A state predissociates

It is known (Herzberg 1960 Ziegler 1985) that ammonia in the A state predissociates via formation of NH2 and H. Verification of the predissociation of NH3 to NH2 + H being a contributor to the mechanism of protonated cluster formation through the A state is seen from the data at v = 2 Figure 6-5. Here, the channel due to predissociation and subsequent ionization becomes readily observable. In the case of v = 0, predissociation in the cluster may be endothermic, as revealed by the appearance potential measurements reported in the literature (Misaizu et al. 1993). 

For the Hg- -Ars cluster the situation is quite different. The experiments observe fluorescence shifted by about 200 cm to the blue (as predicted by our estimates) but none to the red. This is not so easy to understand. All our calculations predict a red-shifted state in the region of -100 to -150 cm". One possible explanation is that for higher clusters the A state predissociates electronically to produce Hg ( Po) atoms. 

Unfortunately, predissociation of the excited-state limits the resolution of our photodissociation spectrum of FeO. One way to overcome this limitation is by resonance enhanced photodissociation. Molecules are electronically excited to a state that lies below the dissociation limit, and photodissociate after absorption of a second photon. Brucat and co-workers have used this technique to obtain a rotationally resolved spectrum of CoO from which they derived rotational... 

The loss of the B state population depends on both the dynamics of the vibrational relaxation and the strength of the coupling of the B state to the repulsive a/a states. In the simulations, the predissociation of the B state molecules is described with the help of the Landau-Zener theory. There, the coupling strength is given by the perturbation energy. Furthermore, the probability for predissociation also depends on the (classical) velocity v t) which relates this process to the vibrational relaxation dynamics of the B state. The theoretical model uses a friction coefficient a to describe the latter process. 

The decay of the B state population due to predissociation is linked to the dynamics of the vibrational relaxation. It could be shown that a longer relaxation time also resulted in a delayed predissociation of the B state iodine molecules. As soon as the vibrational relaxation is completed, the predissociation is solely determined by the coupling strength between the bound B and the repulsive a/a1 states. The coupling of these states is again a function of the interaction with the surrounding cage. 

Figure 9. Femtosecond dynamics of an elementary reaction (I2 — 21) in solvent (Ar) cages. The study was made in clusters for two types of excitation to the dissociative A state and to the predissociative B state. The potentials in the gas phase govern a much different time scale for bond breakage (femtosecond for A state and picosecond for B state). Based on the experimental transients, three snapshots of the dynamics are shown with the help of molecular dynamics simulations at the top. The bond breakage time, relative to solvent rearrangement, plays a crucial role in the subsequent recombination (caging) dynamics. Experimental transients for the A and B states and molecular dynamics simulations are shown.

In order to discuss predissociation dynamics, it is also important to derive a state-specific rate constant based on the measurements of absorption and dissociation cross sections. 

The fact that Callear and Smith found no emission from the C2n state with v > 0 presents strong evidence for predissociation from these states. Furthermore, predissociation must occur for three additional reasons (/) The addition of N2 enhanced the y-band emission much more than predicted for no predissociation (2) the e bands were stronger than the 8 bands, which is contrary to expectation and (5) N2 reduces, though it does not eliminate, the condensable products of the photolysis. If we assume that, in the presence of N2, predissociation still occurs from levels with v > 0, then the product reduction results from quenching of the v = 0 level, which undergoes a weaker predissociation. The ground-vibrational level predissociation step is... 

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. 

The transition to the C B- state of H 0 was achieved by a two photon absorption of KrF laser light near 248 nm (32). The OH(A-X) fluorescence excitation spectrum in the 247.9-248.5 nm range follows the rotational structure of the C B -+ X A transition. However, (i) the OH(A-X) fluorescence spectrum produced by the two photon dissociation of H 0 has a maximum population at N = 14, while single photon absorption near 124 nm generates OH fluorescence spectrum with a maximum population at N = 20 (ii) only absorption to Ka= 1 (and not Ka= 0 where K is the rotational angular momentum about the a axis) of the ClB-L state predissociates into 0H(a2%) + H probably through the B A state. Apparently, the two-photon absorption near 248 nm predominantly populates the c b state, while the single photon process populates the B A near 124 nm. 

C2N2(B Au) have shown that this state predissociates via a radia-... 

The probability of the radiationless transition leading to predissociation is governed by the Franck Condon principle and by a group of selection rules first given by Kronig (1930). One reason why predissociation is so widespread is that, especially for non-linear molecules, the selection rules are extremely permissive. A striking example of a forbidden predissociation from the linear 2H (2I I) Renner state of the HCO radical has been described by Ramsay (1959). 

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