Lifetimes of Electronic Excited States

Radiative decay rates are also affected when an emitting state is comprised of a quantum mechanical mixture of two or more pure electronic states that have different radiative decay rates and different shift behavior with pressure. Many emitting states, for example, consist of two or more electronic states coupled 

In this section we review recent high pressure studies of luminescence lifetimes in transition metal and lanthanide systems. We focus on individual electronic transitions from isolated luminescent centers. Other phenomena, such as changes in energy transfer processes or electronic crossovers induced by pressure, can also influence emission lifetimes and will be discussed in later sections. 

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LIFETIMES OF EXCITED ELECTRONIC STATES OF ATOMS AND MOLECULES... 

In Sections 7.3 and 7.4, the temperature dependence of radiationless transitions and the effect of deuteration on the lifetimes of excited electronic states are examined. In Section 7.5, a contribution to time-resolved spectroscopy is presented. In that section, we will discuss a problem dealing with transport phenomena of electronic excitations in doped molecular crystals. The theory of singlet excitation energy transfer uses an effective Hamiltonian to account for intramolecular excited-state depopulation and energy transfer by multistep migration among guest molecules. 

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,... 

If the exciting laser field interacts with the CNT resonantly, the magnitude of the hyperpolarizabilty tensor is also affected by the lifetimes of the involved electronic levels. Lifetime of electronic states in a CNT is mostly determined by electron-electron scattering events. As far as electrons in a CNT are treated as a Fermi liquid (despite there are experimental evidence of Luttinger liquid behaviour [27]) the lifetime of an electronic state with an energy E above the Fermi level Ef, is given by... 

Selgren et al [29] have taken a more direct approach, forming NNH by neutralizing a beam of NNH" ions. Their experiments indicate that NNH has a lifetime of less than 0.5 Lisec, generally in agreement with the theoretical predictions described above. While the results of these experiments may be compromised somewhat by the formation of excited electronic states of NNH, it appears likely that Tnnh is smaller than the lower limit given by Glarborg et al [8]. 

We now discuss the lifetime of an excited electronic state of a molecule. To simplify the discussion we will consider a molecule in a high-pressure gas or in solution where vibrational relaxation occurs rapidly, we will assume that the molecule is in the lowest vibrational level of the upper electronic state, level uO, and we will fiirther assume that we need only consider the zero-order tenn of equation (BE 1.7). A number of radiative transitions are possible, ending on the various vibrational levels a of the lower state, usually the ground state. The total rate constant for radiative decay, which we will call, is the sum of the rate constants,... 

Time dependent perturbation theory provides an expression for the radiative lifetime of an excited electronic state, given by Tr ... 

Pulsed lasers (Chapter 9) may be used both for photolysis and as a source. Since the pulses can be extremely short, of the order of a few picoseconds or less, species with comparably short lifetimes, such as an atom or molecule in a short-lived excited electronic state, may be investigated. 

Upon excitation of the molecule, one or several electrons are lifted to a higher energy level. When the lifetime of the photoexcited state of molecules j is sufficiently long (i.e., longer than the time required for electronic transitions between particles), this state can be described in terms of a new value of the electrochemical potential, which is higher than the value of nonexcited particles. 

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