Excitation direct

Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

A DIET process involves tliree steps (1) an initial electronic excitation, (2) an electronic rearrangement to fonn a repulsive state and (3) emission of a particle from the surface. The first step can be a direct excitation to an antibondmg state, but more frequently it is simply the removal of a bound electron. In the second step, the surface electronic structure rearranges itself to fonn a repulsive state. This rearrangement could be, for example, the decay of a valence band electron to fill a hole created in step (1). The repulsive state must have a sufficiently long lifetime that the products can desorb from the surface before the state decays. Finally, during the emission step, the particle can interact with the surface in ways that perturb its trajectory. 

Photosensitization is an important alternative to direct excitation of molecules, and this method of excitation usually results in reactions that occur via triplet excited states. If... 

Through direct excitation of a monomeric or polymeric molecule or of a molecular complex (A) followed by a reaction producing an initiating species ... 

The absorption and photoluminescence (PL) spectra of a-6T measured at 80 K are shown in Figure 7-24. There is an onset of absorption at 2.1 eV, with several sharp peaks and a maximum at 2.8 eV. A second absorption band is seen at 4.5 eV and is due to direct excitation of the thiophene ring. We take the first peak as the 0-0 transition and calculate an optical gap j,=2.4eV. The PL spec-... 

In the region near zero thickness, there is no effect of matrix composition, and Equation 7-1 should be valid. The justification for this statement follows. The deviations under discussion rest ultimately upon x-ray absorption. Because x-ray absorption in this region is negligible, its only measurable result being the direct excitation of the analytical line, there should be no deviations from Equation 7-1. 

Sherman compares calculated and observed intensities for a number of known samples in some of which the enhancement components predominate over the intensities by direct excitation. The agreement obtained is usually within a few per cent, and this would be satisfactory even for considerably simpler problems. To be sure, the calculations do not give concentrations from measured intensities. But the fact that intensities can be satisfactorily calculated from known concentrations means that absorption and enhancement effects are thoroughly understood, and that x-ray emission spectrography is on a firm foundation. 

Though theories have been proposed (32-35) to explain this phenomenon, the mechanism of fluorescence is still not yet fully understood. Jankow and Willis (36) proposed a mechanism which involves a direct excitation of the molecule or an impurity to an excited state, followed by internal conversion and then reversion back to the original state with emission of light. This mechanism can be explained as follows A molecule in the lowest vibrational level of the ground state A is transferred to a certain vibrational level in the excited state D. The molecule tends to cascade into the lowest vibrational level of state D by collisions with other excited molecules. It passes from state D to state C and then to state B by radiationless transi-... 

This is an unconventional reflex mediated by capsaicin-sensitive primary afferent neurons. In fact, an adequate stimulus can directly excite a peripheral terminal... 

Oberlin E, Amara A, Bachelerie F et al (1996) The CXC chemokine SDF-1 is the ligand for LESTR/fusin and prevents infection by T-cell-line-adapted HIV-1. Nature 382 833-835 Oh SB, Endoh T, Simen AA, Ren D, Miller RJ (2002) Regulation of calcium currents by chemok-ines and their receptors. J Neuroimmunol 123 66-75 Oh SB, Tran PB, GiUard SE, Hurley RW, Hammond DL, Miller RJ (2001) Chemokines and glycoprotein 120 produce pain hypersensitivity by directly exciting primary nociceptive neurons. J Neurosci 21 5027-5035... 

Oh SB, Tran PB, GiUard SE, Hurley RW, Hammond DL, Miller RJ (2001) Chemokines and glycoprotein 120 produce pain hypersensitivity by directly exciting primary nociceptive neurons. J Neurosci 21 5027-5035... 

Figure 1.3 Some possible basic neurotransmitter-synaptic arrangements for the excitation and inhibition of different neurons, (a) The single NT activates neuron B and inhibits neuron C by being able to activate both excitatory and inhibitory receptors or, more probably, acting on one receptor linked to both events. There is potential, however, for the NT to activate any inhibitory receptors that may be on B or excitatory receptors on C. (b) The same NT is used as in (a) but the excitatory receptors are now only on dendrites and separated from the inhibitory receptors only on the soma. There is less chance of unwanted mixed effects, (c) Neuron A releases distinct excitatory and inhibitory NTs from its two terminals each acting on specific and morphologically separated receptors. But this depends on a neuron being able to release two NTs. (d) Neuron A releases the same NT from both terminals. It directly excites B but inhibits C through activating an inhibitory interneuron (I) which releases an inhibitory NT onto specific receptors on C. This last scheme (d) is clearly more functional and is widely used...

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

The first element on the right-hand side of Eq. (40) describes excitation into a resonance manifold, which, given Eq. (35), is comprised of direct excitation of the i) and excitation via the continuum with which the /) are coupled. The second element describes the dynamics in the resonance manifold and allows for coupling between the resonances. The third element accounts for decay of the resonance into the continuum. 

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature.<44,52) While the fluorescence of /rmy-stilbene is a... 

Thus, upon direct excitation of 4-bromostilbene, cis-trans isomerization takes place in both the singlet and triplet manifolds.<1,16 38,63)... 

Correct experimental conditions are vital. In particular, it is essential to irradiate only the benzophenone chromophore, which can be achieved by employing an appropriate u.v. laser, for direct excitation of the azo compound produces a singlet diradical that collapses to bicyclopentane. Oxygen pressure (150 psi) and reaction time (60-70 h) must be carefully regulated to obtain optimum yields (ca. 20%) of 9. 

Bayer, L., Eggermann, E., Serafin, M. et al. (2001). Orexins (hypocretins) directly excite tuberomammiUary neurons. Eur. J. Neurosci. 14, 1571-5. 

The difference between the total amount of phosphorescence (I ) and the directly excited phosphorescence... 

In order to corroborate the existence of energy transfer we studied the phosphorescence polarization. While the directly excited phosphorescence of MBC in... 

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