Spectral absorptivity

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

In principle, if an estimate could be made of K, the equilibrium concentration ratio of hydrated to anhydrous cations, relation (15) would enable the approximate pA of the anhydrous species to be calculated. Although such an estimate may be derivable from absorption spectral data, no such calculation appears to have been reported. Conversely, if an upper estimate of pA is made from the (pAa)eqm value for the corresponding, appropriately methyl-substituted base, Eq. (15) can be used to furnish a lower limit to the extent of hydration in the cation. Taking quinazoline as an example ... 

Quantitative analysis of the peroxy group of macroinitiators is performed by iodometry [38] and that of the azo group is done by ultraviolet (UV) spectrometry. Recently, type II MAI composed of PU was determined of its azo concentration by UV [20]. When the UV absorption spectral peak of the azo group overlaps other peaks, DSC is available by determining the azo group from the exothermal peak area [1IJ. 

Properties of luciferin. The crystals are microscopic needles, which melt with decomposition at 205-210°C (Bitler and McElroy, 1957). It is a quite stable luciferin compared with some other luciferins, such as Cypridina luciferin and the luciferins of krill and dinoflagellates. It is not significantly affected by lOmM H2SO4 and lOmM NaOH at room temperature in air. The absorption spectral data of luciferin are shown in Fig. 1.3 (McElroy and Seliger, 1961). The molar absorption coefficient of the 328 nm peak in acidic solutions and that of the 384 nm peak in basic solutions are both 18,200 (Morton et al., 1969). Luciferin is fluorescent, showing an emission maximum at 537 nm in both acidic and basic conditions, although the intensity of the fluorescence is lower in acidic solution than in basic solution (fluorescence quantum yields 0.62 in basic condition, and 0.25 in acidic condition Morton et al., 1969). The chemical synthesis... 

Mnemiopsin is inactivated by the exposure to light over its entire absorption spectral range, and the inactivation cannot be reversed by keeping the inactivated material in the dark, which differs from the specimens of a live animal. The photoinactivation is accompanied by a partial loss of the 435 nm absorption band, which is probably due to the decomposition of the peroxidized coelenterazine in the protein. 

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

In order to develop compounds that can selectively target duplex RNA, Sinha et al. [ 194] studied the interaction of berberine with two different conformations of poly(rC) poly(rG) structures. Poly(rC) poly(rG) has been shown [15,215] to exist in two conformations depending on the pH of the solution, the A-form at physiological pH and the protonated form at pH 4.3. These two conformations have been characterized to have clearly defined but distinctly different circular dichroic and absorption spectral characteristics. Both the A-form and the protonated form of the RNA induced moderate hypochromic change and bathochromic shifts in the absorption maxima peaks at 344 nm and 420 nm of the alkaloid with three isosbestic points centered around 357,382 and 448 nm. Binding of berberine to both forms enhanced the fluorescence intensity, which was higher with the protonated form than with the A-from, suggesting clear differences in the nature of orientation... 

TR spectroscopy can be used to investigate the changes in the molecular structure associated with the transient absorption spectral changes observed during the... 

Fig. 6. UV-Vis absorption spectral change of trans-25 (0.126 mM) in acetonitrile under a nitrogen atmosphere upon photoirradiation with three bright lines (Amaj[ = 365, 436, and 546, nm) of a super-high-pressure Hg lamp. The spectra are depicted at 10 min intervals of photoirradiation. The irradiation with each bright line was continued for 30 min in ascending order of wavelength. (Reprinted with permission from Ref. 153.)...

Energy transfer measurements were used, together with fluorescence and absorption spectral data of the donor and acceptor moieties, to calculate the donor-acceptor separation via the Forster equation. The average values of R obtained assuming random donor-acceptor orientations were 21.3 1.6 for (1) and 16.7 + 1.4 for (2). The average separation obtained from molecular models is 21.8 + 2.0 for (1) and 21.5 2.0 for (2). The somewhat low calculated separation between the groups of (2) may be due to nonrandom donor-acceptor orientations. 

Andersson, P.O., T. Gilbro, and L. Fergusson. 1991. Absorption spectral shifts of carotenoids related to medium polarizability. Photochem. Photobiol. 54 353-360. 

Krishnamurty, K. V. et al., At. Abs. Newslett., 1976, 15, 68-70 When preparing soil and sediment samples for atomic absorption spectral analysis for trace metals, pre-oxidation with nitric acid before addition of hydrogen peroxide eliminates the danger of explosion. 

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

A rigorous treatment of the IR-absorption spectral line broadening for valence vibrations of a reorienting group should include, in addition to reorientational... 

