Energy sensitization

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

Secondly, the characteristic X-rays, emitted as the electrons displaced from the inner shells of the atoms are replaced, can be detected by use of an energy-sensitive detector placed close to the specimen. An account of the application of both the energy dispersive spectroscopy (EDS) of the emitted X-rays and EELS to the... 

For each of compounds (7), n = 1-4, the photostationary isomer ratio ([A-T]/[A-C]) is about 0.91. Radical equilibration of n = 1-4 and equilibration with low-energy sensitizers which are thought to operate via Schenck... 

The observations that the pss depends upon the sensitizer concentration (for low-energy sensitizers Et < S3 kcal/mole) and that added azulene (Ef = 30 kcal/mole) alters the [Cy [T], ratio so that the pss contains more fra/w-stilbene requires the addition of the following reactions ... 

Preliminary observations indicate that stilbenes can act as quenchers for piperylene triplets.<69) This, along with the effect of low-energy sensitizers upon piperylene or 2,4-hexadienes, is another area that needs more experimental work before a complete picture can be obtained. 

Another explanation has been offered to explain the large proportion of cyclobutane derivatives produced by low-energy sensitizers, especially for the anthracene derivatives.<17) This is that energy transfer to diene occurs from the second excited triplet state of the sensitizer rather than the first. Experiments using a large number of anthracene derivatives as sensitizers... 

The following generalizations were made for donors that transfer their energy at a diffusion-controlled rate (high-energy sensitizers) ... 

The second mode of addition of alcohols to olefins, to produce ethers, has been found to occur only with cyclic olefins. Thus in 1966 Kropp reported that cyclohexenes (39)-(42) produce tertiary ethers upon photolysis in methanol in the presence of high-energy sensitizers such as benzene, toluene, or xylene<79) ... 

Abstract For the case of small matter effects V perturbation theory using e = 2V E/ Am2 as the expansion parameter. We derive simple and physically transparent formulas for the oscillation probabilities in the lowest order in e which are valid for an arbitrary density profile. They can be applied for the solar and supernova neutrinos propagating in matter of the Earth. Using these formulas we study features of averaging of the oscillation effects over the neutrino energy. Sensitivity of these effects to remote (from a detector), d > PE/AE, structures of the density profile is suppressed. 

The low density of SPHs and SPH composites allows applications as a high-strength, light-weight stractural material as well as a packaging material. They will be also good as insulators and fillers in structures with energy-sensitive applications. 

The lines at 686 and 693 nm with a long decay time of approximately 1 ms in the titanite emission spectrum are not correlated with any other lines and bands (Fig. 4.34). Such lines are very typical for Cr in a high field coordination and may be connected with such a center. The broad luminescence band appears peaking at 765, which may be ascribed to Cr + in a weak field coordination. The band at 765 nm has distinct dips at 749, 762, 793, 798, 804 and 820 nm. Comparison with the titanite absorption spectrum (Fig. 5.19) demonstrates that those lines exactly coincide with the absorption spectrum of Nd (Bakhtin and Gorobets 1992). Cr is a good energy sensitizer, because it has broad, allowed absorption bands with a broad emission spectrum, which overlaps the absorption bands of the lasing ion (Nd " ", Ho " ). 

Nonvertical Energy Transfer. The concept of nonvertical energy transfer was introduced by Hammond and Saltiel to explain the observation that low energy sensitizers would photoisomerize stilbene, despite the fact that energy... 

Ferrocene forms ground-state complexes with dienes such as piperylene and isoprene. Irradiation of these yields a 1.25 trans/cis piperylene ratio in the first case and 92% cyclobutanes in the second,284 both indications of a high-energy sensitizer. However, ferrocene itself quenches anthracene triplets, so that it probably has a fairly low triplet energy. Therefore it has been postulated that an excited ferrocene-diene complex dissociates into an excited diene triplet which then proceeds to decay just as though it had been formed by normal bi-molecular sensitization. 

The nonvertical energy transfer observed between low energy sensitizers and diphenylcyclopropane may thus be similar to that observed with the stilbenes, namely a transition to a triplet level not able to be observed spectroscopically. 

Schenck has shown that the photodimerizations of various substituted maleic anhydrides proceed via triplet states.259 The photoaddition of maleic anhydride to benzene439 is now known to involve excited states of maleic anhydride-benzene complexes.440 The preferred mechanism for the reaction in the presence of high-energy sensitizers is shown at the top of page 125. 

Fluorescein and related quinonelike dyes such as eosin have found extensive use as low-energy sensitizers, especially in photooxidation studies. The efficiency of intersystem crossing in fluorescein varies markedly with pH. For the neutral molecule, the quantum yield of fluorescence is only 0.31, while it is 0.91 for the dianion.449 In strong acid solutions, however, protonated fluorescein is strongly phosphorescent. In this state (dissolved in boric acid) fluorescein was the first compound for which T-T absorption in an excited triplet state was observed.7... 

Azobenzene readily undergoes both sensitized and direct cis-trans photoisomerization. However, the sensitized photostationary cis/trans ratios with high-energy sensitizers do not predict those obtained upon direct irradiation, indicating that much of the direct photoisomerization occurs from states other than the lowest triplet.455 As with stilbene, cyclization of the cis isomer apparently is a singlet-state reaction.456... 

These criteria for an ideal spectroscopic technique have been met, to a large extent, by Mossbauer spectroscopy. UHV conditions are not required since one utilizes y rays with energies in the keV range, and the technique lends itself easily to in situ studies. Furthermore, the technique is ideally suited for studies of small particle systems, and in several instances information about particle size can also be obtained. The unique feature of Mossbauer spectroscopy is the extremely high energy sensitivity of the technique. This allows detailed chemical, structural, and magnetic information to be obtained about atoms on the surface or in the bulk phase. 

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