Steady State Spectroscopy

In the rotating wave approximation (RWA) (i.e., neglecting the terms and Eq. (4.12) becomes 

Here the relations described in Eqs. (4.9) and (4.10) have been used. x(oX) consists of real part y ( u) and imaginary part x (oj) [17, 39, 40] 

From the above discussion, one can see that the central problem in linear and non-linear spectroscopies in the susceptibility method is the calculation of (f). In the linear case one has to solve [22, 38, 39] 

A two-level system (n,m) with En Em shall be used to illustrate the derivation. In this case, Eq. (4.19) becomes 

Here for simplicity it is assumed that Jlnn — flmm — 0. From Eq. (4.17) one obtains 

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In steady state spectroscopy under the condition of slow reorientation of molecules in solution (xr>x), the following peculiarities take place ... 

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

So far presented results are connected with so-called steady-state spectroscopy. Conventionally studied steady state or CW (continuous-wave) liuni-nescence is a process where the excitation sources pump the sample at constant intensity over a time necessary to perform the measurement. The end result is... 

Natural minerals may contain simultaneously up to 20-25 luminescence centers, which are characterized by strongly different emission intensities. Usually one or two centers dominate, while others are not detectable by steady-state spectroscopy. In certain cases deconvolution of the liuninescence spectra may be useful, especially in the case of broad emission bands. It was demonstrated that for deconvolution of luminescence bands into individual components, spectra have to be plotted as a function of energy. This conversion needs the transposition of the y-axis by a factor A /hc (Townsend and Rawlands 2000). The intensity is then expressed in arbitrary imits. Deconvolution is made with a least squares fitting algorithm that minimizes the difference between the experimental spectrum and the sum of the Gaussian curves. Based on the presumed band numbers and wavelengths, iterative calculations give the band positions that correspond to the best fit between the spectrum and the sum of calculated bands. The usual procedure is to start with one or... 

Spodumen is a monoclinic pyroxene, space group C C2 c), with two not equivalent metal cation sites Ml and M2. The aluminum occupies the smaller Ml site, which is approximately octahedral (actual symmetry C2) with an average metal-oxygen distance of 1.92 A. The M2 site, occupied by Li, is also six-fold coordinated with an average metal-oxygen distance of 2.23 A. Both A1 and Li sites may be substitutionally replaced by ions of the transitional metals in various proportions. Both Mn " " and Cr " centers have been identified in luminescence spectra by steady-state spectroscopy (Tarashchan 1978 Walker et al. 1997). At room and lower temperatures only one emission band of Mn + occurs and the excitation spectra taken for the different wavelengths of the luminescence bands are always the same. So it is very probable that Mn + ions in the spodumen matrix present only in one site. The calculated values of 10D,j and B are consistent with the occupation of larger M2(Li) weak-field site. Mn + is mainly in Li-sites rather than Al-sites. 

Two types of Ce centers in calcite were detected by steady-state spectroscopy (Kasyanenko and Matveeva 1987). The first one has two bands at 340 and 370 nm and is connected with electron-hole pair Ce -COj". The second one has a maximum at 380 nm and was ascribed to a complex center with Ce and OH or H2O as charge compensators. Such a center becomes stronger after ionizing irradiation and disappears after thermal treatment. The typical example of Ce luminescence in the time-resolved liuninescence of calcite consists of a narrow band at 357 nm with very short decay time of 30 ns, which is very characteristic for Ce " (Fig. 4.13a). It was found that Ce " excitation bands occurs also in the Mn " " excitation spectrum, demonstrating that energy transfer from Ce to Mn " occurs (Blasse and Aguilar 1984). 

The presence of Pr in apatite samples, up to 424.4 ppm in the blue apatite sample, was confirmed by induced-coupled plasma analysis (Table 1.3). The luminescence spectrum of apatite with a broad gate width of 9 ms is shown in Fig. 4.2a where the delay time of500 ns is used in order to quench the short-lived luminescence of Ce + and Eu +. The broad yellow band is connected with Mn " " luminescence, while the narrow lines at 485 and 579 nm are usually ascribed to Dy and the fines at 604 and 652 nm, to Sm +. Only those luminescence centers are detected by steady-state spectroscopy. Nevertheless, with a shorter gate width of 100 ps, when the relative contribution of the short lived centers is larger, the characteristic fines of Sm " at 652 nm and Dy + at 579 nm disappear while the fines at 485 and 607 nm remain (Fig. 4.2b). It is known that such luminescence is characteristic of Pr in apatite, which was proved by the study of synthetic apatite artificially activated by Pr (Gaft et al. 1997a Gaft... 

Luminescence of Pr + in zircon is very difficult to detect under UV excitation even by time-resolved spectroscopy. The reason is that it has a relatively short decay time similar to those of radiation-induced centers. Visible excitation, which is not effective for broadband luminescence, allows the revealing of Pr + luminescence lines, using high-resolution steady-state spectroscopy. Under such experimental conditions each element has individual lines, enabling confident identification of the spectrum to be possible (Gaft et al. 2000a). Only if radiation-induced luminescence in zircon is relatively weak, the lines of Pr may be detected by UV excitation (Fig. 4.38c). 

