Predissociation spectra

As we have seen in discussing the behavior of diatomic molecules, there is often great difficulty in identifying a predissociation spectrum since the appearance may at one extreme be essentially that of a truly discrete spectrum and at the other that of a true continuum. The facts of photochemistry may be of great use to the spectroscopist in distinguishing between predissociation and truly discrete spectra. 

Carrington A and McNab I R 1989 The infrared predissociation spectrum of H Accounts Chem. Res. 22 218-22... 

The ground state bending frequency V2(Ai) may be -1200 cm a characteristic separation between different bands In the predissociation spectrum of the A Ai transition (see... 

Figure 2. Section of the infrared predissociation spectrum of H+ recorded at high and low beam potentials.

Figure 3. Section of the predissociation spectrum of the D2H+ predissociation spectrum recorded by monitoring H+ (a) and D+ (b) ions.

The H+ infrared predissociation spectrum is so extensive that it is unlikely that a full assignment could ever be made the most that we can currently hope to achieve is an understanding of how such a spectrum could originate. An obvious first step to such an understanding would be to take a low resolution spectrum of the same region, in order to see whether any recognizable structure could be seen. Unfortunately our experiment has an inherently high resolution which prevents us... 

The predissociation spectrum of (SFe) is mainly determined by resonant dipole-dipole forces, which produce clear size-dependent splittings. Therefore, (SFe) possesses... 

B. Heymen, C. Liedenbaum, S. Stolte, and J. Reuss, Hole burning in the IR predissociation spectrum of SFe-dimers, Laser Chem. 8 275 (1988). 

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...

D2-predissociation spectrum of the tripeptide indicated at the top with all C and N. The spectra with heavy isotopes at the indicated positions were subtracted from trace (a) to obtain the color-coded difference spectra (b-e) in the four upper traces. The color coding of the traces and peaks corresponds to the functional groups as indicated in the top schematic structure. Reproduced from [149]... 

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... 

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.

The photodissociation of jet-cooled methyl from a flash pyrolysis source has also been studied at 216.3 nm using the hign-n Rydberg atom time-of-flight technique by Wilson and co-workers.113 The H-atom time-of-flight spectrum indicates that predissociation of Cl l s(/)2 ) at 216.3 nm predom-... 

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. 

Among them, Li-i-HP can be considered a benchmark model system [29, 30] because its low number of electrons makes possible to calculate accurate PES s. Its electronic spectrum has been meassured by Polanyi and coworkers [22], and has been recently very nicely reproduced using purely adiabatic PES s [31]. In the simulation of the spectrum[31], the transition lines were artificially dressed by lorentzians which widths were fitted to better reproduce the experimental envelop. The physical origin of such widths is the decay of the quasibound states of the excited electronic states through electronic predissociation (EP) towards the ground electronic state. This EP process is the result of the non-adiabatic cou-... 

In this work the electronic predissociation from the A,B and B states has been studied using a time dependent Golden rule approach in an adiabatic representation. The PES s previously reported[31 ] to simulate the experimental spectrum[22] were used. Non-adiabatic couplings between A-X and B-X were computed using highly correlated electroiric wavefunctions using a finite difference method, with the MOLPRO package[42]. 

Since the spectrum of the ferricenium cation is so distinctively different from that of ferrocene in halogenated solvents, no ion must be formed as yet and the 307 mu band must be due to a predissociative species probably close to that depicted as 3. 

The beginning of the continuum in the absorption spectrum of simple gas-phase molecules implies the dissociation of the bond in question. (Figure 4.3). In some molecules a blurring of the spectrum appears i before the dissociative continuum suggesting dissociation at a lower energy requirement- This is known as predissociation. An understanding of the... 

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