B,£-Absorption bands

Streltsov A M, Aartsma T J, Hoff A J and Shuvalov V A 1997 Osoillations within the B absorption band of Rhodobacter sphaeroides reaotion oenters upon 30 femtoseoond exoitation at 865 nm Chem. Rhys. Lett. 266 347-52... 

A clear and striking pattern emerges from these data, a u radicals consistently show large up- and downshifts for and V2 while a2u radicals show large shifts in the opposite directions. Since these two bands are the dominant features of spectra obtained in resonance with the B absorption band, their shifts produce a clear indication of radical type, even when the spectra are relatively weak or poorly resolved. Thus, RR spectroscopy appears to be an apt tool for monitoring radical formation in biological-type porphyrins. 

Figures 1.12 and 1.13 readily explain, without quantitative calculation, some key features of the photodissociation of H2O through excitation in the A and in the B absorption bands. Multi-dimensional potential energy surfaces are the cornerstones for a trustworthy analysis of molecular dynamics. Knowing the general topology of the PES often suffices for a qualitative explanation of the main experimental observations. However, in order to perform realistic calculations we need potential energy surfaces which are as accurate and complete as possible.

Figure 1.4. Atomic and molecular spectra a) sharp-line absorption typical for isolated atoms in the gas phase, b) absorption band with vibrational structure typical for small or rigid molecules, and c) structureless broad absorption typical for large molecules in solution (adapted from Turro, 1978).

In Rp. viridis reaction centers an optical signal at 1050 nm, due to stimulated emission of the excited state of P960 (P960 or P ) comparable to the 920-nm emission of P870 in Rb. sphaeroides, was also observed. The times for formation and decay of P in Rp. viridis were found to be identical to those in Rb. sphaeroides reaction centers, namely, less than 100 fs and 2.8 0.2ps, respectively. An eiectrochromic shift of the BChl b absorption band observed at 830 nm was induced by the presence of [P BO ] pro-... 

The existence of even parity states lying above the have been identified by two-photon absorption (TPA), two-photon fluorescence excitation [31], and electroabsorption (EA) spectroscopy. We measured the TPA spectrum of DOO-PPV by Z-scan [32] showing both the real and imaginary components of Figure 7-13 compares the linear absorption spectrum of DOO-PPV to the spectra of the real and imaginary molecular second hyperpolarizability. The imaginary component, y", is proportional to the TPA coefficient a2, while the real component, y, corresponds to the non-linear dispersion, 2- The y" spectrum shows a clear peak at % 3.2,5 eV, 0.5 eV above the peak of the l.B absorption band, and a shoulder at se3.5 eV, which may correspond to a second TPA band. The y spectrum clearly indicates dispersion near the two y" peaks, as expected from Kra-mers-Kronig analysis, and permits us to identify two distinct TPA bands. 

Fig. 1 Typical CD-curve for a relatively simple benzene derivative same signs of the first two CD-bands, split CD (appearance like in CD-couplet) within the B,B -absorption band.

We used QB to probe [bmim][BF ] and [bmim][Tf2N] encapsulated in TX-lOO and BHDC/benzene [151] and found that the microenvironment of [bmim][BF ] and [bmim][Tf2N] encapsulated in TX-lOO and BHDC/benzene RMs depends strongly on the surfactant used to create the organized media. In TX-lOO, both the hypsochromic shift of the B band and trend of the QB B and B absorption band ratio with IT show almost identical trends for [bmim][BF ] and [bnmnJfTfjNj. This suggests that the nature of their interactions within the RM structure is similar and that neither of the anions interact appreciably with the TX-lOO oxyethylene units [92, 96]. 

The optical pumping efficiency and output power of many rare earth lasers can be increased by codoping the medium with other ions which absorb pump radiation and effectively transfer the excitation to the upper laser level. This transfer may be either radiative or nonradiative. In general, sensitization schemes used for phosphors and other luminescence phenomena are also applicable to lasers (Van Uitert, 1966). Requirements for the sensitizer ion include (a) no ground- or excited-state absorption at the laser wavelength, (b) absorption bands which complement rather than compete with absorption bands of the laser ion, since the fluorescence conversion efficiency usually is less for the former, (c) one or more metastable energy levels above the upper laser level,... 

Symmetry-lowering effects of the solvent are referred to as the Ham effect (Ham, 1953 Platt, 1962). Thus, in rigid or fluid solutions, symmetry-forbidden vibrational components of the B2U C b) absorption band of benzene appear with increasing intensity as the polarizability and the polarity of the solvent increase. The fact that for pyrene and 2-methylpyrene the direction of the L, transition moment is inclined on the average 40° and 20° away from the y axis, respectively, has also been ascribed to a symmetrylowering perturbation by the environment, related to the Ham effect (Lang-kilde et al., 1983). The associated intensification of otherwise weak vibronic peaks in the L, band can be used for an investigation of microenvironments such as micelles. 

