Radiationless transitions definition

This review shows how the photochemistry of ketones can be rationalized through a single model, the Tunnel Effect Theory (TET), which treats reactions of ketones as radiationless transitions from reactant to product potential energy curves (PEC). Two critical approximations are involved in the development of this theory (i) the representation of reactants and products as diatomic harmonic oscillators of appropriate reduced masses and force constants (ii) the definition of a unidimensional reaction coordinate (RC) as the sum of the reactant and product bond distensions to the transition state. Within these approximations, TET is used to calculate the reactivity parameters of the most important photoreactions of ketones, using only a partially adjustable parameter, whose physical meaning is well understood and which admits only predictable variations. 

The detailed mechanism of step (8) remains uncertain, although contribution from atoms is definitely exdu(M. Thus the precursor atoms of the emitting B state are states which cannot correlate directly with this molecular state. Combination into an intermediate state which then undergoes a radiationless transition to the B state was suggested by Clyne and Stedman. The intermediate could be a repulsive II(U) state or, conceivably, as suggested by Bader and Ogryzlo, a molecular Os intermediate. 

The population probabilities Pn t) defined in Eqs. (8)-(13) should not be confused with the population probabilities which have been considered in the extensive earlier literature on radiationless transitions in polyatomic molecules, see Refs. 28 and 29 for reviews. There the population of a single bright (i.e. optically accessible from the electronic ground state) zero-order Born-Oppenheimer (BO) level is considered. Here, in contrast, we define the electronic population as the sum of all vibrational level populations within a given (diabatic or adiabatic) electronic state. These different definitions are adapted to different regimes of time scales of the system dynamics. If nonadiabatic interactions are relatively weak, and radiationless transitions relatively slow, the concept of zero-order BO levels is useful the populations of these levels can be prepared and probed using suitable laser pulses (typically of nanosecond duration). If the nonadiabatic transitions occur on femtosecond time scales, the preparation of individual zero-order BO levels is no longer possible. The total population of an electronic state then becomes the appropriate concept for the interpretation of time-resolved experiments. ° ... 

In Chap. 3, wave packet propagation could be observed for nearly all of the alkali dimer and trimer systems considered, over a rather long time compared to the wave packet oscillation period. The wave packet dynamics - a fingerprint of the excited molecule - definitely characterize the excited bound electronic state of these molecules. However, with the results on K3 (excited with A 800 nm), another phenomenon, which often governs ultrafast molecular and cluster dynamics, comes into the discussion photodissociation induced by the absorption of single photons. This photoinduced dissociation permits detailed study of molecular dynamics such as breaking of bonds, internal energy transfer, and radiationless transitions. The availability of laser sources with pulses of a few tens of femtoseconds today opens a direct, i.e. real-time, view on this phenomenon. 

For X = K (no subshell) the number of X-ray photons emitted is given by Nk=N(Dk where N is the total number of K holes involved. Here N is equal to the sum of radiative and radiationless transitions. To a first approximation, K radiationless transition probability is nearly independent of Z, while radiative electric-dipole probability is proportional to Z. It justifies useful semiempirical laws based on q)kOcZ /(Z +c), c=constant. They are discussed in [4] which also gives many references on Auger and related processes up to 1971. Due to Coster-Kronig transitions, experimental and theoretical problems are more complicated for X = L, M,. .. Experimental data depend on the primary vacancy distribution which must remain unaltered before the vacancies are filled. Literature provides either total X-shell data(X = L, M,. ..) or partial Xj-subshelldata (Xj = L., Lg, L3,. ..). Definitions... 

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