Population thermally equilibrated

Principles and Characteristics The term luminescence describes the radiative evolution of energy other than blackbody radiation which may accompany the decay of a population of electronically excited chro-mophores as it relaxes to that of the thermally equilibrated ground state of the system. The frequency of the... 

In the limit of high rotation, it is possible to associate these A-doublet components with states where the half-filled orbital is either in, or perpendicular to, the plane of rotation. If the ions are thermally equilibrated, the e and/parity label states would each represent 50% of the population, implying a possible SIKIE on the order of a factor of two. However, there is no fundamental reason why El caimot have a propensity for producing a particular parity label state that could lead to SIKIE considerably larger (or smaller) than a factor of two. 

ISC from the optically prepared singlet state populates one or two low-lying A" triplet states in a few hundreds of femtoseconds, see Sect. 3. Triplet states are initially populated hot, that is nonequilibrated both in terms of the molecular structure and the medium. Relaxation processes, which occur on the timescale of picoseconds to nanoseconds (depending on the medium), will be discussed in Sect. 5. Herein, we will deal with thermally equilibrated (relaxed) lowest triplet states and their theoretical as well as experimental characterization. 

The traditional theory for the rate of chemical reactions is the transition-state theory [21] (abbreviated as TST). In fact, all the rate constants given so far in previous sections were formulated, in general terms, within the framework of the TST. It is tacitly assumed in this theory that fluctuations in the reactant state are so rapid that all the substates comprising the reactant state are always thermally equilibrated in the course of reaction. According to this assumption, the reactant population in the transition state is always maintained in thermal equilibrium with the population in the reactant state since both states are located on the reactant-state adiabatic (or diabatic) potential. Therefore, calculation of the rate constant is greatly simplified... 

Fulgides (e.g., 53, see Scheme 15) are capable of photochromic reactions. Yokoyama et al. reported the preparation and subsequent photochromic isomerization of a fulgide which was designed to resist enantiotopomerization [103]. A sample of the resolved fulgide was irradiated (405 nm light in toluene solution) and was observed to come to a pss with a ratio of 19 81. Irradiation with visible light (X > 580 nm) led to the complete recovery of the initial conformation. An advance on this system was made by the same authors, who described the process of diastereomeric photochromism, in which a fulgide derivatized with a binaphthyl auxiliary was allowed to thermally equilibrate, and a photocyclization process carried out [104]. As a result of the relative populations of photoreactive... 

In order to elucidate a mechanism, one must first consider the nature of the states initially formed by photoexcitation as well as the natures of other expected states eventually populated by internal conversion/intersystem crossing. Although it is by no means universally true, many transition metal complexes, when excited, undergo efficient relaxation to a bound, lowest energy excited state (LEES) or an ensemble of thermally equilibrated LEESs from which the various chemical processes lead to photoproducts. In such systems, the simplest model of which is illustrated by Figure 9, one can comfortably apply transition state theory to the rates and consider pressure effects in terms of the mechanisms of the individual decay LEES processes. In this case, the quantum yield of product formation would be defined by the ratio of rate constants by which the various chemical and photophysical paths for ES decay are partitioned. For Figure 9, in the absence of a bimolecular quencher Q, this would be... 

Equation (21) may be easily solved yielding a time evolution characterized by biexponential decay. The first (fast) component of the decay corresponds to the initial equilibration of populations between the optically active and the optically nonactive levels. The second (slow) component corresponds to the radiative decay of the thermally equilibrated system. 

In a crystalline host, the potential curves as drawn in Figure 5 describe the total energy, complex and environment. If vibrational relaxation within an electronic state is faster than other competing steps, then photophysical and photochemical processes occur in thermally equilibrated populations. Figure 5 is also applicable for a rigid, noncrystalline medium, but as the solvent melts and solvent relaxation takes place during the excited-state lifetime, a more complex representation is required. 

Like with vibrations (2.5.1), the rotational temperature (Tr) of polyatomic ions drifting in a gas increases at high E/N. That was shown for N2 in He by measuring the rotational state populations at various E/N using LIF spectroscopy (Figure 2.22). The dependence of Tr on E/N revealed by these data agrees with Equation 1.26, proving the thermal equilibration of rotations at T. The rotational... 

Quenching of the A State. Electronic quenching of PH(A rij, v = 0) with a thermally equilibrated rovibrational population was studied at 243, 296, and 415 K for a number of molecular collision partners in all cases, the rate constants decreased with increasing temperature as shown in the following table [4, 5] ... 

A common situation found in condensed phases under illumination is for all levels, except electronic levels, to be thermally equilibrated. Thus, under constant illumination, the sample is a mixture of thermally/vibrationally-equilibrated ground-state(s) with a very small, non-Boltzmann population of the excited electronic state, but which is itself thermally and vibrationally Boltzmann distributed. So the situation is similar to two non-equilibrated chemical species each of which is thermally equilibrated a thermally equilibrated ground-state, and a thermally equilibrated high energy excited-state. 

Other molecules or the surroundings. In gas and solution phases this is usually via collisions, in the solid-state it is to thermal/vibrational states of the solid material. In solution, the frequency of collision with solute is of the order of picoseconds, and therefore thermal equilibration of vibrational states, i.e. the relaxation of non-Boltzmann high-energy vibrational states, vibrational relaxation, is usually faster than aU but the fastest photophysical processes (see Chaps. 3 and 15 for examples where it is not). Hence, population of a higher vibrational level of an electronic... 

Transition metal complexes differ from organic compounds with respect to both the number and the spin multiplicity of accessible electronically excited states which undergo very fast relaxation to thermally equilibrated electronically excited (thexi) states. Thus, depending on the wavelength of irradiation, various electronic states can be excited (see fig. 1). This may result in the population of thexi states of different reactivity. Under... 

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