Anisotropy spectra

Second, we analyze the nature of the next, strong 2PA bands. The positions of their final states correspond to one-photon symmetry forbidden bands and can be found from excitation anisotropy measurements, as illustrated in Figs. 6,19, and 23. Excitation anisotropy spectra for all cyanine-like molecules typically reveal a large alternation of maximum and minimum features suggesting the positions of the 1PA and 2PA transitions. Two-photon excitation into final states involves two dipole moments, fi0i and /i (i. 

The most sophisticated techniques require time-resolved measurements (lifetime, anisotropy, spectra) either in the time or frequency domain ([6-10] for a focused journal issue on the subject see [11]). Thus, the significance of new, versatile, commercially available light... 

Additional information about the nature of the absorption bands of flu-orenes can be revealed from their excitation anisotropy spectra [17,21-23]. The values of fluorescence anisotropy are commonly determined as [13] ... 

The electronic structure of fluorenes and the development of their linear and nonlinear optical structure-property relationships have been the subject of intense investigation [20-22,25,30,31]. Important parameters that determine optical properties of the molecules are the magnitude and alignment of the electronic transition dipole moments [30,31]. These parameters can be obtained from ESA and absorption anisotropy spectra [32,33] using the same pump-probe laser techniques described above (see Fig. 9). A comprehensive theoretical analysis of a two beam (piunp and probe) laser experiment was performed [34], where a general case of induced saturated absorption anisotropy was considered. From this work, measurement of the absorption anisotropy of molecules in an isotropic ensemble facilitates the determination of the angle between the So Si (pump) and Si S (probe) transitions. The excited state absorption anisotropy, rabs> is expressed as [13] ... 

As was shown above, the 2PA spectra of symmetrical and unsymmetrical fluorene derivatives exhibit a complex nature of 2PA bands in the spectral region 280-420 nm (see Figs. 15, 16). In general, the nature of 2PA processes should be reflected in anisotropy spectra. These spectra, T2pa Uexc) can be obtained with Eq. 1 in the same way as riPA(A,exc)> however, with two-fold longer excitation wavelengths (560-840 nm). 

Fig. 18 Two-photon excitation anisotropy spectra (1, 1, top scales) for compound 6 (a, 1), 11 (a, 1 ), 9 (b, 1) in polyTHF and 17 (b, 1 ) in silicon oil. Solid lines (2, 2 ) are the corresponding linear absorption spectra...

Figure 5.2. Fluorescence anisotropy spectra of Lens culinaris agglutinin LCA (squares) and Vida fava agglutinin VFA (triangles) obtained at 20°C. Xem 330 nm. The instrument used is a Perkin-Eliner LS 5B.

Fig. 7.11 reports the fluorescence anisotropy spectra of ai-acid glycoproteiif (a) and ai-acid glycoproteiiE (b) recorded at 20°C between 265 and 305 nm (kem, 330 nm). The general qualitative feature resembles that obtained for Trp residue in proteins. 

Figure 7.11. Fluorescence excitation anisotropy spectra of apacid glycoprotein (a) and ai-acid glycoprotein s (b) at 20°C. Xem = 330 nm. Source Albani, J. R. 1998, Spectrochimica Acta Part A. 54, 175-183.

Fig. 44. Real part of the optical reflectance anisotropy spectra for clean Cu(llO), oxygen covered (2xl)-O-Cu(110) and CO covered (2xl)-CO-Cu(l 10) surface. Horizontal lines indicate zeros of each spectrum [95Hof]...

It is instructive to see how the anisotropy depends on the mode of excitation. Figure 10.34 diows die excitation anisotropy spectra of DPH. For one-photon exdtation... 

As a further com dicating factor, tryptophan dis days complex spectral properties due to the presence of two nearly isoenergetic excited states. and The electronic transitions display distinct absorption, emission, and anisotropy spectra and are differently sensitive to solvent polarity. The complexity of indole photc hysics has stimulated detailed studies of protein fluorescence but has also inhibited interpretation of the data. 

Fipire 20.15. Absorption and emission (------) and eadtation anisotropy spectra (-----) of [Rn(0py)2( ppe)] Ootind to the calf tbymus... 

The excitation anisotropy spectra of chlorophyll a recorded at two different emission wavelengths are distinct and exhibit several crossover points, see figure 4. Similar results were found for chlorophyll b (data not shown). This indicates that at the corresponding wavelengths more than one transition moment is excited. 

Fig. 5.2-W Reflectance anisotropy spectra for GaAs(lOO) grown by MBE at different substrate temperatures in an As4 flux of 2.8x10 ton-. The corresponding reconstructions (as determined by RHEED) are also reported [2.78]...

In order to get the anisotropy spectra it is possible to combine the fluorescence intensity ones either manually or automatically. The manual procedure required the setting of the polarizers in the four combinations reported above and their elaboration with a suitable software. In the case of the automatic acquisition it is the same software that control the instrument that set the sequential acquisition of the spectra controlling the polarizers and giving, at the end, directly the anisotropy spectrum. The direct acquisition of anisotropy is available for several instruments and is surely convenient from the operative point of view. Nevertheless, it is important to examine critically the anisotropy spectra obtained in such automatic modality. 

These quantities can be measured separately. Thus, the so-called trace and anisotropy spectra can be obtained from experiment [264]. The trace spectra are, as a rule, simpler and are due to fully symmetric vibrational transitions only. The lines are narrow, well separated and easy to analyze. Anisotropy spectra originate from non-totally symmetric vibrational distortions in the molecule and usually represent a complicated superposition of strongly overlapped bands. The interpretation of spectra in the gas-phase is also hampered by the presence of a complex rotational structure of the vibrational bands. Trace (I ) and anisotropy (la) spectra can be obtained after careful polarization experiments using the following equations [264] ... 

Fig. 5. Representative spectral patterns of chemical-shielding anisotropy. Spectra A (parallel) and B (perpendicular) were calculated for oriented DNA fibers, assuming Euler angles of 77 and 79 for p and y, respectively, that correspond to the A-DNA model of Amott and Hukins (1972). Spectrum C is a typical powder pattern of asymmetric shielding anisotropy.

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