Inverse predissociation

The emission from molecular halogens in the 3n0u+ state resulting from shock heating could, in principle, arise from both inverse predissociation and direct recombination on account of the large thermal populations of the excited atoms at these high temperatures. Emission from this state has been observed hitherto at high temperatures from iodine, bromine, and chlorine... 

A predissociation, which may or may not be related to the one just discussed, is observed in hot flames147 and in cool atomic flames148. For rotationless states the predissociating curve appears to cross the bound 2E+ state very near v = 2. The corresponding inverse predissociation has been proposed149,150 as an explanation for the observed overpopulation of the first and second vibrational levels of OH(2E+) in flames where there is a considerable excess population (over thermodynamic equilibrium) of O and H atoms. This process may produce a population inversion in nozzle expansion of a dissociated gas 15 x. 

The emission intensity in the (0,0) band of the C(2IJ)-X(2I1) system is independent of pressure, 0.2 < P < 10 torr153, indicating that no third body is involved in the inverse predissociation... 

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. 

Remarkably selective excitation of the A 3n (f = 6) level has been observed in a number of reaction systems. Some years ago, Herzberg [157] suggested that an inverse predissociation might be responsible, but this proposal is incompatible with the modern value of D0 (C2) and has been replaced by the proposal [153] that crossing from the 3S state takes place. The band progression from A 3Na(v = 6), often called the high-pressure bands, is observed when <0.01 % of CO in He is irradiated with a particles [158], The reaction... 

Adams has shown recently that It Is possible for an RRKM calculation based on a reasonable parameterization of the system to reproduce the llnewldth-derlved lifetime for a vdU molecule photodlssoclatlon (25). However, If the llnewldth-derlved lifetimes t actually are the Inverse predissociation rates, then the above data show clearly that the fundamental statistical assumptions of such an approach are qualitatively Incorrect for these vdW systems. Consider the following statistical predictions ... 

The rate constant of the red afterglow which arises from the reaction of atomic hydrogen, H( S), with BCI3 is A =46 s . An inverse predissociation mechanism is believed to be responsible, with a probable fast stage of reaction being ... 

Comparison of the potential-energy curves of O2 and Bri (shown in Figure 7) illustrates that population of vibrational levels up to v = 14 of the B state of O2 (the maximum observed experimentally) is energetically feasible from ground-state atoms. However, population of 02(B) from P + P atoms must occur via an inverse predissociation. Population of the A state of Bt2 from ground-state atoms is straightforward. However, formation of the less hi y populated B state of Bt2 is not so simple, levels above the dissociation limit being observed in emission. ... 

Fig. 29 a—c. Potential energy curves corresponding to different mechanisms of radiative recombination, a Recombination on a repulsive potential curve, b recombination on an attractive potential curve, c recombination via inverse predissociation... 

An example of radiative recombination via inverse predissociation without activation energy is the process... 

---