Vibrational predissociation spectra

Fig. 14 Top panel. Time-of-flight mass spectra illustrating formation of H2 adducts to ESI generated ions in a 10 K ion trap (a) the dipeptide (GlyGlyH ) and (b) the tripeptide indicated in the inset. Bottom panel. Vibrational predissociation spectrum of H2-tagged GlyGlyH (solid black line). The gray overlay in trace (c) is the previously reported IRMPD spectrum [146] of the bare ion at 300 K. Stars in top panel indicate presence of naturally occurring C isotopologues...

There are now a number of experiments which show predissociation in vibrationally excited van der Waals molecules. Smalley, Wharton, and Levy at Chicago have generated l2 He in a supersonic nozzle and excited the iodine in the complex to high-u Vibrational levels of its B IT state. Observed line broadening in the fluorescence spectrum of 12 He corresponded to vibrational predissociation lifetimes x of about lO " to lO " s depending on the u level of 12 Other work at Chicago has shown vibrational predissociation effects in other I2 containing complexes. 

Figure 2. Laser-induced fluorescence and action spectra acquired in the ICl B—X, 2-0 spectral region. The LIF spectrum, panel (a), is dominated by the intense I Cl and I Cl transitions at lower energies. Features associated with transitions of the T-shaped He I C1(X, v" = 0) and hnear He I C1(A, v" = 0) and He I C1(X, v" = 0) conformers are observed to higher energy and are rs 100 times weaker than the monomer features. The action spectra, (b), was recorded by probing the I Cl E—B, 10-1 transition, or the Av = — 1 vibrational predissociation channel.

Sometimes two higher electronic levels, stable state II and unstable state III may exist quite close to each other (Fig. 5.1(d)). By absorption of light, the molecule is raised to higher stable electronic state II. If the oscillations are relatively slow, there is a chance of the molecule of shifting from stable state II to unstable state III. If such a shift takes place, the molecule would dissociate producing atoms or radicals. The spectrum would show fine structure at lower levels of vibrations followed by a continuum. This is called predissociation. 

The stationary excitation spectrum of the predissociated C state of Na is presented in the insert of Fig. 21. It shows a pronounced vibrational... 

Photodissociation cross sections for CH2CHO and CD2CDO are shown in Fig. 1. Both spectra consist of sharp, extended vibrational progressions indicative of predissociation. A comparison of Fig. 1 with the LIF and absorption spectra [5, 7, 8] shows that we observe predissociation all the way to the origin of the B X band this is labeled peak 1 in Fig. 1. We observe photodissociation over the entire range of the absorption band. However, peaks 1-7 are the only peaks seen in the LIF spectrum. Peaks 1, 2,5, and 6 are particularly prominent in the LIF spectrum the latter three peaks are assigned... 

The NH(c ri) lluorescencc excitation spectrum shows diffuse vibrational structure corresponding to the absorption spectrum below 1450 A, while the spectrum is continuous above 1450 A. The results indicate that the NH(< n) state may be formed from predissociation of the electronically excited HN3 below the incident wavelength 1450 A, while the NHtc ri) is dissociated directly above 1450 A. 

In some molecules there is another, slower dissociation path known as predissociation. In this case the crossing to the dissociative state is the rate-limiting step, and this may take place after many vibrations in the absorption spectrum the vibrational sub-levels remain sharp, but the rotational levels are blurred (Figure 4.28). 

If one adopts the correct point of view that the complete wave function of any state of a diatomic molecule has contributions from all other states of that molecule, one can understand that all degrees of perturbation and hence probabilities of crossover may be met in practice. If the perturbation by the repulsive or dissociating state is very small, the mean life of the excited molecule before dissociation may be sufficiently long to permit the absorption spectrum to be truly discrete. Dissociation may nevertheless occur before the mean radiative lifetime has been reached so that fluorescence will not be observed. Predissociation spectra may therefore show all gradations from continua through those with remnants of vibrational transitions to discrete spectra difficult to distinguish from those with no predissociation. In a certain sense photochemical data may contribute markedly to the interpretation of spectra. 

Hi) If, for example, predissociation occurs with high probability at v = a, then excitation to o > a will lead to dissociation provided vibrational energy is lost in small increments and vibrational relaxation is slow relative to dissociation. The latter will necessarily be true if the spectrum is diffuse when o = a. 

If it is correct that the excited states of ethylene are twisted the torsional vibration must be active in the transitions. There is no direct evidence of this, though the possibility exists that the diffuseness of the spectrum owes less to predissociation than to an unresolved thicket of lines resulting from the torsional motion and other vibrational substructure. The torsional vibration does appear in association with the Rydberg transition at 1750 A (Wilkinson and Mulliken, 1955). 

Fig. 15.8. Schematic one-dimensional illustration of electronic predissociation. The photon is assumed to excite simultaneously both excited states, leading to a structureless absorption spectrum for state 1 and a discrete spectrum for state 2, provided there is no coupling between these states. The resultant is a broad spectrum with sharp superimposed spikes. However, if state 2 is coupled to the dissociative state, the discrete absorption lines turn into resonances with lineshapes that depend on the strength of the coupling between the two excited electronic states. Two examples are schematically drawn on the right-hand side (weak and strong coupling). Due to interference between the non-resonant and the resonant contributions to the spectrum the resonance lineshapes can have a more complicated appearance than shown here (Lefebvre-Brion and Field 1986 ch.6). In the first case, the autocorrelation function S(t) shows a long sequence of recurrences, while in the second case only a single recurrence with small amplitude is developed. The diffuseness of the resonances or vibrational structures is a direct measure of the electronic coupling strength.

Bound electronic states exhibit a discrete spectrum of rovibrational eigenstates below the dissociation energy. The interaction between discrete levels of two bound electronic states may lead to perturbations in their rovibrational spectra and to nonradiative transitions between the two potentials. In the case of an intersystem crossing, this process is often followed by a radiative depletion. Above the dissociation energy and for unbound states, the energy is not quantized, that is, the spectrum is continuous. The coupling of a bound state to the vibrational continuum of another electronic state leads to predissociation. 

Vissers GWM, Groenenboom GC, van der Avoird A (2003) Spectrum and vibrational predissociation of the HF dimer. I. Bound and quasibound states. J Chem Phys 119 277-285... 

The reverse process corresponding to the emission of radiation of the same frequency is one of the simplest modes of losing energy, as is observed in emission spectra. Another process is known as predissociation. A third fate of the absorbed energy may lead to direct photodecomposition. It occurs when the vibrational states in the excited state are Continuous and is signalled with a continuous absorption in the spectrum. 

Vibrational Constants and Dissociation Energy.—Apart from the upper limit of 93 kcal./mole set by the above-mentioned predissociation these constants cannot be obtained from the spectrum of SH alone as at least three vibrational bands are necessary for their derivation. By using the different zero point energy of the isotopic molecule, however, another relationship is introduced which makes the calculation possible. If we assume the same force constant for the two molecules it can be shown that... 

Attempts have been made to obtain the spectrum of the SeH radical in the photochemical decomposition of H Se but the only spectrum recorded on the plate was that of Se despite the fact that the amount of decomposition was considerably greater than with HjS. The absence of the spectrum of SeH may be explained if the predissociation occurs below the first vibrational level in this case, or if the SeH radical is chemically less stable, the latter explanation being probable in view of the decrease in stability from OH to SH, the former being observed for as long as i/ioth sec. after the flash. 

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