Predissociation

A vibration rotation level of a diatomic molecule which lies above the lowest dissociation limit may be quasibound and able to undergo spontaneous dissociation into the separate atoms. This process is known as predissociation, and two different cases may be distinguished for diatomic molecules, as we will see shortly. Predissociation does not normally play an important role in rotational spectroscopy but merits a brief discussion here for the sake of completeness. 

The two cases which arise in diatomic molecules are rotational predissociation and electronic predissociation the latter case applies only to excited electronic states. We deal first with rotational predissociation, with can arise for either ground or excited states. The potential energy curve shown for a Morse oscillator in section 6.8 is for a rotationless (./ = 0) molecule. For a rotating molecule, however, we must add a centrifugal term to the potential, 

The second type of predissociation observed for diatomic molecules is known as electronic predissociation the principles are illustrated in figure 6.28. A vibrational level v of a bound state E lies below the dissociation asymptote of that state, but above the dissociation asymptote of a second state E2. This second state, E2, is a repulsive state which crosses the bound state E as shown. The two states are mixed, and the level v can predissociate via the unbound state. It is not, in fact, necessary for the potential curves of the two states to actually cross. It is, however, necessary that they be mixed and there are a number of different interaction terms which can be responsible for the mixing. We do not go into the details here because electronic predissociation, though an important phenomenon in electronic spectroscopy, seldom plays a role in rotational spectroscopy. Since it involves excited electronic states it could certainly be involved in some double resonance cases. 

The primary observable result of predissociation is, of course, line broadening. A transition involving a predissociating level has a width at half-height E given 

We deal first with the potential energy, described by the second and third terms in (6.423). If R is the distance between the two nuclei, we see from figure 6.29 that 

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Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

Another example of current interest is the vibrational predissociation of hydrogen bonded complexes such as (HF) ... 

While monomolecular collision-free predissociation excludes the preparation process from explicit consideration, themial imimolecular reactions involve collisional excitation as part of the unimolecular mechanism. The simple mechanism for a themial chemical reaction may be fomially decomposed into tliree (possibly reversible) steps (with rovibronically excited (CH NC) ) ... 

Pine A S, Lafferty W J and Howard B J 1984 Vibrational predissociation, tunneling, and rotational saturation in the HF and DF dimers J. Chem. Phys. 81 2939-50... 

Sloane C S and Hase W L 1977 On the dynamics of state selected unimolecular reactions chloroacetylene dissociation and predissociation J. Chem. Phys. 66 1523-33... 

Rice O K 1971 On the relation between unimolecular reaction and predissociation J. Cham. Phys. 55 439-46... 

Ewing G E 1980 Vibrational predissociation in hydrogen-bonded complexes J. Cham. Phys. 72 2096-107... 

Huang Z S, Jucks K W and Miller R E 1986 The vibrational predissociation lifetime of the HF dimer upon exciting the free-H stretching vibration J. Cham. Phys. 85 3338-41... 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

A nice example of this teclmique is the detennination of vibrational predissociation lifetimes of (HF)2 [55]. The HF dimer has a nonlinear hydrogen bonded structure, with nonequivalent FIF subunits. There is one free FIF stretch (v ), and one bound FIF stretch (V2), which rapidly interconvert. The vibrational predissociation lifetime was measured to be 24 ns when excitmg the free FIF stretch, but only 1 ns when exciting the bound FIF stretch. This makes sense, as one would expect the bound FIF vibration to be most strongly coupled to the weak intenuolecular bond. 

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

One of the early examples for kinetic studies on the femtosecond time scale is the photochemical predissociation of Nal [74] ... 

Lifetimes of 1 ps translate into linewidths of about 5 cm Thus, Ime-shape methods are ideally suited to measure very fast decay processes, in particular predissociation of excited species. An example is the... 

For complexes such as Ar-H2, Ar-HF and Ar-lTCl, vibrational predissociation is a very slow process and does not cause appreciable broadening of the lines in the infrared spectmm. Indeed, for Ar-ITF, ITuang et al [20] showed that... 

Spectra of two different bands of (1TF)2, showing the difference between predissociated and undissociated spectra [21], are shown in figure Cl. 3.5... 

The occurrence of predissociation opens up a new family of observable quantities. It is possible to measure not only linewidths or lifetimes, but also the internal state distributions of the fragments. All these quantities are sensitive to the intennolecular potential and can be used to test or refine proposed potential surfaces. 

Figure C 1.3.5. Spectra of two different infrared bands of HF dimer, corresponding to excitation of the bound (lower panel) and free (upper panel) HF monomers in the complex. Note the additional line width for the bound HF, caused by vibrational predissociation with a lifetime of about 0.8 ns. (Taken from 1211.)...

Soum and Fontanillet prepared a living polymer of 2-vin yl pyridine using benzyl picolyl magnesium as the initiator. The values of were measured experimentally for polymers prepared with different concentrations of initiator and different initial concentrations of monomer. The results are given below calculate the theoretical molecular weights expected if polymerization proceeds completely from 100% predissociated initiator and compare the theoretical and experimental values ... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

The complex EtCl2SiRhH(Cl)(PPh3)2, unlike the Cl3Si analog, has identical activity to (Ph3P)3RhCl. It has been suggested that both compounds predissociate to afford the same active species (135). 

G. W. Hoffman T. J. Chuang, and K. B. Eisenthal, Picosecond smdies of the cage effect and collision induced predissociation of iodine in liquids. Chem. Phys. Lett. 25(2), 201-205 (1974). 

B. Decay of Metastable State through Tunneling (Predissociation)... 

Needless to say, tunneling is one of the most famous quantum mechanical effects. Theory of multidimensional tunneling, however, has not yet been completed. As is well known, in chemical dynamics there are the following three kinds of problems (1) energy splitting due to tunneling in symmetric double-well potential, (2) predissociation of metastable state through... 

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