Predissociation electronic

The preceding discussion was limited mostly to VP processes occurring by direct coupling of the quasibound state of the complex to the dissociative continuum, which is the simplest and most commonly observed decay route for the complexes. However, these systems also serve as ideal venues for studying an array of more complicated dynamical processes, including IVR, and electronic predissociation. This brief section will focus on the former, underscoring some of the inherent dynamical differences between Rg XY complexes by discussing the IVR behavior of a few systems. 

Among them, Li-i-HP can be considered a benchmark model system [29, 30] because its low number of electrons makes possible to calculate accurate PES s. Its electronic spectrum has been meassured by Polanyi and coworkers [22], and has been recently very nicely reproduced using purely adiabatic PES s [31]. In the simulation of the spectrum[31], the transition lines were artificially dressed by lorentzians which widths were fitted to better reproduce the experimental envelop. The physical origin of such widths is the decay of the quasibound states of the excited electronic states through electronic predissociation (EP) towards the ground electronic state. This EP process is the result of the non-adiabatic cou-... 

The purpose of this work is to study the electronic predissociation from the bound states of the excited A and B adiabatic electronic states, using a time dependent Golden rule (TDGR) method, as previously used to study vibrational pre-dissociation[32, 33] as well as electronic predissociation[34, 35], The only difference with previous treatments[34, 35] is the use of an adiabatic representation, what requires the calculation of non-adiabatic couplings. The method used is described in section II, while the corresponding results are discussed in section III. Finally, some conclusions are extracted in section IV. 

Time-dependent Golden-rule treatment for Electronic predissociation... 

The electronic predissociation from different rovibrational levels of the A and B electronic states has been evaluated, to study the effect of the initial excitation on the process. The bounds states chosen are 1,2,3 and 6 for A, and fc=l,2,3,6 and 12 for B, which correspond to the bending progression of states appearing in the... 

In this work the electronic predissociation from the A,B and B states has been studied using a time dependent Golden rule approach in an adiabatic representation. The PES s previously reported[31 ] to simulate the experimental spectrum[22] were used. Non-adiabatic couplings between A-X and B-X were computed using highly correlated electroiric wavefunctions using a finite difference method, with the MOLPRO package[42]. 

It is found that the products obtained after electronic predissociation are mainly LiE products. The reason is that the non-adiabatic couplings excites the HE vibration, which is the reaction coordinate in the ground electroiric state. 

Electronic Predissociation. We have developed an approach based on the BO approximation (29). In this development the eigenfunctions of the zeroth-order Hamiltonian are BO wave-functions. As was noted above for direct photodissociation, a... 

The Born-Oppenheimer (BO) description is not exact. The deviation from the BO approximation can be treated as an additional nonadiabatic interaction. This interaction does not depend on time and can be the origin of radiationless transitions. Moreover, the nonadiabatic interaction is a main mechanism for one kind of indirect photodissociation, namely, photopredissociation of Type I (electronic predissociation). 

Fig. 15.8. Schematic one-dimensional illustration of electronic predissociation. The photon is assumed to excite simultaneously both excited states, leading to a structureless absorption spectrum for state 1 and a discrete spectrum for state 2, provided there is no coupling between these states. The resultant is a broad spectrum with sharp superimposed spikes. However, if state 2 is coupled to the dissociative state, the discrete absorption lines turn into resonances with lineshapes that depend on the strength of the coupling between the two excited electronic states. Two examples are schematically drawn on the right-hand side (weak and strong coupling). Due to interference between the non-resonant and the resonant contributions to the spectrum the resonance lineshapes can have a more complicated appearance than shown here (Lefebvre-Brion and Field 1986 ch.6). In the first case, the autocorrelation function S(t) shows a long sequence of recurrences, while in the second case only a single recurrence with small amplitude is developed. The diffuseness of the resonances or vibrational structures is a direct measure of the electronic coupling strength.

A large intermolecular isotope effect has been reported in the decomposition of metastable (CH)+ and (CD)+ ions [582]. The decomposition was seen as an electronic predissociation and tunnelling was discussed to explain the magnitude of the observed isotope effect. 

The two cases which arise in diatomic molecules are rotational predissociation and electronic predissociation the latter case applies only to excited electronic states. We deal first with rotational predissociation, with can arise for either ground or excited states. The potential energy curve shown for a Morse oscillator in section 6.8 is for a rotationless (./ = 0) molecule. For a rotating molecule, however, we must add a centrifugal term to the potential,... 

The second type of predissociation observed for diatomic molecules is known as electronic predissociation the principles are illustrated in figure 6.28. A vibrational level v of a bound state E lies below the dissociation asymptote of that state, but above the dissociation asymptote of a second state E2. This second state, E2, is a repulsive state which crosses the bound state E as shown. The two states are mixed, and the level v can predissociate via the unbound state. It is not, in fact, necessary for the potential curves of the two states to actually cross. It is, however, necessary that they be mixed and there are a number of different interaction terms which can be responsible for the mixing. We do not go into the details here because electronic predissociation, though an important phenomenon in electronic spectroscopy, seldom plays a role in rotational spectroscopy. Since it involves excited electronic states it could certainly be involved in some double resonance cases. 

Figure 6.28. Electronic predissociation. The vibrational level v belongs to an electronic state which dissociates into a ground state atom A and an excited atom B. The potential curve for this state crosses that of a second repulsive state which dissociates into ground state atoms A and B. Coupling between the two electronic states leads to predissociation of the level v into ground state atoms.

We may expect that the electronic predissociation of the excited complex will be also highly selective. If the initially excited 5> state is the vibrationless level of the j electronic manifold, the coupling to lower lying vibronic levels of the / manifold will induce the transition ... 

The third situation, figure 3.13(c), is an example of electronic predissociation. That is, the molecule passes from one electronic state, which is bound, to a dissociative state. The reaction rate is then dependent on the strength of the coupling between the two surfaces, a problem that can be understood by curve crossing models... 

Figure 3.13 Examples of unimolecular decomposition from excited electronic states (a) direct dissociation, (b) vibrational predissociation, (c) electronic predissociation, and (d) internal conversion.

The use of the term vibrational predissociation in the above sense is not universal. Herzberg (1967) and others (Kelley and Bernstein, 1986) use VP and statistical RRKM dissociation interchangeably. The suggested use here makes the connection between vibrational and electronic predissociation stronger. In both cases, the reaction can be modeled via coupling of only two or three potential energy surfaces. The full density of vibrational states is not considered in such predissociation models. 

V. Brems, M. Desouter-Lecomte, J. Lievin, Avoided resonance overlapping beyond the energy independent formalism. 2. Electronic predissociation, J. Chem. Phys. 104... 

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