Absorption frozen solutions

Very thick absorbers may be required for applied-field measurements to achieve reasonable absorption depths and measuring times because the Mossbauer spectra are usually split into several hyperfine components. Here the iron content may be as large as 100 pg Fe per cm (1.75 pmol Fe per cm ), which would correspond to t 1 for a two-line spectrum. For smdies of frozen solutions, Fe concentrations of 1 mM are desirable for each nonequivalent iron site [35]. 

Photolysis of DAX in a methylcyclohexane glass at 77 K creates a metastable species detected by its optical absorption spectrum (Table 4). This solution does not exhibit an epr spectrum characteristic of a triplet carbene, nor is one observed when DAX is irradiated in Fluorolube (where triplet carbenes are generally more stable). Warming the frozen solution causes the reaction of the metastable transient and the formation of dimeric xanthone azine. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Biological, chemical. X-ray diffraction, infrared absorption, e.s.r., n.m.r., luminescence, and quantum studies show that dimer formation is universally observed in irradiated frozen solutions of thymine, thymidine, uridine, thymidylic acid and related compounds, and in DNA [560—576]. The purines of DNA, on the other hand, are little affected [577, 578]. Thymine dimers obtained in frozen solution can be converted to the original monomers by ultraviolet... 

Lamola and Eisinger169 have shown that the oriented adjacent pairs of 1,3-dimethyl-thymine monomers formed by photosplitting of the appropriate dimer in frozen solution will photodimerize again with a quantum yield of close to unity, and that the pair of monomers shows no fluorescence, but an exciton splitting in the absorption spectrum. 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

Frozen solutions. In frozen aqueous solutions, the additive may not precipitate but accumulate in ice-free areas in a rather uncontrolled way. Upon irradiation, there are only a few radicals from the radiolysis of ice that reach the solute, and radical formation can occur by direct absorption of the energy of ionizing radia-... 

Its absorption spectrum shows one band at 320 nm (e = 2900 M 1cm 1), assigned to the cti - ct2 transition localized in the Au-Tl moiety. The emission spectrum in the solid state at 77 K shows a band at 602 nm, which is attributable to a transition between orbitals that appear as a result of the metal-metal interaction. In this sense, Fenske-Hall molecular orbital calculations indicate that the ground state is the result of the mixing of the empty 6s and 6pz orbitals of gold(I) with the filled 6,v and the empty 6pz orbitals of thallium(I). In frozen solution, this derivative shows a shift of the emission to 536 nm, which has been explained by a higher aggregation of [AuT1(MTP)2] units in the solid state if compared to the situation in solution. 

Cryoreduction of ferric metalloproteins has been mostly done by y-irradiating of frozen solutions of heme proteins at 77K, unless it is specially stated otherwise. Cryoradiolytic reduction of ferric cytochrome c and its complexes with fluoride, azide, cyanide, and imidazole were studied using optical absorption spectroscopy and magnetic circular dichroism.36,39,46 44 The primary products of cryoreduction of low-... 

Fig. 14. Electron spin resonance spectrum of a frozen solution of rubidium in HMPA, at high machine amplification. The full lines show the variation of resonant field position with A for ge = 1.99800, and a microwave frequency of 9.1735 GHz. The lines are anchored at the crossovers of the MG species (A = 251.3 G). Positions of the Mc, M , ME, Mg, Mh, and M, absorptions are indicated. Reprinted with permission from R. Catterall and P. P. Edwards, Journal of Physical Chemistry, 79, 3010 (1975). Copyright 1975 American Chemical Society.

As is known from the literature [59 a], illumination of frozen solutions containing trapped electrons in the e,7 absorption band results in a decrease in the concentration of e,7. It is also well known that, for water-alkaline glasses, with such illumination, a transition of an electron from a trap into the conduction... 

Near-IR investigations of frozen solutions on the other hand were cited as experimental evidence for the Jahn-Teller effect [35-39]. The fact that the electronic absorption of Q0 has a rich vibrational structure suggests that the symmetry of the Qq ion is lower than Ih. Interestingly it has been found [36]... 

By comparison with conventional pulse radiolysis techniques based on absorption spectra, PAT offer the advantage of not depending on the transparency of the medium. Thus, useful information on, e.g., quasi-free electron scavenging, can be obtained through PAT in non glassy solids or frozen solutions [14, 23, 123). 

Figure 12 Vibrational enhancement selectivity available from resonance Raman spectroscopy. The UV-visible spectrum of a P. aeruginosa azurinis shown together with two different Raman spectra (frozen solution at 77 K) that derive from laser excitation within the S(Cys) — Cu(II) charge-transfer absorption band at 625run (647.1 nm) and away from the absorption (488.Onm). Excitation within resonance leads to dramatically increased Raman scattering from the Cu active site, whereas off-resonance excitation produces a spectrum dominated by bands of nonchromophoric ice...

Fig. 16. Mossbauer spectra of ferric enterobactin as a function of pH at 4.2 °K in a magnetic field with the Happ = 60 mT parallel to the y ray source. All samples were in frozen solution form. The vertical bars indicate 1 % absorption. With decreasing pH a fast relaxing Fe(IIl) high-spin species arises at about 6 = 0.5 mm/s...

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