Luminescence Excitation Spectroscopy

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

There are a large number of cases when the spectra of luminescence center remain broad up to helium temperatures. In certain cases, this is explained by a strong electron-phonon interaction, but more often the inhomogeneous broadening, connected with several types of the same center presence, causes this. In such cases it is possible to simplify the spectmm by selective excitatimi of a specific center. 

In most of luminescence experiments, at least in the minerals luminescence field, excitation is due to absorption of a single photon. However, it is also possible for a luminescence center to absorb two or more long-wavelength photons to reach the excited state. Two-photon excitation occurs by the simultaneous absorption of two lower-energy photons. Such excitation requires special conditions including high local intensities, which can only be obtained from laser sources. 

Thus it was not observed until lasers were invented. In principal, one-photon and two-photon excitation follow different selection mles. For example, the iimer shell one-photon transitions in transition metal, rare earth, and actinide ions are formally forbidden by parity selection mle. These ions have d- or f-shells and transitions within them are either even to even (d — d) or odd to odd (f f). The electric dipole transition operator is equal to zero. The two-photon transition operator is a tensor whose components may be nonzero. Thus an important reason for doing two-photon spectroscopy is that it allows to observe the transitions directly as 

In single beam two-photon spectroscopy, an intense laser beam having a frequency hv = 1/2(E2-Ei) is passed through the crystal, and the attenuation of the 

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Wurtzite-structure ZnO thin films grown by a variety of deposition techniques, as well as commercially available single crystal bulk samples are discussed. Furthermore, data for ZnO thin films intermixed with numerous elements are reviewed. Most of the results are obtained by SE, which is a precise and reliable tool for measurements of the DFs. The SE results are supplemented by Raman scattering and electrical Hall-effect measurement data, as well as data reported in the literature by similar or alternative techniques (reflection, transmission, and luminescence excitation spectroscopy). 

Complex-formation between Eu and some methyl glycosides has also been studied by luminescence excitation spectroscopy. ... 

S. Luzzati, P. Elmino, and A. Bolognesi. Luminescence excitation spectroscopy of highly oriented poly(3-octylthiophene) - polyethylene blends. Synthetic Metals 76, 23-26 (1996). 

Albin M, Horrocks WD Jr., Europium(III) luminescence excitation spectroscopy, quantitative correlation between the total charge on the ligands and the Fq Dq transition frequency in Europium (III) complexes. Inorg Chem. 1985 24(6) 895-900. 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

Time-resolved spectroscopies of various kinds have proven useful in probing the life of an excited state. As an excited state decays, perhaps through a chain of species, time-resolved spectroscopy (e.g., luminescence, excitation, resonance Raman) can provide data for these various steps. Such studies have led, for example, to the view that the first MLCT excited state in [Ru(bipyridine)3]2+, is localized in one bipyridine ring rather than delocalized over all three rings. 

Experimental technique used during these investigations is usual for Raman scattering and photoluminescence spectroscopy. For luminescence excitation He-Cd, He-Ne, and Ar+ ion lasers were used. The exciting light power not exceeds 25 mW in all experiments. 

Comprehensive PL excitation spectroscopy (PLE) investigations of the Fe3+ ( Ti — 6Ai) luminescence in semi-insulating GaN samples reveal intracentre excitation processes via excited states of the Fe3+ centre [10]. NP lines resolved around 2.01 and 2.73 eV are attributed to the 6Ai(S)-4T2(G) and... 

Lanthanide complexes of transferrin have been used for several purposes. Gd3+-transferrin gives a characteristic EPR signal at g = 4.96, quite unlike the spectra for other Gd3+ complexes (165) Eu3+ has been used to probe differences between the two sites by Eu(III) excitation spectroscopy (168) and the luminescence of excited Tb3+ ions bound in one site of mixed-metal Tb3+-Mn3+ and Tb3+-Fe3+ transferrin complexes has been used to determine the intersite distance (169). The value obtained, 35.5 A, compares well with the value of 42 A later obtained from the lactoferrin crystal structure (67). 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two... 

