Annihilation process

Therefore, it is apparent that the efficiency of suppression of the oxidative decomposition at the given G sites in dpflG-containing duplex increased upon increasing the rate of hole transfer to dphG. These remarkable observations could be rationalized by assuming an annihilation process of the dphG... 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

Si - Si Annihilation and Ablation Mechanism. The Si - Si annihilation process is responsible to laser ablation, which was supported by the following experiment. Total fluorescence intensity and the relative intensity of excimer emissions (-15 72 ns gate width) were plotted against the fluence in Figure 3. It is interesting that the relative contribution of excimers showed a similar change to that of total fluorescence intensity. This indicates that the Si - Si annihilation has an important role in the primary processes of laser ablation phenomena, since the relative contribution of excimers is determined by the degree of Si -Si annihilation, and the suppressed fluorescence intensity corresponds to the enhanced ablation. 

The total fluorescence intensity saturated around a few hundreds of mJ/cm2 which corresponds to the irradiation condition where the new plasma-like emission was observed. Above this value fluorescence intensity decreased, which is accompanied with the recovery of the relative intensity of excimer emissions. This means that a quite efficient deactivation channel of excitation intensity opens in this energy range, and the contribution of Si -Si annihilation is depressed. This suggests that fragmentation reactions to diatomic radicals are not induced by the annihilation process. Multi-photon absorption processes via the Si states and chemical intermediates should be involved, although no direct experimental result has as yet been obtained. 

The first simulations of the collapsar scenario have been performed using 2D Newtonian, hydrodynamics (MacFadyen Woosley 1999) exploring the collapse of helium cores of more than 10 M . In their 2D simulation MacFadyen Woosley found the jet to be collimated by the stellar material into opening angles of a few degrees and to transverse the star within 10 s. The accretion process was estimated to occur for a few tens of seconds. In such a model variability in the lightcurve could result for example from (magneto-) hydrodynamic instabilities in the accretion disk that would translate into a modulation of the neutrino emission/annihilation processes or via Kelvin-Helmholtz instabilities at the interface between the jet and the stellar mantle. 

It is the Si state produced by the triplet-triplet annihilation process that is responsible for the delayed fluorescence. Although it is emitted at the same rate as normal fluorescence, its decay is inhibited because it continues to be regenerated via step 3. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

Consequently, for a profile to decay as depicted in Fig. 1, segments of the outermost steps of the side walls must break away from their attracting neighbors, cross the terraces and meet steps of opposite sign thereby initiating a step annihilation process. As a result,... 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

Besides the prompt excimer emission, delayed excimer emission has been observed. The mechanism is through triplet-triplet annihilation process (Section 5.9B) ... 

The femtosecond fluorescence up-conversion setup has been described elsewhere [13,14]. Briefly, a second harmonic (SH) of a home-made chromium-forsterite femtosecond laser tunable from 610 to 660 nm was used to excite the sample (Fig.2) [14]. The pulse duration of the SH pulses was about 50 fs at the full width at half maximum (FWHM). We were successful in the cavity-dumping operation of this laser [14] and kept the repetition rate as low as 4 MHz. Reduction of the repetition rate was necessary to avoid multiple hits of the same location of the sample as small as possible. The excitation intensity, controlled by a neutral density filter before the sample cell, was (0.5-l)xl012 photons/cm2/pulse. Special care was taken to work at the lowest excitation light intensity so that the effect of the exciton-exciton annihilation process was negligible. 

Unlike TBHF and TIHF, the dyes containing the CN-group at C-9 position give triplets which decay by first-order kinetics not influenced by dye concentrations or laser dose. Thus we observed no triplet-triplet annihilation process. This observation may be due to insufficiency of the absorbed energy (optical densities for the CN-containing dyes are much less at Aex than for H-dyes at the same concentration), but we had to carry out our experiments using low dye concentrations in order to avoid the aggregation processes which are well known for xanthene dyes. 

Figure 6.5 displays a typical distribution of A and B particles at t = 100 for a realization of the annihilation process on the Sierpinski gasket of the first kind at the 9th stage. The segregation of dissimilar particles, resulting from initial concentration fluctuations, is clearly visible at this reaction stage. 

We performed numerical simulations of the annihilation process for the Sierpinski gaskets up to the 12th stage, starting with 10 percent of all sites randomly filled with A particles and another 10 percent with B particles. We chose the interaction range ro = 4.27 (in units of the nearest-neighbor distance ao, which leads to the dimensionless initial reactant concentration n(0) = 1 (in units of r d, d = 1.585). Note, that time is still measured in units of... 

For the same parameter values we also evaluated numerically equations (5.1.14) to (5.1.16). Figure 6.7 shows a comparison of the results. The curves a and b show the decay of reactant concentration n(t) for the direct annihilation process on gaskets of type a or b , respectively, each averaged over 6 realizations of the process (the dotted curves indicate the scatter... 

Next we introduce the creation and annihilation process for a bimolecular step. In the previous model without energetic interactions discussed in Section 9.1.1 we have had the two-point transition rates K(aian -A Let us... 

The negation and position that result from the pair-production process or interact to initiate an annihilation process. 

Pair production has a threshold energy of 1.022 MeV because two particles are created, one electron and one positron. Thus, some energy is stored in or used to create the mass of the pair. Notice the total electric charge is conserved because the electron charge is — le and the positron charge is +le. One of the unique features of this process is that the energy that went into the creation of the two particles will be released when the positron comes to rest and annihilates with an electron. The annihilation process is... 

The positron lifetime spectra of polyethylene and glass-filled polyethylene were resolved in four exponentials, representing different annihilation processes. The... 

Another possibility is that two P-M reversal kinks may appear in the same polymer chain. These may translate along the chain to adjacent positions, when both kinks will be mutually annihilated. The annihilation process becomes more likely as the chain becomes longer. Eventually the possibility of backbiting and termination becomes... 

The wave function of the ion that remains after annihilation is a superposition of eigenstates of the Hamiltonian of the ion, the relative probabilities of which may be determined from the wave function used in the calculation of Zeg. The annihilation process takes place so rapidly, compared with normal atomic processes, that it is reasonable to assume the validity of the sudden approximation. Consequently, the wave function of the residual ion when the positron has annihilated with electron 2 at the position r = r2 is... 

The positron-trap technique has been used to measure the annihilation rate of positrons interacting with a wide variety of molecules. The species investigated by Iwata et al. (1995) include many hydrocarbons, substituted (e.g. fluorinated and chlorinated) hydrocarbons and aromatics as mentioned in section 6.1, large values of (Zeff) (in excess of 106) were found for some molecules. Several distinct trends are exhibited in the data of Iwata et al. (1995). Though much of the detailed physics involved in the annihilation process on these large molecules is still unclear, the model of Laricchia and Wilkin (1997), described in section 6.1, may offer a qualitative explanation of the observations. 

Most studies of positronium interactions have depended upon monitoring the annihilation process after positronium has been formed by f3+ particles stopping in relatively dense media (e.g. sections 7.3 and 7.4). Fortunately, as introduced in subsection 1.5.3 and described in more detail below, the availability of positron beams has made it possible to create variable energy positronium atoms under controlled conditions in vacuum. In this section we discuss the development of such beams, in which the positronium atom is considered as a swift atomic projectile. 

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