Excited state radiative

The finite lifetime of each excited state is the reflection of a fundamental law of nature - tendency towards minimum total energy of a system. The quantum mechanical system tends to occupy the state in which its total energy would be minimal. However, the transition of an atom to the lowest (ground) state depends on many circumstances (first of all, on the sort of excited configuration, on the presence of external fields, on the character of the matter itself - density of gas, vapours or plasma, etc.). There are two main channels of decay of the excited states radiative and radiationless. In the first case the electronic transition from the higher to the lower state is connected with the radiation of one or several quanta of... 

The energy stored in an excited state is dissipated by the unimolecular radiative and radiationless relaxations. For strongly allowed electronic transitions, Strickler and Berg have obtained an expression for the rate constant of the excited-state radiative decay (Equation 6.67).31... 

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

Lower state Excited state Radiative lifetime (s) A (nm) Name... 

Afterglow Kinetics.—Introduction, Combination of atoms in their ground electronic states can lead to the formation of the corresponding diatomic molecule in an electronically excited state. Radiative transitions to a lower-lying state may then lead to chemiluminescent ranission, known as an after ow. Examples include the combination of two nitrogen atoms, a nitrogen atom with an oxygen atom, and two chlorine or bromine atoms ... 

Excited state radiative lifetimes tqi for fluorescence were calculated using the relationship tqi = 1/Aqi =... 

It follows from Eq. (14.8) that the decay parameters of the nucleus excited state (radiative lifetime and shift of excited level) are determined by the position of poles cd = co + ico" of integrand function A(ft>) in upper semiplane of complex values Q). From the general structure of the integral (14.8) it follows that the total lifetime of the excited nucleus Ttot corresponds to the condition mco = co" = I /2Ttot-... 

Fig. 6.4 Modified Jablonski diagram for an organic molecule showing ground and excited states and intramolecular photophysical processes from excited states. Radiative ntK esses—fluorescence (hvf) and phosphorescence (hvp) are shown in straight lines, radiationless processes— internal conversion (1C), inter system crossing (ISC), and vibrational cascade (vc) are shown in wavy lines. Adapted with permission fiom (Smith MB, March J 2006 March s Advanced Organic Chemistry Reactions, Mechanisms and Stiucmres, 6th Ed., John Wiley, New York). Copyright (2007) John Wiley Sons...

Recent work by Lim and coworkers [11,12] on small thiones of C, and Cjv excited state symmetry has clearly demonstrated the symmetry requirements of vibronicaUy induced radiationless transitions. In the low pressure gas phase, Cjv molecules such as thioformaldehyde, HjCS, exhibit both strong Sj - Sq fluorescence and strong Tj - Sq phosphorescence. These molecules therefore exhibit rates of S, - Sq internal conversion and Tj - Sq intersystem crossing that are small in comparison with the excited state radiative decay rates, despite the fact that the density of states in Sq is sufficiently large that the molecule s radiationless transitions should fall into the statistical limit case. On the other hand similar thiones of C, excited state symmetry exhibit small quantum... 

Light absorption leading to electronic excitation is commonly accompanied by radiative reemission as the electron decays back to the ground potential energy surface. However, in certain cases (dependent on excited-state radiative lifetime and potential features to be described below) the system returns to the ground electronic surface without optical emission, a so-called radiationless transition. Such non-radiative transition processes are important features of reaction pathways on both ground and excited surfaces. 

In collision processes between slow highly charged ions (HCl) and neutral atoms or molecules, multiple electron capture into excited states of the ions is possible. Double capture is by far the most studied experimentally and theoretically [161] mainly because of the possibility to also reach those doubly-excited states radiatively from the ground state [16, 62]. Note that this research area is still very active, as illustrated by the recent reinvestigation of our pioneer work [161] on radiative transition probabilities... 

In tliese equations and are tire excited state populations of tire donor and acceptor molecules and and are tire lifetimes of tire donor and acceptor molecules in tire excited state tire notation is used to distinguish it from tire radiative constant (in otlier words for tire donor) is given by (C3.4.5) and tire... 

Nonradiative energy transfer is induced by an interaction between the state of the system, in which the sensitizer is in the excited state and the activator in the ground state, and the state in which the activator is in the excited and the sensitizer in the ground state. In the presence of radiative decay, nonradiative decay, and energy transfer the emission of radiation from a single sensitizer ion decays exponentially with time, /. 

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

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