Two-band system

Z states, as indicated on the spectrum (the last two states are almost degenerate). The other two band systems are too weak to be observed. 

The electronic spectrum of S2O has been studied both in absorption and in emission and both in the ultraviolet and the visible regions. The absorption spectrum in the near UV region is extremely intense and well suited to detect S2O in gases even at very low partial pressures. Two band systems are located in the UV region at 340-250 nm and at 230-190 nm [35] while a third system in the visible region at 645-575 nm was discovered only by op-toacoustic detection [36]. The 340-250 nm system has also been observed for matrix-isolated S2O [37]. For more details see [1, 38-47]. 

Magnetic dipole interactions are possible for the 1AJ -3S ," and 129+ <-3E9 transitions in 02, although naturally they are weak. The transitions are, however, known both in absorption, especially in the atmosphere, and in emission indeed, the two band systems are frequently known as the infrared atmospheric and atmospheric bands of oxygen, and the observation of emission from these systems has often been used to demonstrate the presence of excited singlet oxygen both in the laboratory and in the atmosphere. 

Fig. 17. The He(I) UPS of ferrocene (a) full spectrum, (b) high-resolution scan of the first two band systems. (From Ref. 123.)...

The potential energy curves are given in Fig. V- 14. Besides the two band systems already described, the two extremely weak systems blZ +-X3Z (5380 to 7620 A) and Ag-X3Z (9240 to 15,800 A) have been observed in atmospheric absorption. The former is called the atmospheric bands and the latter the infrared atmospheric bands. 

The coherent potential approximation (1, 2) is a consistent theoretical frame, which unifies the different alloy models. In order to account for changes in the electronic nature of the atoms, the coherent potential approximation for a disordered alloy appears at present to be the best. It has been applied to single- and two-band systems (130a 130c). 

Two band systems of methylene have been observed by photolysis of diazomethane in presence of excess nitrogen (Herzberg, 1961 Herzberg and Shoosmith, 1959). The radical is short-lived (no absorption is detected 50 psec after the photolysing flash) and appears to be formed in an excited state from which it decomposes unless deactivated by collisions with the inert gas. 

Figure 8.10 shows the He I spectrum of HBr in which there are two band systems. The low ionization energy system shows a very short progression consistent with the removal of an electron from the doubly degenerate nu lone pair (4px, 4py) orbital on the bromine atom. The lone pair orbital picture and the expectation of only a short progression is further confirmed by the fact that the bond length re is known from high-resolution electronic... 

Similarly, for TiO whose spectrum consists of two band systems, the high-frequency one corresponds to basic, i.e. single-bonded, hydroxyl groups, while the other corresponds to more acidic bridged hydroxyls. Hie anionic character of the type I hydroxyls on alumina and titania is exemplified by the fact that they preferentially exchange with fluoride ions ... 

Studied extensively using Hg-Rg electronic transitions. Kaya and co-workers observed two band systems near the Hg P, <- Sq resonance line, using LIF and dispersed fluorescence spectra of Hg-Rg (Rg = Ne, Ar, Kr, and Xe) complexes [177-180]. Analyses of vibrational level spacings and Franck-Condon intensity patterns led to a determination of the interatomic potentials for the electronically excited (A and B) and ground states [52, 54, 178-180]. LIF spectra of Rg = He, Ne, and Xe complexes were also reported by Soep and co-workers [52-54]. Recently, rotational structures of the A <- X and B <- X systems of Rg = He, Ne, and Ar complexes were observed by Yamanouchi et al., although the spectra were found to be congested by contributions from the six Hg isotopes [181]. Rotational resolution of the A <- X and B <- X systems indicated that the A and B states are 0" and 1, respectively, in the Hund s case (c) representation. 

The absorption spectra of aqueous solutions of MnO, TCO4, and Re04, measured in the VIS and UV (Fig. 11.3.A) demonstrate for each ion essentially two band systems with pronounced vibrational structures. The band systems are shifted to higher wavenumbers from MnOj via TcOj to Re04. In addition, MnOj exhibits a strong band at 52910 cm (189 nm). Tabic 11.7.A summarizes the absorption maxima of TcOj and RCO4. 

The resistivity for these compounds is shown in fig. 7. Havinga et al. (1973) analysed both the resistivity data and the thermodynamic data of these compounds in terms of a two-band system. 

Equation (6.12) can easily be simplified to the case when we take into account only excitations from the valence bands to the conduction bands (two-band system) ... 

The major discernible difference in the two band systems is the length of the bending progressions and consequent width of the electronic transition. The thioketene bands span a narrow region of about 8000 cm centered at 500 nm, whereas the ketene spectrum covers some 17000 cm centered at about 370 nm. This large difference probably reflects to some degree the limited pressure-path attainable with thioketene due to its instability, but also suggests more extensive Franck-Condon activity and a more distorted excited state in the ketene case. Such a conclusion is consistent with the ab initio results. 

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