Lifetimes of Excited States

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. 

Recognise features which relate to the elucidation of the nature, energy and lifetime of excited-state species. 

With this method lifetimes of excited states in diatomic alkali molecules have been measured 122,123) means of the apparatus in Fig. 6. 

Since the lifetimes of excited states are small, by applying the steady state approximation, respective concentrations are obtained,... 

Determination of decay constants or lifetimes of excited states. Decay... 

The natural broadening which results from the finite lifetime of excited states. The energy of a state and its lifetime are related by the principle of uncertainty (section 2.2) which implies a minimal spread of the actual energy of any excited state of finite lifetime this gives an absolute limit to the width of atomic spectral lines. 

In a rigid system such as a glass or a polymer, the molecules M and Q are distributed at random and do not move, at least within the lifetimes of excited states. The distance distribution follows the Perrin law which is based on a very simple model. Take any excited molecule M, and ask if one quencher molecule Q happens to be within the volume of action defined by the centre-to-centre distance r. Should any molecule Q be found within this action volume, the molecule M is quenched instantaneously, but if there is no quencher Q within this space, then M emits as if no quenchers at all were present. Figure 3.39 gives a picture of the Perrin model. The mathemat-... 

The situation for reactions in solids is much more complex and is treated in a separate section (4.7.4, p. 153). Physical diffusion of molecules can be neglected within the lifetimes of excited states, but exciton interactions can become important. These have no counterpart in dark reactions and can lead to unusual photochemical properties in crystals and polymers. 

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]. 

Configurations with vacancies in inner shells possess a number of peculiarities their states are autoionizing they are short-lived relativistic effects are essential for them their energy spectrum has particular characteristics. Short lifetimes of excited states lead to large widths of relevant spectral lines. 

The lifetime of a separate atom in its ground state is infinite, therefore the natural width of the ground level equals zero. Typical lifetimes of excited states with an inner vacancy are of the order 10-14 — 10 16 s, giving a natural width 0.1 — 10 eV. The closer the vacancy is to the nucleus, the more possibilities there are to occupy this vacancy and then the broader the level becomes. That is why T > Tl > Tm- Generally, the total linewidth T is the sum of radiative (Tr) and Auger (T ) widths, i.e. 

Much of the work in this area has centred around efforts to optimize the photochemical and redox properties of the Ru11 complexes which are related to water cleavage reactions, e.g. lifetime of excited state, absorption maxima, etc. A detailed account of these properties is found in Chapter 8.3 and hence it is only intended here to present the results of these studies on hydrogen producing systems. 

If the absorbed energy by the molecule is not sufficient for ionization or disas-sociation of the molecule, it will remain at an excited state for a certain amount of time, which is described as excited-state lifetime. Afterwards, it reemits as heat and/ or light. The light emitted by this process is what we call fluorescence or phosphorescence, depending on the lifetime of the excited state. Short lifetime of excited states HO -10 sj lead to processes called fluorescent, hence long lifetime (>10 6s) of excited states lead to processes called phosphorescent. 

The nanostructure dependence of the excited state dynamics can be derived from the interaction of the electronic excitation with the surrounding environment and its phonon modes. A variety of nanophenomena, particularly, the lifetime of excited states of lanthanide ions in nanostructures may exhibit strong size-dependence (Prasad, 2004). Energy transfer rate and luminescence efficiency in lanthanide activated phosphors are also sensitive to particle size and surrounding environment. 

It is important to remember that the splitting of the t2g orbitals also occurs, but the splitting is smaller than that of the eg orbitals. Although it might be expected that complexes of dl and d2 ions would undergo Jahn-Teller distortion, such distortion would be extremely small. In fact, there are some other problems in studying this type of distortion because of the short lifetimes of excited states and rearrangement of the complexes. 

With the advent of convenient techniques in recent years, it has become popular to measure "lifetimes" of excited states. Decay characteristics are useful adjuncts to quantum yields as they provide further knowledge about the mechanism of reaction, particularly with regard to evaluation of rate constants for excited molecule reactions. Only when the decay is exponential can a unique lifetime be defined. 

Photochemical reactions involving photo-excited states can also be catalyzed as well as the thermal reactions of ground states. However, the lifetimes of excited states are usually very short, particularly for the singlet excited states, and accordingly reactions of the excited state should be fast enough to compete with the decay of the excited state to the ground state (typically the lifetime is 10" -10 " s). Hence there seems to be little chance of catalysis to accelerate the reactions of excited states, which are already fast. There are many cases, however, such that photochemical reactions can be accelerated by some added substances which act as catalysts in the photochemical reactions [62-65]. Photoinduced electron transfer reactions can also be accelerated by the presence of an appropriate catalyst [52]. 

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