Photoexcited singlet state

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

That there is magnetic resonance of only spin 1/2 excitations eliminates most of the theoretical models for the HE band in trans-iCH),. These include singlet excitons [53,541 and breather modes [55,56] for which 5 = 0, as well as triplet excitons [57] for which 5 = 1. Of all the models for the HE photoabsorption band in the literature, only 5 has spin 1/2. Thus, the only viable explanation for the HE band in the photomodulation spectrum is photogenerated 5". However, 5 cannot be directly photogenerated because the photoexcited singlet state cannot directly decay into an pair. There... 

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

Fig. 1. Relaxation of molecule-medium system subsequent to photoexcitation where Sq is the singlet ground state the fkst excited singlet state and...

Photoexcitation of the long wave bands of diazo compounds results in an excited singlet state of the diazo compound, which may then react as follows ... 

For several compounds probabilities of excited singlet state formation as a result of TTA processes have been determined in our previous paper ( ) with the help of a method based on measurement of the ratio of the integrated intensities of ADF and prompt fluorescence (PF) under the conditions of powerful photoexcitation... 

Signals recorded at probe wavelengths between 500 and 700 nm showed similar decays as the 570 nm transients shown in Figs. 2-4. These results mirror previous ones for the monomeric bases [4], and we assign these transients to excited-state absorption by singlet states created by UV photoexcitation. We consider alternative assignments in section 2.3. AMP and the two homopolymers show an ultrafast decay component, but the polymers also... 

A typical example of this is in the preparation of extraordinary divalent carbon intermediates (carbenes) by application of ultrafast laser techniques. Thus, photoexcitement of diphenyldiazomethane (DPDM) to an excited singlet state breaks the C=N bond, releasing diphenvlcarbene (DPC) in a singlet state and ultimately allows its stabilization in the triplet ground state,... 

Bis-(2,4,6-tricholophenyl)oxalate (TCPO) reacts with hydrogen peroxide to form a proposed energetic intermediate, 1,2-dioxethanedione, which transfers its energy to a lluorophor via a proposed electron transfer and excites it to the first excited singlet state followed by the emission of a photon. The reason for the interest in CL detection is that chemical excitation, as opposed to conventional photoexcitation, is lower, because of the lack of problems from scattering and fluctuations of the source. 

Quite recently, the reaction of photoexcited (singlet or triplet) silylene centers with dihydrogen has also been studied346. These excited states abstract hydrogen atoms from H2 directly. For the excited singlet state, the process takes place with no discernable barrier. 

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