Luminescence experiments

Luminescence experiments in dichloromethane solution indicated that the fluorescence of the phenylacetylene branches is quenched, whereas intense emission is observed from the binaphthol core. This antenna effect represents the first example of efficient (>99%) energy migration in an optically pure dendrimer. The fluorescence quantum yield increases slightly with increasing generation the values of 0.30,0.32, and 0.40 were obtained, respectively, for 10-12. 

In a luminescence experiment, only a fraction of the total emitted light is measured. This fraction depends on the focusing system and on the geometric characteristics of the detector. Therefore, in general, the measured emitted intensity, (/em), can be written in terms of the incident intensity /q as ... 

The authors would like to thank Dr J.L. Halary for valuable discussions and help in the field of polarized luminescence experiments. Scholarship support to I. Iliopoulos from the Greek State Scholarship Foundation is also gratefully acknowledged. 

In most luminescence experiments, at least in the mineral 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. 

Figure 18-20 Essentials of a luminescence experiment. The sample is irradiated at one wavelength and emission is observed over a range of wavelengths. The excitation monochromator selects the excitation wavelength (X ) and the emission monochromator selects one wavelength at a time (Xem) to observe.

One respresents evolution with respect to universal time controlled by the local system Liouvillian L0, the other part describes evolution with respect to intrinsic time being the irreversible component of the process determined by the interaction Liouvillian Ly. The average <9r(t)/2r°> then corresponds to any deviation of the usual density changes due to the population or depopulation (decay) of the states. For a stationary luminescence experiment, this term therefore describes the photon fluctuations from a vibronic state of the local system H0, caused by environmental interactions. 

SIMS (61,64,86), microscopy (65), XPS (56), electron microprobe techniques (14,66), electron paramagnetic resonance (EPR) (67) and luminescence experiments (68) have been successfully employed to probe and study V mobility and reactivity on a catalyst surface. TEM, STEM and energy dispersive X-ray emission (EDX) measurements have indicated that V interaction with REY-crystals induced vanadate clusters formation (65). Vanadium was also found capable of reacting with rare-earths outside the zeolite cavities to form LaVQ4... 

The presence of a shallow acceptor level in GaN has been attributed to C substituting on an N site by Fischer et al [7], In luminescence experiments on GaN from high temperature vapour phase epitaxy in a C-rich environment donor-acceptor and conduction-band-to-acceptor transitions have been distinguished in temperature dependent experiments. From the separation of both contributions an optical binding energy of 230 meV close to the value of effective mass type acceptors was obtained. Hole concentrations up to 3 x 1017 cm 3 were achieved by C doping with CCU by Abernathy et al [10], In addition Ogino and Aoki [17] proposed that the frequently observed yellow luminescence band around 550 nm should be related to a deep level of a C-Ga vacancy complex. The identification of this band, however, is still very controversial. 

Recombination is either radiative or non-radiative. The radiative process is accompanied by the emission of a photon, the detection of which is the basis of the luminescence experiment. The radiative transition is the inverse of optical absorption and the two rates are related by detailed balance. Non-radiative recombination is commonly mediated by the emission of phonons, although Auger processes are sometimes important, in which a third carrier is excited high into the band. The thermalization process occurs by the emission of single phonons and is consequently very rapid. Non-radiative electron-hole recombination over a large energy requires the cooperation of several phonons, which suppresses the transition probability. 

Photoluminescence is the radiation emitted by the recombination process and as such is a direct measure of the radiative transition. Information about non-radiative recombination can often be inferred from the luminescence intensity, which is reduced by the competing processes (Street 1981a). The most useful feature of the luminescence experiment is the ability to measure the emission spectrum to obtain information about the energy levels of the recombination centers. The transition rates are found by measuring the transient response of the luminescence intensity using a pulsed excitation source. Time resolution to about 10 s is relatively easy to obtain and is about the maximum radiative recombination rate. The actual recombination times of a-Si H extend over a wide range, from 10 s up to at least 10- s. 

The calculated curves in Fig. 3.19 were obtained using a traps depth of 0.25 eV below the conduction band. (The low-current discrepancies between the model and experiment are either due to some field-effect component of the mobility or else to a non-negligible energy barrier for electron in ection.) This value is consistent with independently measured trap levels determined by thermally stimulated luminescence experiments.84 It is also corresponds well with the difference in reduction potential of Hq and Alq3, about 0.2 eV, as suggested in Figs. 3.7 and 3.8. 

In Sect. 4.2 the luminescence and electrical conductivity of uranium-activated sodium-fluoride single crystals will be discussed, whereas in Sect. 4.3 the laser excited luminescence experiments will be dealt with. 

The luminescence experiments show that the preparation conditions of the crystals provide a tool to isolate the various line series. It is therefore concluded that the... 

Transient flash photolysis and time-resolved luminescence experiments were performed using a system similar to one described previously (92), which is diagramed in Figure 1. Aqueous suspensions of ca. 100 mg of solid sample... 

---