Induced absorption spectral bands

There are three main EA spectral features in the energy range of band I a derivative-like feature with zero-crossing at 2.29 eV, followed by vibrational features, and an induced absorption band between 2.9 and 3.2 eV. The features below 2.5 eV are the results of a redshifted 1 Bu exciton energy, and its phonon sidebands (Stark shift). These features are more easily observed in EA than in absorp-... 

The induced absorption band at 3 eV does not have any corresponding spectral feature in a(co), indicating that it is most probably due to an even parity state. Such a state would not show up in a(co) since the optical transition IAK - mAg is dipole forbidden. We relate the induced absorption bands to transfer of oscillator strength from the allowed 1AS-+1 (absorption band 1) to the forbidden 1 Ak - mAg transition, caused by the symmetry-breaking external electric field. A similar, smaller band is seen in EA at 3.5 eV, which is attributed to the kAg state. The kAg state has a weaker polarizability than the mAg, related to a weaker coupling to the lower 1 Bu state. 

The pump and probe pulses employed may be subjected to a variety of nonlinear optical mixing processes they may be prepared and characterized by intensity, duration, spectral band width, and polarization. They may arrive in the reaction chamber at a desired time difference, or none. The probe pulse may lead to ionizations followed by detections of ions by mass spectrometry, but many alternatives for probing and detection have been used, such as laser-induced fluorescence, photoelectron spectroscopic detection, absorption spectroscopy, and the like. 

Induced spectra actually consist of contributions arising from free-to-free, free-to-bound, bound-to-bound, and bound-to-free transitions. At temperatures much greater than the well depth of the intermolecular potential, kT e, the observed induced absorption is nearly fully due to free-to-free transitions as Welsh and associates suggest, but individual dimer lines or bands may still be quite prominent unless pressure broadening and perhaps other processes (like ternary interactions) have obliterated such structures. However, at lower temperatures, kT collision-induced is a poor choice. 

Spectral moments of allowed molecular absorption bands vary in general nearly linearly with the gas density, y /q constant. At sufficiently high pressures, a small, linear increase with density of the ratio y /g is, however, discernible, e.g., [8]. This quadratic absorption component is largely due to apparent induced absorption, resulting from the long range interaction of dipoles induced by the incident radiation field [400], Moreover, a true induced absorption component is believed to exist which arises from collision-induced dipole components (Chapter 4) [146, 210]. It was argued, however, that in most measurements true induced absorption was too weak to be identified positively in this way. Recent experimental and... 

We have not attempted to exhibit in great detail the effects of the rotational excitations on the induced dipole components B and those of vibrational excitation on the interaction potential because this was done elsewhere for similar systems [151, 63,295,294], The significance of the j,f corrections is readily seen in the Tables and need not be displayed beyond that. The vibrational influence is displayed in Fig. 6.20 first and second spectral moments are strongly affected, especially at high temperatures, similar to that which was seen earlier for H2-He [294], Fig. 6.23. The close agreement of the measurements of the rotovibrational collision-induced absorption bands of hydrogen with the fundamental theory shown above certainly depends on proper accounting for the rotational dependences of the induced dipole moment, and of the vibrational dependences of the final translational states of the molecular pair. 

On pp. 31 Iff., a preliminary discussion of the symmetry of induced line profiles was given. The spectral lines encountered in collision-induced absorption show a striking asymmetry which is described roughly by a Boltzmann factor, Eq. 6.59. However, it is clear that at any fixed frequency shift, the intensity ratio of red and blue wings is not always given exactly by a Boltzmann factor, for example if dimer structures of like pairs shape the profile, or more generally in the vibrational bands. We will next consider the latter case in some detail. 

The induced bands mentioned are known from spectral studies with liquid, solid and compressed gaseous oxygen. Condensed oxygen is blue, because of the red 0-0 absorption bands near 760 and 630 nm, and their 1-0 counterparts, that is, because of interaction-induced absorption. 

Figure 1 Comparison of observed ( , ) and computed ternary spectral moments of collision-induced absorption of compressed hydrogen in the H2 fundamental band. The dashed curve represents the moments computed on the basis of the pairwise additive dipole components only. The solid line also accounts for the irreducible dipole components from Ref. [53]. (This figure is an update of Fig. 3.46.)...

Ternary spectral moments of collision-induced absorption in hydrogen gas are analyzed in the H2 fundamental band in terms of pairwise additive and irreducible contributions to the interaction-induced dipole moment, Eqs. (1 - 7) [51]. Numerical results show that irreducible dipole components, especially of the exchange quadrupole-induced ternary dipole component, are significant for agreement with spectroscopic measurements, such as ternary spectral moments (Fig. 1) [53], an observed diffuse triple transition 3<3i centered at 12,466 cm-1 [52, 54, 55], and the intercollisional dip in compressed hydrogen gas, pp. 188 -190. 

The significance of collision-induced absorption for the planetary sciences is well established (Chapter 7) reviews and updates appeared in recent years [115, 165, 166, 169-173]. Numerous efforts are known to model experimental and theoretical spectra of the various hydrogen bands for the astrophysical applications [170, 174-181]. More recently, important applications of colhsional absorption in astrophysics were discovered in the cool and extremely dense stellar atmospheres of white dwarf stars [14, 43, 182-184], at temperatures from roughly 3000 to 6000 K. Under such conditions, large populations of vibra-tionally excited H2 molecules exist and collision-induced absorption extends well into the visible region of the spectrum and beyond. Numerous hot bands, high H2 overtone bands, and H2 rotovibrational sum and difference spectral bands due to simultaneous transitions that were never measured in the laboratory must be expected. Ab initio calculations of the collisional absorption processes in the dense atmospheres of such stars have yet to be provided so that the actual stellar emission spectra may be obtained more accurately than presently known. 

In native VCPO, the position and intensity of cofactor-induced absorption are sensitive to pH change at pH 5, the band is blue-shifted (Xmax 308 nm), and the intensity is decreased to e = 2.0 mM-1 cm-1. The exact correlation between the above-mentioned spectral features and the known X-ray data are complex, but a further UV-VIS analysis of several mutants of this enzyme showed the importance of several structural elements. A His496Ala mutant showed no optical spectrum (Hemrika et al. 1999), whereas crystal structure of this mutant showed the presence of tetrahedral vanadate in the active site (Macedo-Ribeiro et al. 1999), indicating that the V— N bond is crucial for the observed spectrum. The effect of other mutations can be seen in Table 1.1. 

The absorption spectral change in the fuUy oxidized state induced by cyanide is characterized by a red shift of the Soret band peak position to 428 nm, regardless of the original position (418-424 nm) (Yoshikawa and Caughey, 1990). Fe +—CN showing absorption near 590 nm is autoxidizable. Cyanide strongly stabilizes the ferric state of Foaj so that excess amounts of dithionite cannot reduce Fe -CN of the enzyme at the concentrations appropriate for visible-Soret measurements at neu-... 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

In fact, the authors found that in different proteins, the second derivative spectra below 270 nm were essentially the same as the second derivative spectrum of phenylalanine (Ichikawa and Terada, 1979). The absorption at certain peaks and troughs of the second derivative spectrum of phenylalanine are found dependent on the microenvironment of the phenylalanine. Denaturation of the proteins by urea or guanidine modifies the intensities of the spectral bands of phenylalanine without inducing a significant shift in their positions. Table 1.3 shows the difference between peaks and troughs in the second derivative spectrum of phenylalanine under various conditions and Table 1.4 shows the difference observed for four proteins in the native and denatured states. 

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