Predissociation diatomic molecules

FIGURE 3.8 Potential energy curves for the ground state and two electronically excited states in a hypothetical diatomic molecule. Predissociation may occur when the molecule is excited into higher vibrational levels of the state E and crosses over to repulsive state R at the point C (from Okabe, 1978). 

Figure 7.3 Situations for predissociation of a diatomic molecule on photoexcitation.

There is a possibility that an FC state will react before complete thermal equilibration. In the case of diatomic molecules, the process is usually known as predissociation — a dissociative state crosses the excited state potential surface. The situation is more complicated in the case of a coordination compound, but one can imagine an FC state relaxing along some nuclear coordinate leading to bond breaking. A state capable of such a process has been called a DOSENCO state, an acronym for Decay On SElected Nuclear Coordinates .21 The same authors use the term DERCOS (DEcay via Random Coordinate Selection) for a thexi state. 

Vibrational Predissociation, in this section we discuss the case of a transition from a predissociative state to the photofragment state that occurs on a single adiabatic pes. Such processes cannot occur for diatomic molecules, but they can be observed for polyatomic systems. The transition is caused by intramolecular energy transfer, that is, by internal redistribution of vibrational energy. 

If one adopts the correct point of view that the complete wave function of any state of a diatomic molecule has contributions from all other states of that molecule, one can understand that all degrees of perturbation and hence probabilities of crossover may be met in practice. If the perturbation by the repulsive or dissociating state is very small, the mean life of the excited molecule before dissociation may be sufficiently long to permit the absorption spectrum to be truly discrete. Dissociation may nevertheless occur before the mean radiative lifetime has been reached so that fluorescence will not be observed. Predissociation spectra may therefore show all gradations from continua through those with remnants of vibrational transitions to discrete spectra difficult to distinguish from those with no predissociation. In a certain sense photochemical data may contribute markedly to the interpretation of spectra. 

Photochemical dissociation of diatomic molecules may occur either (a) by absorption in a spectral region showing a continuum or (b) by absorption in a region with bands which have more or less discrete appearance followed by crossover from one upper electronic state to another, which results in dissociation. The latter phenomenon is called predissociation and has been discussed at some length. 

When a diatomic molecule dissociates photochemically it yields atoms either in their ground or in excited states. When the optical transition leading to dissociation is not forbidden, and does not arise from predissociation, one of the two atoms formed will not be in its ground state. 

As we have seen in discussing the behavior of diatomic molecules, there is often great difficulty in identifying a predissociation spectrum since the appearance may at one extreme be essentially that of a truly discrete spectrum and at the other that of a true continuum. The facts of photochemistry may be of great use to the spectroscopist in distinguishing between predissociation and truly discrete spectra. 

Nevertheless, predissociation in polyatomic molecules can be visualized as qualitatively similar to a unimolecular reaction in the Rice-Ramsperger-Kassel sense. With diatomic molecules possessing only one degree of vibrational freedom,... 

A vibration rotation level of a diatomic molecule which lies above the lowest dissociation limit may be quasibound and able to undergo spontaneous dissociation into the separate atoms. This process is known as predissociation, and two different cases may be distinguished for diatomic molecules, as we will see shortly. Predissociation does not normally play an important role in rotational spectroscopy but merits a brief discussion here for the sake of completeness. 

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. 

When more than one state correlates with the electronic states of the separated species, collisions populate the various molecular states at statistically controlled relative rates. If more than one such state is bound, then it may be stabilized in a third-order process. For complex species, the rate of predissociation of the energy-rich complex, that is, k i[Rt], depends on its dissociation energy, so the ground-state complex will survive longest and have the highest chance of being collisionally stabilized. For diatomic molecules, for example, N2, 02, and NO, dipole transitions from these excited states to the ground state are not fully allowed and the excited species are almost certainly quenched in collisions. 

Recombination may also proceed via an electronically excited state if during the course of a bimolecular collision the system may transfer from the nonquantized part of the potential curve associated with one electronic state to a second state from which emission is allowed. This process is called preassociation or inverse predissociation, and the selection rules that control the probability of crossing in both directions are well known [109]. In such encounters total angular momentum must be conserved. For diatomic molecules, the system can pass only into the rotational level of the excited bound state which corresponds to the initial orbital angular momentum in the collision. 

Spectroscopic evidence on D SO) in the diatomic molecule leads to 4 00 or 5 18 eV. Herzberg 217 favours the lower, Gaydon the higher. The result depends on the assumed products of a predissociation. 

Figure 2.9. Potential energy curves for a predissociative process in a diatomic molecule.

Interesting in this context are also theoretical studies of vibrational predissociation in van der Waals complexes by Beswick and Jortner. They conclude that the rates of vibrational predissociation of the rare gas atom-diatomic molecule complexes should be enhanced with decreasing mass of the rare-gas atom. If one could view relaxation of the guest molecule as a predissociation in a polyatomic van der W lals complex involving the guest and the nearest-neighbor rare-gas atoms, the observed trends would again be correctly predicted. 

If the transition probability between adiabatic curves is denoted as P. and that between diabatic curves P, it can be shown (Nikitin, 1974) that Pg = 1 Pa- For a diatomic molecule initially in the diabatic state (see fig. 3.10), the predissociation rate constant is... 

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