As shown in Figure 3.2a,b, absorption bands due to water vapor and carbon dioxide exist in the spectra ( ) Bf(v). In principle, these bands should cancel themselves... 

Figure 2.4 (a) An absorption experiment, (b) A broad and (c) a narrow absorption band with the... 

Substance A has an absorption spectmm in one or more regions of the ultraviolet or visible spectral range. Irradiation of A at a wavelength corresponding to one of the absorption bands results in formation of substance B, which has a visible absorption spectmm different from A. Most commonly, substance A is uncolored or only slightly colored, whereas substance B is colored or appears darker than A. 

A. Cations, on the other hand, which are a mixture of hydrated and anhydrous species have an intermediate spectrum of the type B B. The long-wavelength absorption band B is due to the anhydrous cation, and the short-wavelength absorption band B to the hydrated... 

As mentioned in Section II,B, solutions of y9-hydroxypyridines in the nonpolar solvents chloroform and carbon tetrachloride show sharp infrared absorption bands near 3600 cm indicating that they exist in the hydroxy form. Infrared spectral data also led Mason to conclude that -hydroxypyridines probably exist largely as such in the solid state and exhibit O— 0 hydrogen bonding, a conclusion which is contrary to an earlier proposal favoring a zwitterion structure. 

Unstable chlorides were converted to stable SnCU complexes. In their IR spectra there is an intense absorption band in the 1900 cm region, which is consistent with the band of allenic system (structure C). Unlike unstable chlorides A and B, the SnCl4 complexes are stable and, when kept in an inert atmosphere, remain intact for several days. The allenic structure of the immonium salt was confirmed by studying the mercuration of the same aminobutenynes (74DIS). 

The ethereal filtrate and washings are distilled under reduced pressure (Note 9) with the use of a 6-in. Vigreux column, and pure ketene di(2-methoxyethyl) acetal (Note 10) is obtained b.p. 81-84° (2.0 mm.), m25d 1.4411, yield 98-132 g. (56-75%). The infrared spectrum of the product shows a very strong C=C absorption band at 1640 cm.-1. 

The b-polarized absorption bands (see Fig. 6-3b) at 20945 cm 1, 22250 cm-1 and 23535 cm-1 are assigned to a vibronic progression built on the bu Davydov component with the totally symmetric mode at 1275 cm-1. 

If we consider an absorption band showing a normal (Gaussian) distribution [Fig. 17.13(a)], we find [Figs. (b) and (d)] that the first- and third-derivative plots are disperse functions that are unlike the original curve, but they can be used to fix accurately the wavelength of maximum absorption, Amax (point M in the diagram). 

Moreover, an absorption band near 1375 cm-i is detected and it is assigned to the CH bending vibration present in cellulose and hemicellulose chemical structures (Sim et al., 1998). The prominent band at 1044 cm-i is also associated with hemicelluloses and is attributed to the C-OH bending. Finally, a sharp band at 897 cm-i, which is typical of b-glycosidic linkages between the sugar units in hemicelluloses, was detected in the anomeric region (Sun et al., 2005). 

In order to clarify the reason why the Rap-Stoermer reaction proceeds so efficiently under solvent-free conditions, the K2C03-assisted reaction of 89a and 90a was studied by IR spectral monitoring in Nujol mulls [ 16]. Firstly, the potassium salt formation of 89a was studied (Scheme 16). One minute after mixing of 89a with twice the molar amount of K2CO3, a vC=0 absorption band (B) appeared at 1692 cm" (spectrum II in Fig. 1), although 89a itself shows vC=0 absorption (A) at 1664 cm (spectrum I in Fig. 1). After 10 min, B disappeared and a new vC=0 absorption band (C) appeared at 1670 cm (spectrum III in... 

Fig. 1). The absorption (C) was assigned to vC=0 in the potassium salt (93) by comparison of the spectrum III with that of an authentic sample. Absorption band B was assigned to vC=0 in a 1 1 complex (92) of 89a with 93, by comparison of the spectrum II with that of an authentic sample, prepared by mixing 89a with half the molar amount of K2CO3. Unfortunately, the structure of 92 could not be determined, since 92 did not form suitable crystals for X-ray analysis. Treatment of 92 with K2CO3 in the solid state gave 93.

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