Optical spectral transitions

The copper system appears to behave similarly to the silver system, and it may be used here in order to illustrate the idea of "selective, naked-cluster cryophotochemistry 150,151). A typical series of optical-spectral traces that illustrate these effects for Cu atoms is given in Fig. 15, which shows the absorptions of isolated Cu atoms in the presence of small proportions of Cu2, and traces of Cus molecules. Under these concentration conditions, the outcome of 300-nm, narrow-band photoexcitation of atomic Cu is photoaggregation up to the Cus stage. The growth-decay behavior of the various cluster-absorptions allows unequivocal pinpointing of UV-visible, electronic transitions associated with Cuj and Cus 150). With the distribution of Cui,2,3 shown in Fig. 15, 370-nm, narrow-band excitation of Cu2 can be considered. Immediately apparent from these optical spectra is the growth (—10%) of the Cu atomic-resonance lines. Noticeable also is the concomitant... 

An example of an application to actual spectroscopy is shown in Figure 6.9. This shows the states computed in the range of the 0 —> 3 CH spectral transition. The intensities were computed as a Franck-Condon overlap of the optically bright CH overtone state (Holme and Levine, 1989 Levine and Berry, 1989) with the relevant eigenstate. 

Benson RC, Hoffinan RC, Potember RS, Bourkoff E, Poehler TO (1983) Spectral dependence of reversible optically induced transitions in organometallic compounds. Appl Phys Lett 42 855-857 and references therein... 

The 77-SCF MO Cl method has also been used446 to interpret spectral transitions of a series of possible intermediates in the reaction of uracil and cytosine with the solvated electrons eaq, produced by radiolysis of water. Experimentally this reaction has been investigated by Hayon,447 who used the technique of flash radiolysis. Hayon measured the optical-absorption spectra of the transient species in the UV range to obtain information on the site of attack of eaq on the pyrimidine base. At pH 5.0 the solvated electrons react with the pyrimidine molecules mainly at the C-2 and C-4 carbonyls, and the intermediates are rapidly protonatcd to give the corresponding ketyl radicals. For uracil Hayon found two absorption maxima (at 305 and < 280 nm) at pH 5.1 and one peak at 310 nm at pH 11.7. In this last case, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. Several species, 44, 45, 46, have been suggested by... 

It may be shown that there are optically allowed transitions from the ground state (1) to excited states formed by combining expressions (3) only for k = 0. There are just a small number of spectral lines in each band of k values. The number of allowed transitions is equal to or less than the number of molecules in the... 

The synthesis of mixed-valence corrpounds with optical IT transitions in the visible and near infrared region provides an interesting way to low-energy spectral sensitization. However, experimental results of photochemical reactions initiated by excitation of IT states are very scarce, see e.g. (21, 22). 

Trivalent transition metal ions substituting for Al3+ occur in several aluminosilicate minerals, including epidotes, yoderite and the Al2Si05 polymorphs andalusite, kyanite and sillimanite, often producing spectacular colours and pleochroism. As a result, numerous optical spectral measurements have been made of several natural and synthetic Al2Si05 minerals (Kerrick, 1990). 

Other transition metal-bearing pyroxenes Optical spectral measurements have been made on a variety of natural and synthetic pyroxenes containing a number of transition metal ions (Rossman, 1980). Crystal field splitting and stabilization energy parameters derived from these spectra are summarized in table 5.7. 

Considerable interest centres on the Mantle constituting, as it does, more than half of the Earth by volume and by weight. Attention has been focussed on several problems, including the chemical composition, mineralogy, phase transitions and element partitioning in the Mantle, and the geophysical properties of seismicity, heat transfer by radiation, electrical conductivity and magnetism in the Earth. Many of these properties of the Earth s interior are influenced by the electronic structures of transition metal ions in Mantle minerals at elevated temperatures and pressures. Such effects are amenable to interpretation by crystal field theory based on optical spectral data for minerals measured at elevated temperatures and pressures. 

Table 4.22. Optical spectral (one-electron) transition energies ( ) calculated for manganese oxides using the MS-SCF-ATa method, compared with experimentally observed transitions assigned to charge transfer, crystal field, valence band conduction band and crystal field conduction band transitions...

This expression is analogous to the general Drude formula (10), but it suggests that, in a spectral region distant from the optically active transitions of a helical polymer, the dispersion will be complex rather than simple. 

There are technical causes characteristic to the instrument and collectively known as instrumental parameters width of entrance slit, quality of the optics, focal distance, diffraction phenomena through narrow orifices. However, there are also causes due to quantum mechanics, which ensure that the spectral transitions have a natural width . The radiations emitted by the atoms are not quite monochromatic. In particular with the plasmas, a medium in which the collision frequency is high (this reduces enormously the lifetimes of the excited states), Heisenberg s uncertainty principle plays a large role (Figure 14.7). Moreover, the elevated temperatures increase the speed of the atoms, enlarging line widths by the Doppler effect. Finally for all of these reasons, the width of the lines at 6000 K reaches several picometres. 

This system in many cases can be simplified further. For example, if we have a broad spectral line excitation with a not very intense laser radiation, we have a situation for an open transition when 7 Ti, H. In practical cases this condition is often fulfilled at excitation with cw lasers operating in a multimode regime. If the homogeneous width of spectral transition usually is in the range of 10 MHz, then the laser radiation spectral width broader than 100 MHz usually can be considered as a broad line excitation. In this case we can use a procedure known as adiabatic elimination. It means that we are assuming that optical coherence pi2 decays much faster than the populations of the levels puJ = 1,2. Then we can find stationary solution for off-diagonal elements for the density matrix and afterwards find a rate equations for populations in this limit. For the two level system we will have... 

Here fA presents the oscillator strength [1,3] of the optical absorption transition on A, E the energy of maximum spectral overlap, and SO the spectral overlap integral as given in Eq. (3.1). Equation (3.2) makes it possible to calculate Rc from spectral data. 

In each system there exist spectral transitions in the optical region between high and low configurations. Since the optical transitions which connect the two systems are very few (see next paragraph), each of the systems effectively generates a separate spectrum. The overlap of these two spectra is one of the reasons for the abundance of lines in lanthanide spectra. 

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