Indirect excitation

Richmond, M. D. and Yeung, E. S., Development of a laser-excited indirect fluorescence detection for high-molecular weight polysaccharides in capillary electrophoresis, Anal. Biochem., 210, 245, 1993. 

A rather interesting observation is that the neodymium-decay time measured in the coactivated glass when excited directly (0.58 fi) is different than when excited indirectly through the cerium (0.30 fi). This they explain by assuming that, under direct excitation, one measures the average decay... 

Whereas under photoexcitation the exciplex is excited indirectly via energy transfer from the excitons, it is the primary neutral excitation in electroluminescence. This is shown in Fig. 2.24, parts (a) and (b), where the EL emission for both TFB and PFB blends is dominated by the exciplexes. This becomes particularly clear when comparing the EL spectra with the delayed emission spectra in Fig. 2.23, parts (c) and (d). In contrast, the time-integrated PL from similarly prepared blend films (also plotted in Fig. 2.24) is primarily due to bulk excitons. We note that exciplex EL emission has been observed previously, which suggests that these exciplexes may also be formed by the mechanism of direct electron-hole capture at the interface [37, 41, 42]. 

Spectroscopic methods such as uv and fluorescence have rehed on the polyene chromophore of vitamin A as a basis for analysis. Indirectly, the classical Carr-Price colorimetric test also exploits this feature and measures the amount of a transient blue complex at 620 nm which is formed when vitamin A is dehydrated in the presence of Lewis acids. For uv measurements of retinol, retinyl acetate, and retinyl palmitate, analysis is done at 325 nm. More sensitive measurements can be obtained by fluorescence. Excitation is done at 325 nm and emission at 470 nm. Although useful, all of these methods suffer from the fact that the method is not specific and any compound which has spectral characteristics similar to vitamin A will assay like the vitamin... 

If the cross-coupling is strong enough this may include a transition to a lower electronic level, such as an excited triplet state, a lower energy indirect conduction band, or a localized impurity level. A common occurrence in insulators and semiconductors is the formation of a bound state between an electron and a hole (called... 

Significant stimulated emission is only found for the pristine side of the sample. From these results it was concluded that the photoinduccd absorption that suppresses die stimulated emission is directly or indirectly caused by the presence of oxygen-related defects. It was shown earlier that the effect of photooxidation on the emission properties of PPV can be explained by the formation of carbonyl-groups that act as sLrong electron acceptors leading to an efficient dissociation of the plioh excited slate 29). It can be concluded that the dissociated pair near the defcci leads to the strong photoinduccd absorption. The observation that... 

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. 

The exact process(es) by which a2-adrenoceptors blunt release of transmitter from the terminals is still controversial but a reduction in the synthesis of the second messenger, cAMP, contributes to this process. a2-Adrenoceptors are negatively coupled to adenylyl cyclase, through a Pertussis toxin-sensitive Gi-like protein, and so their activation will reduce the cAMP production which is vital for several stages of the chemical cascade that culminates in vesicular exocytosis (see Chapter 4). The reduction in cAMP also indirectly reduces Ca + influx into the terminal and increases K+ conductance, thereby reducing neuronal excitability (reviewed by Starke 1987). Whichever of these releasecontrolling processes predominates is uncertain but it is likely that their relative importance depends on the type (or location) of the neuron. 

The mu, delta and kappa opioid receptors are coupled to G° and G proteins and the inhibitory actions of the opioids occur from the closing of calcium channels (in the case of the K receptor) and the opening of potassium channels (for /i, d and ORL-1). These actions result in either reductions in transmitter release or depression of neuronal excitability depending on the pre- or postsynaptic location of the receptors. Excitatory effects can also occur via indirect mechanisms such as disinhibition, which have been reported in the substantia gelatinosa and the hippocampus. Flere, the activation of opioid receptors on GABA neurons results in removal of GABA-mediated inhibition and so leads to facilitation. 

Figure 15.9 Peptide modulation of striatal input to the globus pollidus. Enkephalin released from axon terminals of neurons of the indirect pathway (see Fig. 15.2 for details) is thought to inhibit GABA release from the same terminals so that feedback (auto) inhibition is reduced. This will free the neurons to inhibit the subthalamic nucleus (SThN) and its drive to GPint and SNr which in turn will have less inhibitory effect on cortico-thalamic traffic and possibly reduce akinesia. Dynorphin released from terminals of neurons of the direct pathway may also reduce glutamate release and excitation in the internal globus pallidus and further depress its inhibition of the cortico-thalamic pathway. High concentrations of these peptides may, however, result in dyskinesias. (See Henry and Brotchie 1996 and Maneuf et al. 1995)...

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