Electronically non-adiabatic processe

The slow peak at 6 30 has a fast shoulder. A similar feature appears in other TOP spectra that sample v= 3 product near 6 180 , and this could be fit only by assuming it was due to HF(v>3) from reactants with approximately 1 kcal/mol internal excitation. This product, designated as v 3, could originate from spin-orbit excited F( P1/2) which lies 1.16 kcal/mol above the P3/2 ground state and constitutes 21Z of the F beam at 920 K, but the reaction between F( Px/2) And H2 can only occur by an electronically non-adiabatic process and is expected to be inefficient (25-26). It is more likely that the v 3 product is from the reaction of F(2p3/2> with H2(J=2) which is 1.03 kcal/mol above H2[Pg.485]

Spin-forbidden reactions are a subset of the broader class of electronically non-adiabatic processes, which involve more than one PES. The fundamental theory of how such processes occur is well understood (7-9), and a very large amount of research is being performed with the aim of elucidating more details in all the areas of nonadiabatic chemistry. It is not possible to present this work here, so I will instead provide an outline of the most important theoretical insights in the... 

In an electronically non-adiabatic process the description of the nuclear motion involves more than one PES. Electronic spectroscopy and photochemical reactions involve transitions between two or more PES in critical regions (avoided crossings, conical intersections, crossings) where the nature of the electronic wave function may change rapidly as a function of the nuclear displacement. This is illustrated in Scheme 4 which represents two different... 

The efficiency of the transition is governed by the non-zero interaction matrix elements between electronic states, the BO approximation being no longer valid. In order to determine the electronic structure aspects of an electronically non-adiabatic process it is necessary to determine i) the critical regions of the PES ii) the coupling between the states in these regions. The first point has been discussed in the previous section whereas the second point will be developed in the next section. 

Before dealing with electronically non-adiabatic processes, it is useful to consider the treatment of vibrational relaxation in encounters between species with closed electronic shells, say A( 5o) + The collision dynamics will be con-... 

The rate of the electronically non-adiabatic process which appears to remove HCI(v = 1) should approximately double when HQ is raised to =2. In addition, the endothermic reaction ( -65) becomes energetically possible and results that confirm that this reaction takes place are referred to in Section 1. However, there does appear to be a discrepancy between the various kinetic results that have been obtained for... 

Our purpose in this section is to suggest what special features in the electronic structure of the reagents allow electronically non-adiabatic processes, with their nominally very high gap, to occur efficiently. The required feature is not invariably present. Many processes are electronically adiabatic. The exponential gap principle is valid. We will sketch a mechanism that, if present, allows the gap to be smaller than what you would expect. 

A comer-stone of a large portion of quantum molecular dynamics is the use of a single electronic surface. Since electrons are much lighter than nuclei, they typically adjust their wavefiinction to follow the nuclei [26]. Specifically, if a collision is started in which the electrons are in their ground state, they typically remain in the ground state. An exception is non-adiabatic processes, which are discussed later in this section. 

In this chapter, we look at the techniques known as direct, or on-the-fly, molecular dynamics and their application to non-adiabatic processes in photochemistry. In contrast to standard techniques that require a predefined potential energy surface (PES) over which the nuclei move, the PES is provided here by explicit evaluation of the electronic wave function for the states of interest. This makes the method very general and powerful, particularly for the study of polyatomic systems where the calculation of a multidimensional potential function is an impossible task. For a recent review of standard non-adiabatic dynamics methods using analytical PES functions see [1]. 

Expression of the Electron Transfer Rate for a Non-adiabatic Process... 

It is necessary to worry about the relative merits of these two approaches if one is concerned with non-adiabatic processes (Some references on this point are 5-9), but, as our primary concern is with the adiabatic potential energy surfaces, we can ignore the difference, which effectively means that we assume the mass of the electron to be zero. 

Adiabatic Process.—This term is often seen in spectroscopic and photochemical literature and used in a different sense than its usual thermodynamic meaning. In Herzberg s opinion (15) adiabatic processes should be defined as reactions or processes in which no change of electronic state occurs and in which the velocity of the partners is sufficiently small that at every point the electronic energy takes on the value corresponding to the particular values of the coordinates. A non-adiabatic process is one in which there is a change in electronic state. ... 

Next, the GLRT shall be applied to analyze the femtosecond time-resolved spectra. Suppose that the probing process corresponds to SE, the relations between the different electronic levels in this case are shown in Figure 5.1. The non-adiabatic process is represented by a -> b and SE is from a to g. Notice that in the BOA... 

This section briefly introduces the generalized coupled master equation within the Born-Oppenheimer adiabatic (BOA) approximation. In this case, the non-adiabatic processes are treated as the vibronic transitions between the vibronic manifolds. Three types of the rate constant are then introduced to specify the nature of the transitions depending on whether the electronically excited molecular system achieves its vibrational thermal equilibrium or not. The radiationless transitions can occur between two... 

See -> charge transfer reaction, -> electron transfer at liquid-liquid interfaces, - Marcus theory, -> adiabatic process, -> non-adiabatic process... 

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