52] and nonlinear [53-55] spectroscopy were used, as well as theoretical methods such as quantum chemistry [56] and molecular dynamics simulation [57]. A fair understanding of the dissociation and recombination dynamics resulted. However, a fascinating challenge remained to film atomic motions during the reaction. This was done in the following way. 

Most of the latest experimental techniques have been applied to the study of the photodissociation dynamics of these molecules. Part of the reason for the great interest in these molecules is a result of their long history in photochemistry and partly because there is a low lying absorption that is accessible to the available lasers. Formaldehyde is particularly important since it is the simplest molecules in the group and has been studied theoretically. 

Besides its practical importance, photodissociation — especially of small polyatomic molecules — provides an ideal opportunity for the study of molecular dynamics on a detailed state-to-state level. We associate with molecular dynamics processes such as energy transfer between the various molecular modes, the breaking of chemical bonds and the creation of new ones, transitions between different electronic states etc. One goal of modern physical chemistry is the microscopical understanding of molecular reactivity beyond purely kinetic descriptions (Levine and Bernstein 1987). Because the initial conditions can be well defined (absorption of a single monochromatic photon, preparation of the parent molecule in selected quantum states), photodissociation is ideally suited to address questions which are unprecedented in chemistry. The last decade has witnessed an explosion of new experimental techniques which nowadays makes it possible to tackle questions which before were beyond any practical realization (Ashfold and Baggott 1987). 

Photophysical processes can be used to probe the local structure of organized systems that are difficult to characterize by conventional analytical protocols. Photophysical probes provide valuable structural information for monolayers assembled on metal supports and for polymeric composites. In the latter case, it has been possible to determine the level of thermal residual strain and to monitor architectural deformations. Fluorescence from single molecules continues to attract great attention while the dynamics of photodissociative processes have been reviewed, with emphasis given to experimental techniques. ... 

The development of experimental techniques has allowed studies of clusters at the initial state of nucleation to be carried out, starting from the single molecule or ion through further formation of consecutive clusters with the size increasing toward the bulk phase [8,21-23]. The most extended sets of thermodynamic data have been obtained via the high-pressure mass spectrometry technique [24,25]. The use of lasers in the cluster ion Association spectroscopy allows the dynamics of photodissociation at the molecular level [21] to be unraveled. The high resolution spectroscopic studies are another important source of information concerning the properties of molecular and ionic extended complexes [26,27]. 

One of the most important experimental techniques to appear in the last few years has been transient grating spectroscopy and this methodology has numerous important applications. The technique has been used to measure the quantum yield for photodissociation of diphenyl disulfide, and compared to photoacoustic and transient absorption spectoscopic methods. Additional studies have used transient grating spectroscopy to monitor the energetics and dynamics of charge separation of an ion pair into free ions. The approach permits direct measurement of the enthalpy change accompanying formation of the free ions... 

While the dipole absorption features [114, 124] and photodissociation dynamics of small sodium clusters are rather well known [112, 113, 126, 127, 407-409], there is very little knowledge about potassium clusters larger than the dimer. The lack of experimental data might be caused by ultrafast fragmentation processes within the potassium clusters, so that conventional stationary spectroscopic techniques might fail. Hence, the goal of this section is to determine the photodissociation probability of small potassium clusters as a function of cluster size as well as excitation energy. 

This chapter is concerned with experimental investigations of the dynamics of the dissociation of polyatomic neutral molecules carried out by the technique of laser Doppler spectroscopy, in bulk and under crossed-beam condition. Photodissociation is a basic process in the interaction of light with molecules, of interest in itself as an elementary molecular process and also with respect to a variety of applications in different fields. The interest has increased considerably in recent years, first, because the experimental investigation of photodissociation is rapidly advancing by the use of the laser, and second, because the laser makes possible to achieve photodissociation, state, and isotope selectively, by new excitation mechanisms. These are, aside from the common one-photon absorption, stepwise... 

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