Electronically excited species

For electronically excited species, the emitted light can be used for spectroscopic purposes, as in fluorescence analysis. 

Emission of light from electronically excited species produced in a chemical reaction. 

Chemiluminescence is the term used to describe chemical reactions that result in the formation of vibrationally or electronically excited species that subsequently undergo emission... 

Where the products are not indicated the rate coefficient refers to total removal of the electronically excited species. 

The quantised energy levels of matter have a separation that is of the same order as the energy of visible or ultraviolet light. Thus the absorption of visible or ultraviolet light by matter can excite electrons to higher energy levels, producing electronically-excited species. 

An electronically-excited species is usually associated with an excess of vibrational energy in addition to its electronic energy, unless it is formed by a transition between the zero-point vibrational levels (v = 0) of the ground state and the excited state (0 —> 0 transition). Vibrational relaxation involves transitions between a vibrationally-excited state (v > 0) and the v = 0 state within a given electronic state when excited molecules collide with other species such as solvent molecules, for example S2(v = 3) - Wr> S2(v = 0). 

Luminescence is an emission of ultraviolet, visible or infrared photons from an electronically excited species. The word luminescence, which comes from the Latin (lumen = light) was first introduced as luminescenz by the physicist and science historian Eilhardt Wiedemann in 1888, to describe all those phenomena of light which are not solely conditioned by the rise in temperature , as opposed to incandescence. Luminescence is cold light whereas incandescence is hot light. The various types of luminescence are classified according to the mode of excitation (see Table 1.1). 

In addition to providing a novel approach to the preparation of chiral compounds, this type of chemistry may allow one to inquire into the subtle stereochemical details of some crystal-state reactions. For example, what are the approach geometry and the preferred side of attack in the addition of bromine to a chiral olefin (259) What can be learned of the geometry of the labile electronically excited species involved in (2 + 2) photocycloaddition reactions (260) ... 

C. C. C. Vidigal, A. Faljoni-Alario, N. Duran, K. Zinner, Y. Shimizu, and G. Cilento, Electronically excited species in the peroxidase catalyzed oxidation of indoleacetic acid. Effect upon DNA and RNA, Photochem. Photobiol. 30, 195-198 (1979). 

The transfer of excitation energy present in an atom or molecular species in an excited singlet state to another atom or molecular species, generating an electronically excited species in a triplet state. 

Because electron transfer chemiluminescence is the only kind of chemiluminescence where chemical bond breaking or bond forming is not evident,18 it affords an opportunity to obtain evidence, under new conditions, as to how reactions of ground-state molecules produce electronically excited species. It may also provide the opportunity for investigating excited states not reached efficiently by absorption of radiation and it should permit the comparison of excited states produced photochemically and nonphotochemically. 

In complete equilibrium, the ratio of the population of an atomic or molecular species in an excited electronic state to the population in the groun d state is given by Boltzmann factor e — and the statistical weight term. Under these equilibrium conditions the process of electronic excitation by absorption of radiation will be in balance with electronic deactivation by emission of radiation, and collision activation will be balanced by collision deactivation excitation by chemical reaction will be balanced by the reverse reaction in which the electronically excited species supplies the excitation energy. However, this perfect equilibrium is attained only in a constant-temperature inclosure such as the ideal black-body furnace, and the radiation must then give -a continuous spectrum with unit emissivity. In practice we are more familiar with hot gases emitting dis-... 

Due to differences in charge distribution in the various energy states of a molecule, the electronically excited species can be completely different in its chemical and physical behaviours as compared to the normal ground state molecule. Molecular geometries and internuclear force constants also change. 

The ionization potential of an electronically excited species may be lower than that of the ground-state species by as much as the excitation energy. Determination of the appearance potential for the mass 32 peak... 

If electronically excited species can be made to give up their excess energy to some active material, then their concentration may be determined by measurement of the rate of heat liberation. The method is, of course, well established for the measurement of oxygen and hydrogen atom concentrations, and the most accurate experimental technique is to use the isothermal hot-wire calorimeter developed by Tollefson and LeRoy.42 The amount of power needed to maintain a catalytic probe at a constant temperature is reduced if heat is liberated at the probe, and no correction is needed for heat losses. The flow of energy-rich species, [Pg.325]

Vibrationally excited species appear. Ion-molecule reactions. Dissociation of electronically excited species to give radicals. 

The translational temperature Tt plays an important kinetic role. At high temperatures chemical reactions are fast, and — in view of the decreasing rate of surface recombination of atoms — the energy exchange of the system with the environment becomes slower. Consequently, the theoretical model can be applied to such systems (for comparison see Table 1 in43)). The actual equilibrium concentration of the volatile reaction products — CN in the present case - may be reduced by dissociative de-excitation of electronically excited species (cf. also the system C/H2). 

The observation of molecular luminescence at electrode solution interfaces results from high-energy annihilation reactions between electrochemically generated radical ions that result in the formation of an electronically excited species [6-16], The radical ions can be generated at two separate electrodes in close proximity to one another or at the same electrode by alternating between reductive and oxidative potentials. This is particularly useful when the radical ions are unstable since they can be produced in situ immediately prior to, or during, the reaction. The general mechanism of an ECL reaction is as follows. 

The nitric oxide reacts with ozone to produce an electronically excited specie (N02 ) that decays back to its ground state emitting light in the infrared spectral region (600-2800 nm). 

Another type of reaction that may be used to create reactants is the collisional dissociation of a molecule by some electronically excited species, usually an inert gas atom [28]. Uranium atoms have been generated [29] by the collisional dissociation of uranocene by metastable argon atoms. This is an attractive alternative to the problems associated with the vaporisation of uranium. 

Electronically excited species are generally produced by direct excitation or as the products of photolysis. In addition to conventional light sources, fixed frequency or tuneable visible and ultraviolet lasers are regularly used for single photon excitation or photolysis, as well as... 

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