Dissociative excited states

Both VUV light and ionizing radiation can lead to the formation of dissociative excited states, e.g. 

Absorption and Emission Spectra of Small Molecules. In diatomic molecules the number of vibrational and rotational levels is small, so that their energy spacing remains relatively large. Their absorption spectra are therefore line spectra which correspond to transitions to stable , associative excited states, but if a dissociative excited state is reached then the absorption spectrum becomes a continuum since such states have no vibrational levels. 

Figure 3.11 Potential energy diagram of a diatomic molecule. E is the potential energy and r the internuclear distance. The theoretical bond dissociation energy is in the ground state and E in the associative excited state Si, while T] is a dissociative excited state...

Because of the rapidity with which the first-formed excited states are usually converted to the Sx or Tx state, most photochemical reactions start from these states. There are exceptions. An obvious one is when an upper dissociative excited state, in which the molecule immediately fragments, is populated.49 Other exceptions occur when a molecule contains two different chromophores. For example, 20 reacts analogously to franj-stilbene (21) from its state but also undergoes photoreduction, a process typical of an n,n state (see p. 719).60... 

The angular distributions of recoiling iodine atoms were also measured for all four alkyl iodides studied. As is discussed elsewhere, such distributions provide information about the symmetry, configuration, and lifetime of the parent dissociative excited state. The details of these results will be presented in a future paper. Briefly, the angular distributions show that the transition dipole moment lies along the C—I bond and that the excited state breaks up on a time-scale short compared toarotational period. 

Photochemical processes are chemical reactions in which excited states resulting from absorption of light are involved. A primary photochemical process arises when decomposition or dissociation of the molecule occurs as a direct consequence of the absorption of radiation. In this case the potential energy curves are usually schematized as shown in Fig. 4. Figure 4(b) shows electronic excitation resulting from the formation of a dissociative excited state such that the atoms repel each other at any separation distance R. Figure 4(a) shows that a stable excited state is reached but decomposition of the molecule occurs because absorption is... 

Fig. 4. Photodissociation of a molecule (a) by excitation above the dissociative limit of an upper state (b) through a dissociative excited state.

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.

Contrary to DEA and DD, no anion is produced during the DI process, but this latter involves an electronic transition of the molecule toward a dissociative excited state by nonresonant inelastic scattering of the incident electron, as in DD. This mechanism does not show any sharp maximum in the yield function as observed with DEA. In fact, the yield functions from Dl and nonresonant DD processes increase monotonically with electron energy from 12-15 eV and reach a broad maximum at higher energies (30-60 eV) (Bass and Sanche 2003, Sanche 2002). DEA, DD, and Dl are the main processes producing ionic desorption by electron impact below 70 eV. 

About 75% of the bleached absorptions recover within 200 ps, which shows that 25% loss of the parent molecules were driven to the CO loss channel, since no long-lived non-dissociative excited states were present. This result was consistent with earlier quantum yield measurements. The naked 16-valence-electron complex Rh(Cp)(CO) was not directly observed. The early time ps-TRIR spectra show broad featureless transient absorptions due to the formation of electronically and vibrationally excited states. On the nanosecond timescale, Rh(Cp)(CO)(alkane) converts to Rh(Cp)(CO)(alkyl)H. We have recently repeated these experiments and these data are shown in Figure 9, from which the rate of C-H activation from the solvated intermediates can be measured. 

The nature of the dissociative excited state is still being discussed. For instance, for Cr(CO)6, the dissociative excited state was initially believed to be a dd-state however, calculations by DFT and TD-DFT methods indicated that the lowest excited state may have a Cr-to-CO MLCT character. 

Phenolate. The photoionization of phenols and phenolates continues to be the subject of studies to understand the photophysics and photochemistry involved. This interest is related to the application of phenols as antioxidants in organic materials and polymers,as well as to the fact that phenols are important constituents of many biochemical systems," and that OH-bond dissociation of the aromatic amino acid tyrosine may play an important role in protein photodegradation. Essential questions in these studies are whether the photoionization is mono- or bi-photonic and what is the nature of the dissociative excited state. 

The fluorescence induced by the second laser allows the accurate determination of the molecular parameters for those excited states on which the fluorescence transitions from are terminating. The LIF method can therefore be extended by stepwise excitation to the investigation of many molecular states which may not even have been found before. Of particular interest are dissociating excited states with repulsive potential curves below bound states E. These continuous states often cannot be studied by direct absorption from the ground state because the Franck-Condon factors for the transitions may be quite small. As an example of such investigations we mention the two-step excitation of the iodine molecule I2 (Fig.8.29). Selected (V, J ) levels in the B rig state are populated by optical pumping with a cw dye laser. Starting from these levels a krypton laser excites further levels in a... 

Since all of the low-lying singlet states of ammonia are planar (Ds] ), the two types of dissociation are most easily considered in terms of C2v symmetry. The first excited state of NH3 is 1 A, and thi s state correlates with Bx NH2 and 1 H. On the other hand, the 1 A 2 state correlates either with the ground state (X of NH and the dissociative excited state (b of H2 or... 

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