Vertical transitions 518 Subject

The classical picture of photodissociation closely resembles the time-dependent view. The electronic transition from the ground to the excited electronic state is assumed to take place instantaneously so that the internal coordinates and corresponding momenta of the parent molecule remain unchanged during the excitation step (vertical transition). After the molecule is promoted to the potential energy surface (PES) of the upper electronic state it starts to move subject to the classical equations... 

Electron-transfer reactions, which occur either by the inner-sphere or by the outer-sphere mechanism, are subject to restrictions defined by the Franck-Condon principle. The principle explains the distribution of relative intensities of the vibrational structure of an electronic transition. Namely, since the nuclei have a much larger mass than the electrons, electron transition takes place much faster than the nuclei can respond. Thus, the electron transition takes place in about 10" to 10 s, while the nuclei need about 10 s to respond (which is approximately the time of one vibration). This is the reason why electron density is rapidly built up in the new molecular region. The new electron density distribution acts upon the nuclei, which become subject to a new force field, causing them to vibrate. Since the nuclear framework remains unchanged with respect to its geometry during a very fast electronic transition, the expression vertical transition is used to describe this situation. 

The determination of nitrate in seawater is not subject to interferences. It has been claimed that nitrate might occur together with small amounts of hydrogen sulphide. For thermodynamic reasons this can only be true for waters from the transition layers between oxic and anoxic environments where intense vertical turbulent mixing processes occur. In this case the hydrogen sulphide is precipitated on top of the reductor as copper or cadmium sulphide and does not interfere in the nitrate analysis. Nitrate values observed together with hydrogen sulphide should be interpreted with care. 

In this chapter we will also consider the first basic aspects of the dynamical, i.e., nonequilibrium, behavior of liquid media. The subject embraces a very large variety of phenomena, requiring different formulation of the continuum models. We cite here an aspect, relatively simple, related to a phenomenon occurring in a span of time relatively short with respect to the characteristic relaxation times (CRT) of the solvent the vertical electronic transitions in solutes (Basilevsky and Chudinov 1990 BQm and Hynes 1990 Marcus 1992). 

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