Inverse electronic relaxation

The inverse electronic relaxation from the / manifold to lower lying j,n—l),..., 5,o— > states. 

The importance of this mechanism is certainly reduced in polyatomic molecules, where direct vibrational relaxation is efficient and the inverse electronic relaxation is less probable. 

Puretzky, A. A., and Tyakht, V. V. (1989). Inverse electronic relaxation under IR multi-photon excitation of molecules. In Laser spectroscopy of highly vibrationally excited molecules (ed. V. S. Letokhov), pp. 329-378. Adam Hilger, Bristol. 

Such an inverse electronic relaxation is indeed observable [A. M. Ronn, Adv. Chem. Phys. 47, 661 (1981) H. ReislerandC. Wittig,/Idv. Chem. Phys. 47, 679 (1981)]. 

Figure Bl.13.3. The inversion-recovery experiment. (Reproduced by pennission of VCFI from Banci L, Bertini I and Luchinat C 1991 Nuclear and Electron Relaxation (Weinlieim VCFI).)...

In order to visualize the effects of water exchange, rotation and electronic relaxation as well as of magnetic field on proton relaxivity, we have calculated proton relaxivities as a function of these parameters (Fig. 2). The relaxivity maximum is attained when the correlation time, tc1, equals the inverse proton Lar-mor frequency (l/rcl = l/rR + l/rm + l/Tle = a>j). The most important message of Fig. 2 is that the rotational correlation time, proton exchange and electronic relaxation rates have to be optimized simultaneously in order to attain maximum relaxivities. If one or two of them have already an optimal value, the remaining parameter starts to become more limitative. The marketed contrast agents have relaxivities around 4-5 mM1 s 1 contrary to the theoretically attainable values over 100 mM 1 s1, which is mainly due to their fast rotation and slow water exchange. 

Crystal structures of Ln complexes of DTPA and DOTA derivatives show that Ln +-bound diethylenetriamine moieties in these complexes always occur either in the 88 or in the X.X. conformation (Scheme 5.10). In these complexes, the steric interactions are always minimized. Upon coordinated by a metal ion, the central nitrogen atom is chiral. The inversion is precluded if the nitrogen is coordinated to the metal. A common feature of cyclen-based macrocyclic Gd (and other Ln ) is the formation of various isomers which display dynamic behavior (interconvert/exchange) on the NMR time scale in aqueous solution [174,175]. However, the long electronic relaxation time of Gd ion prevents the observation of the NMR spectra of its complexes and its solution structure has been inferred from the H and NMR spectra of its related Ln complexes. 

Exponential decay often occurs in measurements of diffusion and spin-relaxation and both properties are sensitive probes of the electronic and molecular structure and of the dynamics. Such experiments and analysis of the decay as a spectrum of 7i or D, etc., are an analog of the one-dimensional Fourier spectroscopy in that the signal is measured as a function of one variable. The recent development of an efficient algorithm for two-dimensional Laplace inversion enables the two-dimensional spectroscopy using decaying functions to be made. These experiments are analogous to two-dimensional Fourier spectroscopy. 

Silane and hydrogen show relaxation patterns with the same characteristic time t, however, inverse signs. The fragmentation of silane induced by collisions with electrons, yields molecular hydrogen in an order of magnitude faster than the time resolution of the mass spectrometry setup, i. e. faster than 1 ms. Two possible pathways of silane fragmentation can be regarded ... 

The radiationless transition between two states of same spin is called internal conversion, the one occuring with inversion of spin being termed intersystem crossing. In both processes the excess energy is liberated as heat. All these transitions between different electronic states are customarily preceded by vibrational relaxation, i.e. the deactivation from a higher vibronie level to the v0-level of the same electronic state (Fig. 5). 

An improved and direct correlation between the experimental rate constant and [obtained using Eq. (49)] is observed if v = /zd is used instead of v = 1/Tt, the solvent-dependent tunneling factor is utilized, and only AG (het) of Eq. (8) is used in Eq. (49) (see triangles in Fig. 18). Furthermore, the inverse of the longitudinal solvent relaxation time Xi is not necessarily the relevant one to use as the frequency factor v (see empty circles in Fig. 18). Similar conclusions were reached by Barbara and Jerzeba for the electron transfer reaction in homogeneous solutions. Barbara and Jerzeba measured the electron transfer time... 

The question then remains as to why the relaxivity is relatively high for ions whose ground state does not have an S configuration. An inversion between the H4g and Eog states as suggested by Glebov et al. (42) appears unlikely in view of recent theoretical investigations of the absorption frequencies and intensities in the electronic spectrum of NpO (38,41). An admixture of states is more likely as Matsika et al. (38) found that the ground state of NpO is only 84% H4g (5u< )i,) and as Ismail et al. (40)... 

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