Optical excitation spectra

Fig. 7. Calculated optical excitation spectra (left) and exciton absorption spectra (right) of a semiconducting CNT for a parallel polarisation.

The photoluminescence spectra, optical excitation spectra and decay curves of anthophyllite from Canada were obtained at 300 and 10 K (Sidike et al. 2010a). In the PL spectra obtained under 410-nm excitation, bright red bands with peaks at 651 and 659 nm were observed at 300 and 10 K, respectively. The origin of the red luminescence was ascribed to Mn in anthophyllite from the analysis of the excitation spectra and PL decay times of 6.1-6.6 ms. 

The photoluminescence (PL) and optical excitation spectra of baratovite in aegirine syenite from Dara-i-Pioz, Tien Shan Mts., Tajikistan and katayamalite in aegirine syenite from Iwagi Islet, Ehime, Japan were obtained at 300 and 80 K (Sidike et al. 2008). Under short wave (253.7 nm) ultraviolet light, baratovite and katayamalite exhibited bright blue-white luminescence. The PL spectrum of baratovite at 300 K consisted of a wide band with a peak at approximately 406 nm and a full width at half maximum (FWHM) of approximately 6.32 kcm The excitation spectrum of the blue-white luminescence from baratovite at 300 K consisted of a prominent band with a peak at approximately 250 nm. The PL and excitation spectra of katayamalite were similar to those of baratovite. The luminescence from these minerals was attributed to the intrinsic luminescence. 

The energy eigenvalues in DF calculations cannot be related directly to the optical excitation spectra, but the electronic density of states (DOS) and its projections often lead to useful insight and are shown in Fig. 17.7. The former are characteristic of materials with average valence five the two lowest bands can be assigned to s-electrons (a-band), and the broad band between -5 eV and the Fermi energy... 

W. E. Spicer, Possible non-one-electron effects in the fundamental optical excitation spectra of certain crystalline solids and their effect on photoemission, Phys. Rev. 154, 385-94 (1967). 

In view of this apparent contradictory outcome from the transport and magnetic properties, we were motivated to investigate the dynamics of the charge excitation spectrum by optical methods. In fact, the optical measurement is a powerful contactless experimental tool which should in principle allow to unfold the disagreement between and p(7 since the optical response of a metal... 

Figure 10-8. Emission spectra of a free standing film of a blend system consisting of 0.9% MEH-PPV in polystyrene with ca. I011 cm 3 TiOj-particlcs. The nanoparlicles act as optical scattering centers. The emission spectrum is depicted for two different excitation pulse energies. Optical excitation was accomplished with laser pulses of duration I Ons and wavelength 532 nm (according to Ref. 171).

Since the 4550 cm-1 state is the first excited state of PuF6, its radiative lifetime can be determined to be a reasonable approximation by integrating the optical absorption spectrum of PuF6 over the wavelength range where absorption due to the 4550 cm- state occurs. Some uncertainty arises since optical absorption from the next higher state undoubtedly overlaps that due to the 4550 cm-1 state. 

The calculated relative energies of the most important MO s are given in Fig. 9, together with the excitation energies, derived from the optical absorption spectrum. 

More fluorescence features than just the emission intensity can be used to develop luminescent optosensors with enhanced selectivity and longer operational lifetime. The wavelength dependence of the luminescence (emission spectmm) and of the luminophore absorption (excitation spectrum) is a source of specificity. For instance, the excitation-emission matrix has shown to be a powerful tool to analyze complex mixtures of fluorescent species and fiber-optic devices for in-situ measurements (e.g. 

Figure 8. Optical spectroscopy of a indocyanine green dye solution (10 5 M in water) (a) fluorescence spectrum obtained by excitation around 780 nm (b) excitation spectrum obtained on top of a microresonator the rectangle in the inset shows the area from which the fluorescence signal is collected.

The excitation spectrum of a fluorescent material, i.e., the incident radiation spectrum required for the induction of fluorescence, is determined by the absorption spectrum of the fluorescent material, which it often closely resembles, and by the efficiency with which the absorbed energy is transformed into fluorescence. Normally, the excitation spectrum is of higher photon energy (shorter wavelength) than that of the corresponding fluorescence emission, and in sensor schemes this has an effect in the choice of preferred fluorescent agent, compatible with appropriate optical detection devices. 

Now we will assume in addition that the total measured fluorescence is proportional to the above volume average. This can be accomplished experimentally by suspending the fluorescent particle in an integrating enclosure and monitoring the fluorescence with an optical fiber which is pushed through a small hole in the side of the enclosure. Our interest in what follows is to use Eq. (8.4) to simulate a fluorescence excitation spectrum. 

In the previous section we have seen how to determine the energy levels of an optically active center. Optical spectra result from transitions among these energy levels. For instance, an optical absorption spectrum is due to different transitions between the ground energy level and the different excited energy levels. The absorption coefficient at each wavelength is proportional to the transition probability of the related transition. 

The excitation spectrum demonstrates that for an effective luminescence not only the presence of an emitting level is important, but also the presence of the upper levels with a sufficiently intensive absorption. The excitation spectra enable us to choose the most effective wavelength for luminescence observation. The combination of excitation and optical spectroscopies enable us to determine the full pattern of the center s excited levels, which may be crucial for luminescence center interpretation, energy migration investigation and so on. The main excitation bands and fines of luminescence in minerals are presented in Table 2.2. 

A principal obstacle to identification of defects is the difficulty of comparing the results from EPR, luminescence, absorption, and deep state experiments. Probably the least ambiguous is that between EPR and luminescence when, as for transition metal impurities, it is possible for optical Zeeman measurements of a sharp luminescence line to determine the ground state g factor. If the optical and EPR measurements give the same value, then the correlation is made (Watts, 1977). In some cases, when optical excitation enhances or quenches the EPR signal, there may be a similar response in the photoconductivity or luminescence excitation spectrum. 

The 196 cm"1 mode is antisymmetric and thereby optically inactive and does not appear in the Raman spectrum. Since a direct optical excitation of the mode is excluded by symmetry selection rules we conclude that it is solely excited by the single proton transfer which breaks the symmetry. This demonstrates for the first time that the coherent excitation of a vibrational mode results exclusively from an ultrafast reactive process. 

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

Surface electromagnetic waves or surface polaritons have recently received considerable attention. One of the results has been a number of review articles1, and thus no attempt is made here to present a comprehensive review. These review articles have been concerned with the surface waves, per se, and our interest is in the use of surface electromagnetic waves to determine the vibrational or electronic spectrum of molecules at a surface or interface. Only methods using optical excitation of surface electromagnetic waves will be considered. Such methods have been the only ones used for the studies of interest here. 

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