Single vibrational level fluorescence

If emission is from only one vibrational level of the upper electronic state it is referred to as single vibronic level fluorescence (or phosphorescence). 

Electronic relaxation in different excited vibronic levels corresponding to the same electronic configuration can be experimentally studied, provided that, as mentioned above, (1) single vibrational levels within the initial electronic state are populated and (2) the excited molecule decays nonradiatively on a timescale much shorter than the mean time between deactivating collisions or by other means such as infrared fluorescence [115]. For typical polyatomic molecules in the gas phase, a narrow-band optical excitation pulse (as small as 1 and shorter relative to the genuine decay times wiH result in the selection of a single vibronic state. U nder these conditions,... 

Alves, A.C., Hollas, J.M., Musa, M., and Ridley, T. (1985) The 370-nm electronic spectrum of tropolone evidence from single vibronic level fluorescence spectra regarding the assignment of some vibrational fundamentals in the X and A states. J. Mol. Spectrosc., 109, 99. 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

In the optical fluorescence quenching experiments on mixtures of NO( 4 2 +)+ N2, the former was produced in vibrational levels, v = 3,2 and 1, and transfers involving each of these levels were followed. It was found that more than 85 % of the transfers observed involved exchange of a single quantum, e.g. 

The laser-excited fluorescence from dibenzofuran in a biphenyl host at 4.2 K and three different sites in the lattice have been identified. The two-photon excitation spectrum of single crystals of carbazole at 4.2 K has been analysed in detail. Very extensive vibrationally unrelaxed fluorescence is observed following dye-laser excitation of single vibronic levels of naphthazanine and the rates and pathways of relaxation are examined by picosecond emission spectroscopy. The photophysics of rubrene peroxide have been measured for the first time by Bayrakceken. Picosecond fluorescence has been used to study the spectra and kinetics for Sj— So and Si transitions for... 

Because fluorescence and phosphorescence originate exclusively from the lowest vibrational level of the lowest excited singlet and triplet states, respectively, and terminate in any of a number of vibrational levels of the ground electronic state, there will be only one possible fluorescence band and one possible phosphorescence band observable from a single chemical species capable of fluorescing and/or phosphorescing. More than one of either is indicative of extraneous luminescing species, either impurities or products of photochemical reaction. 

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