Molecule dissociative electronic states

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... 

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... 

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, ... 

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

An excimer is a dimer which is stable only in an excited electronic state but dissociates readily in the ground state. Examples of these are the noble gas dimers such as Hc2, discussed in Section 7.2.5.6. This molecule has a repulsive ground state but a bound... 

All heteronuclear diatomic molecules, in their ground electronic state, dissociate into neutral atoms, however strongly polar they may be. The simple explanation for this is that dissociation into a positive and a negative ion is much less likely because of the attractive force between the ions even at a relatively large separation. The highly polar Nal molecule is no exception. The lowest energy dissociation process is... 

All of the atomic species which may be produced by photon decomposition are present in plasma as well as the ionized states. The number of possible reactions is therefore also increased. As an example, die plasma decomposition of silane, SiH4, leads to the formation of the species, SiH3, SiHa, H, SiH, SiH3+ and H2+. Recombination reactions may occur between the ionized states and electrons to produce dissociated molecules either direcdy, or tlrrough the intermediate formation of excited state molecules. 

Consider now the behaviour of the HF wave function 0 (eq. (4.18)) as the distance between the two nuclei is increased toward infinity. Since the HF wave function is an equal mixture of ionic and covalent terms, the dissociation limit is 50% H+H " and 50% H H. In the gas phase all bonds dissociate homolytically, and the ionic contribution should be 0%. The HF dissociation energy is therefore much too high. This is a general problem of RHF type wave functions, the constraint of doubly occupied MOs is inconsistent with breaking bonds to produce radicals. In order for an RHF wave function to dissociate correctly, an even-electron molecule must break into two even-electron fragments, each being in the lowest electronic state. Furthermore, the orbital symmetries must match. There are only a few covalently bonded systems which obey these requirements (the simplest example is HHe+). The wrong dissociation limit for RHF wave functions has several consequences. 

Transitions occur constantly in nature molecules change from one tautomeric form to another, radioactive nuclei decay to form other nuclei, acids dissociate, proteins alter their shapes, molecules undergo transitions between electronic states, chemicals react to form new species, and so forth. Transition rules allow the simulation of these changes. 

A dilute I2/CCI4 solution was pumped by a 520 nm visible laser pulse, promoting the iodine molecule from its ground electronic state X to the excited states A,A, B, and ti (Fig. 4). The laser-excited I2 dissociates rapidly into an unstable intermediate (I2). The latter decomposes, and the two iodine atoms recombine either geminately (a) or nongeminately (b) ... 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

In addition to the previously mentioned disadvantages, all of these methods have another drawback in the large molecule photofragment velocity measurements. For example, in the studies of UV photon photodissociation of polyatomic molecules, like alkene and aromatic molecules, molecules excited by the UV photons quickly become highly vibrationally excited in the ground electronic state through fast internal conversion, and dissociation occurs in the ground electronic state. 

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