Dissociative electronic states

The chemisorption process has its maximum cross section at 11 eV in contrast with cross sections for the radiolysis of bulk ice. This difference was understood as being dependent on the selective quenching of dissociative electronic states of water due to the resonant charge exchange between the substrate and adsorbate and the absence of multiple inelastic scattering in the H2O bilayer on Si(lll). 

An electronic R-matrix radius a is chosen such that exchange can be neglected for r > a. An upper limit qd for the intemuclear coordinate q is chosen so that a dissociating electronic state ,/ is bound for q > qd- This defines a vibronic hypercylinder [284] with two distinct surface regions an electronic wall with r = a for 0 < q < qd and a dissociation cap defined by the enclosed volume of... 

Figure 23 Predissociation of the v = 2 vibrational level of the bound electronic state by a vibrational continuum wave function of the dissociative electronic state after radiative excitation (arrows) from the electronic ground state Po- The circle around the potential curve crossing point indicates an area of large overlap between the vibrational wave functions.

In a photodissociation reaction it is usual for the initial state of the molecule to be the ground vibrational state of the ground electronic state. The incident radiation is resonant with an excitation to an electronic state that is dissociative (repulsive potential energy surface) or predissociative (the optically allowed transition is to a bound-state potential energy surface that intersects a repulsive surface). In the Franck-Condon picture, the electrons respond instantaneously to the incident light, while the relatively massive nuclei respond only slowly. Hence, on absorption of a photon the nuclear wave-function retains its shape but is projected up to the dissociative electronic state. In the traditional approach to the calculation of the photodissociation... 

Fig. 9. Potential energy diagram for breaking chemical bonds in an energetic molecule. The specific coordinate R shown here is identified as the reaction coordinate. In ascending energy these levels are the electronic ground state, a bound excited state and a dissociative excited state. Thermal cleavage of a bond in the electronic ground state requires a minimum energy Dq. In bound electronic states the bond dissociation energy Do is usually smaller than Do, so thermochemistry often has a lower barrier electronic excited states. Chemical bonds can also be broken by electronic excitation to predissociative or dissociative electronic states.

On the other hand, we may consider a model in which represents a bound electronic state of the diatomic molecule, whereas >/ represents a dissociative electronic state of the diatomic molecule. The dissociative state then has an energy level spacing that is zero, e, 0, while the radiative lifetime is nonzero (albeit possibly rather small in many cases). Thus the parameter x, of (2.1) is infinite for this example, and the conditions for the... 

Dissociation. Electronic states of the dissociation products P" + H2, PH" + H, and PH + H" have been correlated with states of PHJ in [11 ] (in a figure of bending potential energy curves). 

Such diagnostic information is often required for electronically-excited bound or quasi-bound molecular states formed, e.g. in charge-exchange reactions of molecular beams with other species or with surfaces . A short (subnanosecond) laser pulse dissociating the molecule from such a state can allow to monitor the CF with sufficient temporal resolution, in order to probe the properties of this state. Another foreseeable application of CF effects is the study of molecular S3mimetry (related to the geometrical configuration) of clusters in laser-excited quasi-bound or dissociative electronic states. 

Simple Energy-Level Model. Figure 2.35 sketches a simplified case of TPI spectroscopy of a dissociative electronic state. Fragmentation with a probability l/rfrag is regarded as the only relaxation channel. In a real-time TPI experiment, first an ultrashort pump pulse transfers an ensemble of e.g. molecules or clusters to an excited state (2.18a). Then, either the excited... 

Predissociation is the breaking of a chemical bond by intersystem crossing to a dissociating electronic state from a non-dissociating state. 

A diatomic molecule AB is excited from the v = 1 vibrational level of the ground electronic state to a photo-dissociating electronic state that falls apart into A and B, ... 

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