Excited state, long lived

Stable and metastable states have been characterized for several negatively charged atoms and molecules both in their ground and/or electronically excited states. Long-lived electronically excited molecular systems, where the anionic electronic ground state is not bound, do exist and have been characterized experimentally. A detailed discussion and a full presentation of the examples known are reviewed in Refs. [5,6]. In this section, we are treating the case of the CS- anion. 

The development of techniques to prepare well-characterized targets of species in excited states (long-lived metastable atoms and molecules, electronically excited atoms and molecules, and vibrationally excited, hot molecules) for electron and ion collisions, including attachment and dissociative attachment studies and spectroscopic measurements. 

Bimolecular Reactions from Upper Triplet States. Other cases of sensitization by second excited triplet states have not yet come to light however, several bimolecular reactions of this sort have been reported. Since an upper excited state that lives long enough to undergo a bimolecular reaction should also be capable of transferring energy, these reactions will be discussed briefly. 

In this connection, the suggestion by Aquilanti et al. (1967) that the excited species formed in the deuteron transfer to alkanes from are produced either in a state that undergoes a fast, pressure-independent dissociation, or in states long-lived enough to allow collisional... 

According to this mechanism, the absorption of light produces an excited singlet state of the monomer which may either fluoresce [Eq. (6.67)] or be converted to an excited (and long-lived) triplet state [Eq. (6.68)]. The latter may be regarded as a diradical, that is, CH2-C(H)X. Attack on the monomer by this diradical ultimately yields two monoradicals [Eqs. (6.69) and (6.70)], which, in turn, initiate polymerization. For both types of direct photoinitiation the rates are proportional to the light intensity and to the extinction coefficient of the monomer (see later). 

The two main limitations of the current implementation of the nuclear-ensemble approach are clear in the simulations for furan First, the lack of vibrational resolution while the experimental results show a vibrational structure near the maximum, the simulations predict only the envelope of the band (the apparent oscillations are numerical noise). Second, the wrong band width for long-lived states long-lived states give rise to very thin peaks in the spectrum, which are not correctly described in the simulations. Both limitations are caused by the overlap-function approximation (Eq. 15), which neglects the excited-state wave packet evolution (see discussion in Sect. 2.6). 

Final state analysis is where dynamical methods of evolving states meet the concepts of stationary states. By their definition, final states are relatively long lived. Therefore experiment often selects a single stationary state or a statistical mixture of stationary states. Since END evolution includes the possibility of electronic excitations, we analyze reaction products in terms of rovibronic states. 

The first excited singlet state, 2 Sq, is also metastable in the sense that a transition to the ground state is forbidden by the Af selection rule but, because the transition is not spin forbidden, this state is not so long-lived as the 2 Si metastable state. 

We will first consider possible assignments for the fluorescing states in laser-excited PuF6(g) based on available energy level structure and thermodynamic information. We will then consider some of the implications of the long-lived PuF6 fluorescence we have observed in terms of potential photochemical separation processes. 

Research Opportunities. The presence of a long-lived fluorescing state following either 532 nm or 1064 nm excitation of PuF6(g) provides a valuable opportunity to study the extent to which electronic energy in a 5f electron state is available in photochemical and energy transfer reactions. Such gas phase bimolecular reactions would occur in a weak interaction limit governed by van der Waals forces. Seen from the perspective of potential photochemical separations in fluoride volatility... 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

For nuclear y-resonance absorption to occur, the y-radiation must be emitted by source nuclei of the same isotope as those to be explored in the absorber. This is usually a stable isotope. To obtain such nuclei in the desired excited meta-stable state for y-emission in the source, a long-living radioactive parent isotope is used, the decay of which passes through the Mossbauer level. Figure 3.6a shows such a transition cascade for Co, the y-source for Fe spectroscopy. The isotope has a half-life time //2 of 270 days and decays by K-capmre, yielding Fe in the 136 keV excited state ( Co nuclei capmre an electron from the K-shell which reduces the... 

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