Electronically nonadiabatic influences

From the point of view of associative desorption, this reaction is an early barrier reaction. That is, the transition state resembles the reactants.46 Early barrier reactions are well known to channel large amounts of the reaction exoergicity into product vibration. For example, the famous chemical-laser reaction, F + H2 — HF(u) + H, is such a reaction producing a highly inverted HF vibrational distribution.47-50 Luntz and co-workers carried out classical trajectory calculation on the Born-Oppenheimer potential energy surface of Fig. 3(c) and found indeed that the properties of this early barrier reaction do include an inverted N2 vibrational distribution that peaks near v = 6 and extends to v = 11 (see Fig. 3(a)). In marked contrast to these theoretical predictions, the experimentally observed N2 vibrational distribution shown in Fig. 3(d) is skewed towards low values of v. The authors of Ref. 44 also employed the electronic friction theory of Tully and Head-Gordon35 in an attempt to model electronically nonadiabatic influences to the reaction. The results of these calculations are shown in... 

Fig. 3. Vibrational population distributions of N2 formed in associative desorption of N-atoms from ruthenium, (a) Predictions of a classical trajectory based theory adhering to the Born-Oppenheimer approximation, (b) Predictions of a molecular dynamics with electron friction theory taking into account interactions of the reacting molecule with the electron bath, (c) Born—Oppenheimer potential energy surface, (d) Experimentally-observed distribution. The qualitative failure of the electronically adiabatic approach provides some of the best available evidence that chemical reactions at metal surfaces are subject to strong electronically nonadiabatic influences. (See Refs. 44 and 45.)...

Fig. 3(b). The stunning agreement between experiment and theory suggests that electronically nonadiabatic effects strongly influence this reaction. 

We next address the question if the inclusion of another electronic state influences the control yield. Therefore, the first excited state 11) is incorporated in the theoretical description. From Fig. 43 it can be taken that the respective potential curve is well separated from the ground state and thus nonadiabatic couplings are small (this, however, does not hold for the coupling between the states 1) and 2)). We therefore employ the Hamiltonian... 

One should not be left with the impression that electronically nonadiabatic processes are limited to predissociation. Figure 1(a) shows a crossing between two excited repulsive curves in the photolysis of methyl iodide. If the surface hopping process is efficient enough, it can even influence a dissociating molecule that passes the curve crossing within a few femtoseconds, as is the case for methyl iodide photodissociation. 

R to P is slow, even when the isoenergetic conditions in the solvent allow the ET via the Franck-Condon principle. The TST rate for this case contains in its prefactor an electronic transmission coefficient Kd, which is proportional to the square of the small electronic coupling [28], But as first described by Zusman [32], if the solvation dynamics are sufficiently slow, the passage up to (and down from [33]) the nonadiabatic curve intersection can influence the rate. This has to do with solvent dynamics in the solvent wells (this is opposed to the barrier top description given above). We say no more about this here [8,11,32-36]. 

These theoretical considerations also gave a basis for the consideration of the optimal distance of discharge, which is a result of competition between the activation energy AG and the overlap of electronic wave functions of the initial and final states. The reaction site for outer-sphere electrochemical reactions is presumed to be separated from the electrode surface by a layer of solvent molecules (see, for instance, [129]). In consequence, the influence of imaging interactions on AGJ predicted by the Marcus equation is small, which explains why such interactions are neglected in many calculations. However, considerations of metal field penetration show that the reaction sites close to the electrode are not favored [128], though contributions to ks from more distant reaction sites will be diminished by a smaller transmission coefficient. If the reaction is strongly nonadiabatic, then the closest approach to the electrode is favorable. 

When the distance of the reactant from the surface increases beyond r, the electronic coupling will decrease, leading consequently to nonadiabaticity. For such nonadiabatic reactions, with a small overlap of orbitals, the net pre-exponential factor KV should not be influenced by solvent dynamics [206, 207], because k is inversely proportional to Vn. 

The behavior of CTTS states is dependent on energy levels of the ion-solvent molecular couphngs. These levels can lead to internal relaxation and/or complete electron detachment via adiabatic or nonadiabatic electron transfer. The ultrafast spectroscopic investigations of electronic dynamics in ionic solutions would permit us to learn more about the primary steps of an electron-transfer reaction within a cationic atmosphere. The influence of counterions on early electron photodetachment trajectories from a hahde ion can be considered as prereactive steps of an electron transfer. 

Aqueous ionic solutions represent a paradigm for the study of early branching between ultrafast nonadiabatic and adiabatic electron transfers. The very recent experimental observations of specific counterion effects on electronic dynamics provide direct evidence of complex influences of inhomogeneous ion-ion distributions on ultrafast electron-transfer processes. These microscopic effects are particularly evident in IR electronic traiectories in sodium chloride solution (86, 92). 

In many cases the sudden changes in the electronic state, i.e., the nonadiabatic transitions from a lower to a higher potential energy surface, have to be taken into account /3/- The reflection in the curvilinear part of the reaction path may also considerably influence the reaction probability. Therefore, the introduction of a transmission coefficient ( < 1) in the rate equation ( A) is necessa-... 

Femto/picosecond time-resolved absorption spectroscopy (see section 2.A) traces the pathway of the electron from P to P+H and constitutes so far the only experimental approach leading to the various rates of the reaction schemes (1) and (2). However, this is only true for extensive data sets acquired under special excitation and probing conditions. Then, the measurement and evaluation of the temperature dependence of the kinetics may yield the electronic matrix elements V23 or 3, provided that the nonadiabatic ET theory [12,13] is applicable and thermal contraction effects influencing the couplings are negligible. With these assumptions, the weak increase of the time constant of H formation at low temperatures has been attributed to an activationless behaviour of the primary ET [4] leading to a... 

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