Lifetime electronically excited states

The lifetime of an analyte in the excited state. A, is short typically 10 -10 s for electronic excited states and 10 s for vibrational excited states. Relaxation occurs through collisions between A and other species in the sample, by photochemical reactions, and by the emission of photons. In the first process, which is called vibrational deactivation, or nonradiative relaxation, the excess energy is released as heat thus... 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

The energy q of a nuclear or electronic excited state of mean lifetime t cannot be determined exactly because of the limited time interval At available for the measurement. Instead, q can only be established with an inherent uncertainty, AE, which is given by the Heisenberg uncertainty relation in the form of the conjugate variables energy and time,... 

Of the different kinds of forbiddenness, the spin effect is stronger than symmetry, and transitions that violate both spin and parity are strongly forbidden. There is a similar effect in electron-impact induced transitions. Taken together, they generate a great range of lifetimes of excited states by radiative transitions, 109 to 103 s. If nonradiative transitions are considered, the lifetime has an even wider range at the lower limit. 

One can expect that the analysis of continuous distributions of electronic excited-state lifetimes will not only provide a higher level of description of fluorescence decay kinetics in proteins but also will allow the physical mechanisms determining the interactions of fluorophores with their environment in protein molecules to be elucidated. Two physical causes for such distributions of lifetimes may be considered ... 

As described in Chapter 3, the products of some chemical reactions are initially produced in electronically excited states. If the excited state has a sufficiently short radiative lifetime, it will emit light faster than collisional quenching by air molecules can occur (see Problem 1). The effective concentration of the emitting species (and hence emitted light intensity) is proportional to the concentrations of the reactants. As a result, the chemiluminescence intensity can be used to monitor one of the reactants if the second reactant is kept at a constant (excess) concentration. 

So far the methods described for measuring excited state lifetime, and hence reactivity, have been indirect methods that rely on a comparison with some standard le.g. actinometer quantum yield or quenching rate constant) that has already been measured. A direct method for measuring the lifetime of short-lived species produced photochemically is flash photolysis. This is a very important technique in photochemistry, though only the basic ideas as they apply to mechanistic studies are outlined here. In flash photolysis a high concentration of a short-lived species (electronically excited state or... 

Table V-3. Lifetimes of Some Electronically Excited States of N2...

Electronically excited states of ions produced by electron-impact ionization, for which various ion-neutral reactions have been studied, are listed in Table I. Additional information included in Table I, such as ion lifetimes, are discussed in subsequent sections. 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

Utilizing ionization efficiency curves to determine relative populations of vibrationally excited states (as in the photoionization experiments) is a quite valid procedure in view of the long radiative lifetime that characterizes vibrational transitions within an electronic state (several milliseconds). However, use of any ionization efficiency curve (electron impact, photon impact, or photoelectron spectroscopic) to obtain relative populations of electronically excited states requires great care. A more direct experimental determination using a procedure such as the attenuation method is to be preferred. If the latter is not feasible, accurate knowledge of the lifetimes of the states is necessary for calculation of the fraction that has decayed within the time scale of the experiment. Accurate Franck -Condon factors for the transitions from these radiating states to the various lower vibronic states are also required for calculation of the modified distribution of internal states relevant to the experiment.991 102... 

The energy levels and eigenfunctions, obtained in one or other semi-empirical approach, may be successfully used further on to find fairly accurate values of the oscillator strengths, electron transition probabilities, lifetimes of excited states, etc., of atoms and ions [18, 141-144]. 

There are numerous needs for precise atomic data, particularly in the ultraviolet region, in heavy and highly ionized systems. These data include energy levels, wavelengths of electronic transitions, their oscillator strengths and transition probabilities, lifetimes of excited states, line shapes, etc. [278]. 

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