The electronic spin-state crossover in [Fe(HB(pz)3)2] has also been observed in the fine structure of its fC-edge x-ray absorption spectrum [38]. The changes in the x-ray absorption spectra of [Fe(HB(pz)3)2] are especially apparent between 293 and 450 K at ca. 25 eV, as is shown in Fig. 5. The 293 K x-ray absorption spectral profile observed in Fig. 5 for [Fe(HB(pz)3)2] has been reproduced [39] by a multiple photoelectron scattering calculation, a calculation that indicated that up to 33 atoms at distances of up to 4.19 A are involved in the scattering. As expected, the extended x-ray absorption fine structure reveals [38] no change in the average low-spin iron(II)-nitro-gen bond distance of 1.97 A in [Fe(HB(pz)3)2] upon cooling from 295 to 77 K. 

EXO 0748-676, Cottam et al. (2002) have found absorption spectral line features, which they identify as signatures of Fe XXVI (25-time ionized hydrogenlike Fe) and Fe XXV from the n = 2 —> 3 atomic transition, and of O VIII (n = 1 —> 2 transition). All of these lines are redshifted, with a unique value of the redshift z = 0.35. Interpreting the measured redshift as due to the strong gravitational field at the surface of the compact star (thus neglecting general relativistic effects due to stellar rotation on the spectral lines (Oezel Psaltis 2003)), one obtains a relation for the stellar mass-to-radius ratio ... 

In 1899, Lowry discovered the change in the rotatory power over time of a solution of nitrocamphor in benzene, an effect previously encountered only with aqueous solution of sugars. He named this effect "mutarotation," and its discovery was taken as a prominent achievement for Armstrong s laboratory research group. 50 Lowry ascribed the phenomenon to tautomeric conversion (from a CH-N02 form to a C = NO-OH form), that is, the shift of a hydrogen atom and the shift of a double bond. In 1909, he and Desch concluded that this reversible transformation occurs very quickly because they could not find an ultraviolet absorption spectral band characteristic of either isomer. 51 But what triggered this reversible transformation ... 

Figure 2. Electronic absorption spectral changes during 366-nm irradiation of an isooctane solution of [ReH5(PMe2Ph)s] under an Ht atmosphere.

Figure 3 Absorption spectral changes of t,c,t-TPCB recorded immediately after y-R of t,c,t-TPCB (5 X 10 M) in glassy BC matrices at 77 K and after warming (a), or after y-R and subsequent photoirradiation at I longer than 390 nm (1) and after warning (in the order of 2 and 3) (b), and difference spectra between spectra after photoirradiation and after warming (A = /Ino. i No. 2) (c).

Figure 4 Absorption spectral changes of recorded immediately after y-R of t,t,t-TPCB...

Figure 5 Absorption spectral changes of 9 and 10 recorded immediately after 7-R of 9 at 3 x 10 M (a) and 10 at 1 x 10 M (b) in BC rigid matrices at 77 K and after warming. Arrows show decrease in the peak upon warming.

Figure 7 Transient absorption spectral changes of lla during PR of 11a at 1.0 x 10 HMPA at r.t. Insets time profiles of AO.D.450 and AO.D.soo-...

Figure 8 Absorption spectral changes of 11a upon warming up to 90 K after 7-R of 11a at 5.0 X10 M in MTHF rigid matrix at 77 K.

Figure 10 Transient absorption spectral change of 14b in the NIR region (CR band) obtained from decay curves during PR of 14b in DMF.

Of these four properties, spectral shifts are the most sensitive to environmental changes and also the most readily measured. As a result the majority of investigations into electronic absorption spectral changes resulting from surface adsorption have been confined to measurements of spectral shifts. While the shift of the 0-0 bands is the most meaningful measurement to make, these 0-0 bands are not always discernible, especially when the molecules are adsorbed on polar surfaces, so it has become common practice simply to measure the shift of the absorption maximum. In most cases this measurement would correspond to the shift of the 0-0 band, in others, however, adsorption processes can produce unequal displacement of the ground and excited state potential curves, resulting in a different vibronic band shape. 

One further comment regarding noise in absorption spectral data, it is the signal-to-noise ratio that affects the quality of the results, not the peak-height -to-noise or information-to-noise ratio. This statement assumes that the data to be deconvolved are principally 10-30% absorbing and that the signal-to-noise ratio satisfies the requirements of Eq. (40). That this is a reasonable observation follows from the physically meaningful constraints that are imposed and the deconvolution process as discussed in Chapters 4 and 7. 

Photochromic dithenylmaleimide 32 contains two ferrocene units.41 Electrochemical absorption spectral studies indicate that oxidation of ferrocenyl units can induce ring-opening reaction for the ring-closed isomer of 32. 

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