Depopulation optical

In Chapter 5, we discuss in a simple way static (crystalline field) and dynamic (coordinate configuration model) effects on the optically active centers and how they affect their spectra (the peak position, and the shape and intensity of optical bands). We also introduce nonradiative depopulation mechanisms (multiphonon emission and energy transfer) in order to understand the ability of a particular center to emit light in other words, the competition between the mechanisms of radiative de-excitation and nonradiative de-excitation. 

The luminescence lifetime is the average time the molecule remains in its excited state before the photon is emitted. From a kinetic viewpoint, the lifetime can be defined by the rate of depopulation of the excited (singlet or triplet) states following an optical excitation from the ground state.9 Luminescence generally follows first-order kinetics and can be described as follows. 

Fig. 3.9 shows the V J) and C(J) dependences for the types of transitions discussed. As can be seen for Q Q i transitions the quantum mechanical solutions already at J > 5 differ little from the J —> oo limit. At the same time, in the case ofPj. Pj.or.Pj. RI. type transitions, even at such high values as J = 50 the difference from the classical limit still constitutes a value of the order of 1% of that of the degree of polarization or of anisotropy. This may lead to considerable error if one uses classical expressions from Table 3.5 for V x) dependences of the type presented in Fig. 3.8 for the determination of parameter x Table 3.6 also presents the values of 7Z(x), P(x) and C(x) in the limit of very strong optical depopulation for x > oo- It is noteworthy that in this case the corresponding values for P-excitation are identically equal to zero, independently of J, whilst in the other cases they behave either as 1/J or as 1/J2 see Fig. 3.9. 

For concrete estimates of the parameters of a reaction (3.1) let us turn to diatomic molecules, such as Na2, K2, Te2, which have been studied most in experiments on optical pumping of molecules via depopulation. A number of data characterizing the states and transitions in these objects under conditions typical for such experiments are given in Table 3.7. These parameters are, to a certain extent, characteristic of diatomic molecules in thermal vapors of the first, sixth and seventh group of the periodic system of elements, such as alkali diatomics, S2, Se2,12, etc. These molecules may... 

If we compare these values with the corresponding ones, obtained in the optical polarization of angular momenta of the lower level by depopulation (see Figs. 3.3, 3.4 and 3.5), then we will see that similar values can be obtained only in the case of a large non-linearity parameter, x — 10 and more. 

In other words, one may speak here of a certain fly-through lifetime Tq of the optically depopulated lower level. 

Fig. 3.20. Signals of fluorescence kinetics representing fly-through relaxation of an optically depopulated initial level (a) rectangular profile of the beam (b) limited Gaussian profile (c) unlimited Gaussian profile (d) experimentally registered signal. Values of the non-linearity parameter Bwpvp/ro are shown in brackets.

The example discussed considers the case of weak light excitation, where the first cycle J" —> J (see Fig. 3.14) does not produce ground state optical polarization via depopulation of the J" level. If this is not so, then the signal is described, accounting for depopulation effects, and naturally assumes a more complex shape [30]. 

Auzinsh, M.P., Tamanis, M.Ya. and Ferber, R.S. (1986). Zeeman quantum beats during the transient process after optical depopulation of the ground electronic state of diatomic molecules, Sov. Phys.—JETP, 63, 688-693. 

Clarke (326) has studied the optical electron spin polarization in triplet anthracene and has observed ESR emission at 1.5°K which was attributed to a non-Boltzman distribution over the triplet spin levels at low temperature. The dynamics of optical spin polarization in triplet naphthalene at 1.6°K was also reported by Sixl and Schwoerer (327a) and van der Waals et al. (327b). have used a general method to study dynamics of populating and depopulating triplet spin levels by microwave-induced delayed phosphorescence. These experiments enable measurements of the lifetimes of each triplet spin state and thus can provide important information about intramolecular decay processes and intermolecular triplet energy transfer. 

In addition to the hypsochromic shift of the Soret and Q bands of diamagnetic Ni TPP, there are various optically invisible lower-energy electronic states in which an electron is promoted from the metal to the porphyrin ring or vice versa and those that correspond to d-d excitation within the metal orbitals. Calculations have shown that there is a multitude of CT states below the first optically allowed state and explain its lack of fluorescence (42). The radiationless depopulation of the Q state... 

The rate constants for radiationless decay of the triplet state of mesoporphyrin IX dimethyl ester at 77 are 26 s-1 in EtOD and 57 s-1 in EtOH.352 The most probable cause is a decrease in the rate of tautomerism in the porphyrin due to deuteriation of the N—H hydrogen. However, for TPPH2 triplet state in n-octane matrices 363 such tautomerism does not appear to be an important mechanism for radiationless deactivation. Several recent reports deal with the low-temperature e.s.r. spectra of triplet states of porphyrins.364-368 The zero field splittings and depopulation rates of the various spin sub-levels of the triplet state of Zn-chlorophyll-a have been determined by an optically detected magnetic resonance method.359... 

A second direct optical-detection method for selective population and depopulation is microwave-induced delayed phosphorescence in zero field (Bq = 0) [25]. Figure 7.26 shows the phosphorescence intensity from quinoline in a durene (tet-ramethyl benzene) host crystal at T= 1.35 K as a function of the time after the end of the UV excitation. The phosphorescing zero-field component here is Tz). Its lifetime is considerably shorter than those of the other two zero-field components, from which furthermore no phosphorescence is emitted. If the zero-field transition... 

The differences in the population and depopulation rate constants and the phosphorescence probabilities of the three components of the triplet states form the basis of all the methods for Optical Detection of Magnetic Resonance in triplet states of jr-electron systems. These methods were developed after the discovery of optical spin polarisation and extended to inorganic solids. The essential physical difference from the optical double resonance in atoms developed by Alfred Kastler is to be found in the selection mechanism in optical double resonance, the polarisation of the resonant UV light, i.e. the symmetry of an applied field, is responsible for the selection. In optical spin polarisation, the selection is due to the spin-orbit coupling, and thus to an internal field. 

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