Excitation probability

Two-photon excited fluorescence detection at the single-molecule level has been demonstrated for cliromophores in cryogenic solids [60], room-temperature surfaces [61], membranes [62] and liquids [63, 64 and 65]. Altliough multiphoton excited fluorescence has been embraced witli great entluisiasm as a teclmique for botli ordinary confocal microscopy and single-molecule detection, it is not a panacea in particular, photochemical degradation in multiphoton excitation may be more severe tlian witli ordinary linear excitation, probably due to absorjDtion of more tlian tire desired number of photons from tire intense laser pulse (e.g. triplet excited state absorjDtion) [61],... 

The emission yield, Ra, defined as the radiation of the spectral line, k, of an element, i, emitted per unit sputtered mass must be determined independently for each spectral line. The quantities g, and Ry are derived from a variety of different standard bulk samples with different sputtering rates. In practice, both sputtering rates and excitation probability are influenced by the working conditions of the discharge. Systematic variation of the discharge voltage, L/g, and current, I, leads to the empirical intensity expression [4.185] ... 

In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also - with the advent of X-ray detectors of increased sensitivity - by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Because two electrons are involved, incident and the excited, the shape of the spectral structure is proportional to the self convolution of the unoccupied state densities. 

The total excitation probability from the ground state is approximated as... 

First we consider the electronic excitation probability Pekc for a single molecule during a single laser pulse. When a molecule has the absorption coefficient Eabs(dm mol cm ), its absorption cross section Gabs is given by 3.81 x lO Sabs cm molecule". Since the probability Peiec is proportional to the light intensity (photons s cm ) under the objective lens, it is given by... 

Fig. 9.34 Monitoring of inelastic excitations by nuclear resonant scattering. The sidebands of the excitation probability densities for phonon creation, S(E), and for annihilation, S —E), are related by the Boltzmann factor, i.e., S(—E) = S E) tTvp —Elk T). This imbalance, known as detailed balance, is an intrinsic feature of each NIS spectrum and allows the determination of the temperature T at which the spectrum was recorded...

Observed angular distributions were quasi-specular and scattered rotational distributions were strongly dependent upon the incidence energy, both observations indicating the direct nature of the interaction. The most important observation of the work was the approximately Arrhenius surface temperature dependence of the vibrational excitation probability, exhibiting an effective activation energy close to the vibrational excitation energy of the scattered molecule (see Fig. 2). The authors also showed that the... 

Another type of complex for which it is obvious thatQlarge structural changes occur on excitation (probably 0.2-0.3 A) along particular coordinates are the infinite-chain mixed-valence complexes of platinum and palladium (class II, or localized mixed-valence complexes) (11).. These Pti [/PtIV, PdIVPtIV or Pdi [/PdIV... 

Taking into account the excitation probability, i.e. cos2 6, the number of excited molecules whose transition moment is oriented within angles 6 and 0 + d9, and and tj> + dip, is proportional to cos2 6 sin dddd. The fraction of molecules oriented in this direction is... 

Each equivalent site i of a given crystal has the same probability I of being occupied by an electronically excited molecule, immediately after irradiation with a Dirac pulse. The excitation probability of site i is the th element of a vector P that we call excitation distribution among the sites. We distinguish between the low intensity case in which at maximum one dye molecule per crystal is in an electronically excited state and cases where two or more molecules in a crystal are in the excited state. Where not explicitly mentioned we refer to the low-intensity case. 

We do not know which donor D has been excited. We therefore assume that immediately after irradiation at t = 0 all sites i have the same excitation probability Pt(0), while all traps T are in the ground state, hence P/(0) = 0. These probabilities change with time because of energy migration, relaxation processes, and trapping. The excitation probability Pt(t) is governed by the following master equation ... 

The trapping efficiency Pt(oo) is equal to the sum of the excitation probabilities of all trapping sites J at infinite time after irradiation. 

Both Pzm(t) and Pzt f) express the excitation distribution among the slabs. For front-back trapping, traps are found in the first and the last slab, which we call slabs 0 and mmax. Therefore T (t) is equal to the sum of the excitation probabilities in both slabs at time t. 

Figure 1.24. (a) Excitation distribution along the channel axis of a zeolite L crystal consisting of 90 slabs under the condition of equal excitation probability at t = 0 calculated for front-back trapping. Fluorescence of the donors is assumed. (1) t = 5 ps, (2) t— 10 ps, (3) t — 50 ps, and (4) t = 100 ps after irradiation. (b) Luminescence decay of the donors in absence of traps (dotted), in the presence of traps at both ends (solid), and luminescence decay of the acceptors (dashed). 

These data led to the model already described several times above. The enzyme executes a search for a tunneling sub-state, apparently 13 kcaFmol in energy above the principal state from this state the hydrogen atom tunnels with no further vibrational excitation. Probably motion of the secondary center is coupled into the tunneling coordinate. The result is large, temperature-independent primary and secondary isotope effects in the context of an isotope-independent activation energy. 

The different pumping methods, such as the commonly used current injection or optical pumping electron beam pumping and avalanche breakdown have been studied in detail (for further refs, see and information has been obtained regarding the excitation probabilities of the different interband transitions. The very short laser pulses (less than 10 sec) obtained enable rapid processes and their time dependence to be studied. 

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