Moreover, by means of the method of luminescent-kinetic spectroscopy we have shown that Eu(III) ion in excited state forms more stable complex (up to two orders of magnitude) with sulfoxides, sulfones, ketones and amines then that in ground state. Since it is known that excitation of the europium is caused by electron transitions in the inner metal-centered 4f-states, our findings testify that f-electrons participate in the chemical bonding. 

The absorption of the G and D lines of the Znin acceptor in InP have been reported by Causley and Lewis [30], and this appears to be the only known acceptor absorption result for this compound. Their positions are 241.5 cm-1 (29.94 meV) for the G line (2P3/2 (Ts-) and 286.0 cm-1 (35.46 meV) for the D line (2P5/2 (Ts-). The scarcity of absorption results is due to the fact that such spectra would be close to the one-phonon absorption of InP (37.7 and 42.8meV for the TO and LO phonons, respectively). However, shallow acceptor transitions have been identified in InP by selective pair luminescence (SPL) and excitation spectroscopy (Dean et al. [48]). The separation from the IS3/2 ground state of some S and P acceptor states of Znin and Cdin have also been measured by Raman scattering [188]. 

Electronic Absorption and Emission Spectroscopy. UV and visible spectra were recorded on Cary 14, Cary 171, or Perkin-Elmer 576 ST spectrophotometers. Luminescence excitation and emission spectra. were recorded on an Hitachi-Perkin-Elmer MPF-2A spectrofluorimeter equipped with a red-sensitive Hamamatsu R-446 photomultiplier tube. Conventional flash photolysis experiments were performed as described previously (41). The samples were degassed by several cycles of freeze-pump-thaw and sealed under vacuum. 

To investigate and understand this type of energy transfer processes, one needs to make use of many different spectroscopic techniques, in particular absorption, luminescence, and excitation spectroscopy in the visible and the ultraviolet spectral regions, with which one can obtain information about the electronic excited states. These methods are assumed here to be known to the reader. 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two measurable quantities, which are obtained in arbitrary units and related to the circular polarization condition of the luminescence. It is appropriate to consider CPLS spectroscopy as a technique that combines the selectivity of CD with the sensitivity of luminescence. The major limitation associated with CPLS spectroscopy is that it is confined to emissive molecules only. 

Luminescence, Excitation, and Depolarization Spectroscopy, and Measurement of Lifetimes... 

Detailed research of titanite REE luminescence under different cw laser excitations was done by Lenz et al. (2015) including the study of luminescence and its excitation of artificial titanite samples activated by different REE, such as Sm, Nd, Pr and Eu. Relative emission intensities of individual REEs depend strongly on the excitation wavelength. The Raman spectra of titanite obtained using a 473 nm laser excitation shows emissions of Pr, Sm " and Nd ", whereas green excitation (532 nm) excites preferentially the PL of Sm ", and Nd ", red excitation (633 nm) predominantly Cr " and Nd ", and NIR excitation (785 run) Nd " only. Those results have been confirmed by excitation spectroscopy of artificially activated titanite samples. Under 785 mn excitation, Nd " emissions are exceptionally strong (whereas Raman scattering is weaker under NIR excitation when compared to visible excitatirui). Therefore, Raman spectra of titanite samples obtained with IR excitation typically are obscured vastly by Nd " emissions. 

The possibilities of time-resolved laser based spectroscopies have been demonstrated, combining such techniques as luminescence, Raman, breakdown and second-harmonic generation. New type of luminescence excitation mechanism. Plasma Induced Luminescence, was found. UV Gated Raman spectroscopy proved to be an effective tool for minerals Raman detection on the strong luminescence background. 

The analysis of the CPL spectra constitutes a straightforward method for the study of the chirality of molecules in their luminescent excited states. By means of comparative CD/CPL measurements one can investigate the geometrical differences between the ground and excited states. The observation of CPL has the problems and limitations already described in the previous sections. In particular, the molecular or supramolecular species must contain a luminophore exhibiting a sufficiently high emission quantum yield. CPL spectroscopy, however, has a number of advantages in terms of specificity and selectivity that can be extremely useful in supramolecular chemistry, namely ... 

Horrocks WD Jr, Sudnick DR. Time-resolved europium(III) excitation spectroscopy - luminescence probe of metal-Ion binding-sites. Science. 1979 206(4423) 1194—1196. 

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