Divalent europium is detected in apatite as a shoulder of Ce luminescence when studied by steady-state spectroscopy (Tarashchan and Marfunin 1969 ... 

After a delay of several ps, the luminescence of Eu " is already very weak, and narrow long-lived lines of trivalent RE dominate in the spectrum. The lines at 589, 617, 651, and 695 nm (Fig. 4.1c) have never been detected in natural apatite by steady-state spectroscopy. According to their spectral position they may be ascribed to Eu ", but they are different from known lines in synthetic apatites activated by Eu (Jagannathan and Kottaosamy 1995 Morozov et al. 1970 Piriou et al. 1987 Piriou et al. 2001 Voronko et al. 1991). In order to clarify this problem we studied artificially activated samples by laser-induced time-resolved luminescence spectroscopy. 

The spectral-kinetic parameters of the narrow band at 375 nm enable its confident identification as Eu + luminescence, which is confirmed by emission of synthetic BaS04 artificially activated by Eu (Fig. 5.15a). Such emission was also detected and interpreted by steady-state spectroscopy (Tarashchan 1978). It is interesting to note that very often such a band is absent in natural barite and appears only after heating in air at 600-700 °C (Fig. 4.31b). Such a transformation is reversible, at least partly. Under X-ray excitation the intensity of the UV band diminishes, and a new blue-green emission appears (Fig. 5.16). This shows some kind of transformation, which takes place in the barite lattice under these conditions. Several possibihties exist. It is possible that in barite the luminescence is quenched by the components with high-energy phonons. The water and organic matter may represent the latter. They are removed after... 

The narrow band at 437 nm with a decay time of 650 ns in the danburite luminescence spectrum belongs to Eu " luminescence (Fig. 4.15a), which was also detected by steady-state spectroscopy (Gaft et al. 1979). Besides that, under 308 nm excitation narrow lines appear at 580, 592, 611, 618, 655 and 692 nm (Eig. 4.15c) with a long decay time, which confidently may be ascribed to Eu. Under 266 nm excitation another group of hnes appear with the main line at 575 nm connected with a different type of Eu " (Fig. 4.15d). 

Spectra with narrow gates where the centers with a short decay time are emphasized enables us to detect broad bands at 794 nm with a decay time of 5 ps and broad band at 840 nm with a decay time of 190 ps (Fig. 4.42). These bands maybe ascribed to Cr luminescence centers, in addition to Cr with narrow -lines at 720 nm, detected by steady-state spectroscopy (Min ko et al. 1978). Wollastonite structure has three different types of six-coordinated calcium-oxygen groups, which enables the formation of several types of Cr " limiinescence centers. Nevertheless, luminescence of Cr " as result of Ca substitution has not been confidently found yet and another interpretation is also possible. For example, ions of V maybe considered, which have similar luminescence properties with Cr + and may substitute in Ca + sites. 

Time-resolved Photoacoustic Spectroscopy. In photoacoustic spectroscopy (PAS) the heat evolved by the absorption of light in the sample is transformed into sound waves which are detected by a microphone. In steady-state spectroscopy the light is continuous, but it is also possible to use a pulsed laser and to observe the change in the intensity of the sound signal with time. In this respect time-resolved PAS is somewhat similar to thermal lensing, but both techniques have different limitations and advantages. 

D. V. Matyushov and M. D. Newton,/. Phys. Chem. A, 105, 8516 (2001). Understanding the Optical Band Shape Coumarin-153 Steady-State Spectroscopy. 

B. O Regan M. Graetzel D. Eitzmaurige, Optical Electrochemistry I Steady-state spectroscopy of conduction band electrons in a metal oxide semiconductor electrode. Chem. Phys. Lett. 1991, 383, 89-93. 

In the first section, steady-state spectroscopy is used to determine the stoichiometry and association constants of molecular ensembles, emphasize the changes due to light irradiation and provide information on the existence of photoinduced processes. Investigation of the dynamics of photoinduced processes, i.e. the determination of the rate constants for these processes, is best done with time-resolved techniques aiming at determining the temporal evolution of absorbance or fluorescence intensity (or anisotropy). The principles of these techniques (pulse fluorometry, phase-modulation fluorometry, transient absorption spectroscopy) will be described, and in each case pertinent examples of applications in the flelds of supramolecular photophysics and photochemistry will be presented. 

They also directly observed the sequential energy transfer from individual DSB units to their aggregates and then from the loose aggregates to the relatively more compact aggregates by time-resolved fluorescence spectroscopy. This observation explained that the emission of the concentrated solutions and films of PDSBs I-IV is entirely or almost the aggregation emission in the steady-state spectroscopy. 

Steady-State Spectroscopy Although the simplest tool available to probe the solution behavior of polyelectrolytes, data derived from steady-state spectroscopy has proven extremely informative in terms of revealing the conformational switch of PMAA in aqueous media. Steady-state measurements encompass a variety of forms of spectroscopic analysis ranging from examination of shifts in the spectral profile [6,19-22,24] to changes in the intensity of luminescence sampled at a particular wavelength [6,17,18,22-24,60-62] of a label or probe. 

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