Polarization, electronic transition

Linear response polarizability theory of spectral bandshapes was applied to the numerical analysis of the chiroptical spectra obtained for DNA-acridine orange complexes [85]. After analysis of various models of conformation, it was concluded that a dimer-pairs repeating sequence model was best able to account for the observed spectral trends. In another work, the CD induced in the same band system was studied at several ionic strengths [86]. The spectra were able to be interpreted in terms of the long-axis-polarized electronic transitions of the dyes, with the induced CD being attributed to intercalated and non-intercalated dye species superimposed by degenerate vibronic exciton interactions between these. 

It is thought that purines owe their maximum in the 260-nm region to a polarized electronic transition in the direction C-2 — C-4, and 8-azapu-rines should behave similarly. However those ionic species of 8-azapurine that are subject to hydration exhibit (when compared to purines) a spectral shift of about 15-75 nm (to lower wavelengths) without loss of intensity. 3.4.45,48... 

A very weak peak at 348 mn is the 4 origin. Since the upper state here has two quanta of v, its vibrational syimnetry is A and the vibronic syimnetry is so it is forbidden by electric dipole selection rules. It is actually observed here due to a magnetic dipole transition [21]. By magnetic dipole selection rules the A2- A, electronic transition is allowed for light with its magnetic field polarized in the z direction. It is seen here as having about 1 % of the intensity of the syimnetry-forbidden electric dipole transition made allowed by... 

The example we consider is the two-photon fluorescence excitation specfrum of 1,4-difluorobenzene, shown in Figure 9.29 and belonging to the >2 point group. The transition between the ground and first singlet excited state is Table A. 3 2 in Appendix A shows that 82 = r(T ) and, therefore, according to Equation (7.122), the electronic transition is allowed as a one-photon process polarized along the y axis which is in-plane and... 

The three bands in Figure 9.46 show resolved rotational stmcture and a rotational temperature of about 1 K. Computer simulation has shown that they are all Ojj bands of dimers. The bottom spectmm is the Ojj band of the planar, doubly hydrogen bonded dimer illustrated. The electronic transition moment is polarized perpendicular to the ring in the — Ag, n — n transition of the monomer and the rotational stmcture of the bottom spectmm is consistent only with it being perpendicular to the molecular plane in the dimer also, as expected. 

We now discuss the annular tautomerism of azoles. For 5-phenyltetra-zole, the 2//-tautomer was found predominant in polyvinyl alcohol film (based on the determination of polarization angles of electronic transition... 

Direct probe of ligand field and charge transfer excited states Greater sensitivity than ABS in observing weak transitions and greater resolution due to differences in circular polarization complimentary selection rules aiding in assignment of electronic transitions... 

This assignment is confirmed f)y the fact that the measured depolarization ratio of the V2 band of the [Re2Clg]2 and [Re2Brg]2" ions at resonance is 1/8, a situation which can only arise if the resonant electronic transition is xy polarized. 

The optical transition moments for vibrational or electronic transitions between defect states have specific orientations with respect to the defect coordinates. The absorption strength of polarized light for each of the differently oriented centers is proportional to the square of the component of the transition moment that is along the polarization direction. Hence, a stress-induced redistribution of the defects among their different orientations will be detected as an anisotropy in the polarized optical absorption. A convenient measure of the anisotropy is the dichroic ratio, defined as... 

Inspecting the character tables of Figure 13, we notice that the first (lowest-energy) excited state, corresponding to the electronic transition jr -> ji, is, for the s-trans isomer, Bu, for the s-cis, Hi and finally for the skewed form, B. This transition is therefore electrically allowed in all three cases, being in-plane polarized for the former two cases as shown in Scheme 9. 

In spectroscopy we may distinguish two types of process, adiabatic and vertical. Adiabatic excitation energies are by definition thermodynamic ones, and they are usually further defined to refer to at 0° K. In practice, at least for electronic spectroscopy, one is more likely to observe vertical processes, because of the Franck-Condon principle. The simplest principle for understandings solvation effects on vertical electronic transitions is the two-response-time model in which the solvent is assumed to have a fast response time associated with electronic polarization and a slow response time associated with translational, librational, and vibrational motions of the nuclei.92 One assumes that electronic excitation is slow compared with electronic response but fast compared with nuclear response. The latter assumption is quite reasonable, but the former is questionable since the time scale of electronic excitation is quite comparable to solvent electronic polarization (consider, e.g., the excitation of a 4.5 eV n — n carbonyl transition in a solvent whose frequency response is centered at 10 eV the corresponding time scales are 10 15 s and 2 x 10 15 s respectively). A theory that takes account of the similarity of these time scales would be very difficult, involving explicit electron correlation between the solute and the macroscopic solvent. One can, however, treat the limit where the solvent electronic response is fast compared to solute electronic transitions this is called the direct reaction field (DRF). 49,93 The accurate answer must lie somewhere between the SCRF and DRF limits 94 nevertheless one can obtain very useful results with a two-time-scale version of the more manageable SCRF limit, as illustrated by a very successful recent treatment... 

UV-visible (UV-vis) spectroscopy detects valence electron transitions. One may detect the electronic state of metal ions from d-d transitions. Identification of unusual ligands—that is, Cu(II)-SR, Fe(III)-OPh, Fe(III)-0-Fe(III)—may be possible. UV-vis spectroscopy on single crystals using polarized light may yield geometric information. 

---