Emission yields

Photoexcited fluorescence from spread monolayers may be studied [158,159] if the substance has both a strong absorption band and a high emission yield as in the case for chlorophyll [159]. Gaines and co-workers [160] have reported on the emission from monolayers of Ru(bipyridine)3, one of the pyridine ligands having attached C g aliphatic chains. Ruorescence depolarization provides information about the restriction of rotational diffusion of molecules in a monolayer [161], Combining pressure-area... 

The emission yield from the horseradish peroxidase (HRP)-catalyzed luminol oxidations can be kicreased as much as a thousandfold upon addition of substituted phenols, eg, -iodophenol, -phenylphenol, or 6-hydroxybenzothiazole (119). Enhanced chemiluminescence, as this phenomenon is termed, has been the basis for several very sensitive immunometric assays that surpass the sensitivity of radioassay (120) techniques and has also been developed for detection of nucleic acid probes ia dot-slot. Southern, and Northern blot formats (121). 

The quantification algorithm most commonly used in dc GD-OES depth profiling is based on the concept of emission yield [4.184], Ri] , according to the observation that the emitted light per sputtered mass unit (i. e. emission yield) is an almost matrix-independent constant for each element, if the source is operated under constant excitation conditions. In this approach the observed line intensity, /ijt, is described by the concentration, Ci, of element, i, in the sample, j, and by the sputtering rate g, ... 

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 contrast with the dc source, more variables are needed to describe the rf source, and most of these cannot be measured as accurately as necessary for analytical application. It has, however, been demonstrated that the concept of matrix-independent emission yields can continue to be used for quantitative depth-profile analysis with rf GD-OES, if the measurements are performed at constant discharge current and voltage and proper correction for variation of these two conditions are included in the quantification algorithm [4.186]. 

In the early years of this century the periodic table ended with element 92 but, with J. Chadwick s discovery of the neutron in 1932 and the realization that neutron-capture by a heavy atom is frequently followed by j6 emission yielding the next higher element, the synthesis of new elements became an exciting possibility. E. Fermi and others were quick to attempt the synthesis of element 93 by neutron bombardment of but it gradually became evident that the main result of the process was not the production of element 93 but nuclear fission, which produces lighter elements. However, in 1940, E. M. McMillan and P. H. Abelson in Berkeley, California, were able to identify, along with the fission products, a short-lived isotope of... 

Jones, G., Jackson, W.R., Choi, C.Y., and Bergmark, W.R. (1985) Solvent effects on emission yields and lifetime for coumarin laser dyes. Requirements for the rotatory decay mechanism.. Phys. Cbem. 89, 294. 

The long lifetime has important consequences on the decay rates. First, we consider what affects the nonradiative rates (knr) which change the yields of fluorescence and phosphorescence. The nonradiative decay rate is often enhanced in molecules which have flexible constituents (the so-called loose-bolt effect). Therefore, both fluorescence and phosphorescence yields are generally larger for rigid molecules than flexible molecules. For the same reason, a rigid environment will increase the emission yields hence both fluorescence and phosphorescence yields often increase with increasing viscosity. 

Because of the Stokes shift for vibrationally relaxed systems (the rate of transfer < the rate of vibrational relaxation), transfer between like molecules is less efficient than that between unlike molecules when acceptor is at a lower energy level (exothermic transfer). No transfer is expected if the acceptor level is higher than the donor level. If (i) the acceptor transition is strong (Emaz —- 10,000), (ii) there is significant spectral overlap, and (iii) the donor emission yields lie within 0.1 — 1.0, then R0 values of 50-100 A are predicted. 

Early work on peridinin demonstrated that its structure leads to breaking of the idealized C2h symmetry resulting in relatively strong fluorescence from the Si state [16], Recent studies demonstrated that the intensity of the peridinin Si emission depends on solvent polarity [8,9], and time-resolved studies revealed that the polarity-dependent change in the Si emission yield... 

This assertion of experimental fact is not extracted simply from superficial examination of experimental evidence. Emission yields do vary considerably, but the effects can usually be associated with quenching by other solute molecules in condensed systems. Since a variety of common solvents do not function as quenchers in this sense, the quenching phenomenon is usually associated with special effects. The commonest explanations are transfer of electronic excitation to the quencher and some kind of chemical reaction. However, other more subtle quenching action has also been observed. 

Absolute emission yields for substances in the gas phase are very difficult to measure and such data as exist often show quite large variations from one laboratory to another. The efficiency of emission from... 

The determination of the triplet-state yield when benzene vapor is irradiated depends not only on the emission yield of biacetyl as determined by Almy and Gillette32 but on the absolute emission efficiency of benzene itself. When all corrections are applied, the biacetyl method gives a triplet-state yield at incident wavelengths 2530-2590 A of about 0.63 with an uncertainty hard to estimate but perhaps 0.1. Below 2530 A the emission efficiency decreases, and by 2400 A the emission efficiency has fallen to zero. 

The recombination of free radicals can be highly exergonic, since in general a new bond is formed by the formerly non-bonding electrons. In the example of eqn. (4.59) the closed-shell molecule, N02, is then formed in an excited state which can decay through luminescence emission. Some organic free radicals show similar reactions but it must be stressed that in most cases the emission yields are low. 

Auger Electron-emission Yields from Metal Surfaces67... 

Like all first-order processes, radioactive decay is characterized by a half-life, f]/2, the time required for the number of radioactive nuclei in a sample to drop to half its initial value (Section 12.5). For example, the half-life of iodine-131, a radioisotope used in thyroid testing, is 8.02 days. If today you have 1.000 g of I, then 8.02 days from now you will have only 0.500 g of remaining because one-half of the sample will have decayed (by beta emission), yielding 0.500 g of MXe. After 8.02 more days (16.04 total), only 0.250 g of will remain after a further 8.02 days (24.06 total), only 0.125 g will remain and so on. Each passage of a half-life causes the decay of one-half of whatever sample remains, as shown graphically by the curve in Figure 22.2. The half-life is the same no matter what the size of the sample, the temperature, or any other external condition. 

Another example is found with benzene vapor at room temperature. The emission yield (fluorescence only) is about 0.20 0.04 and essentially independent of pressure100,101. There are products formed, although there is some disagreement on this subject and the yields in the gas phase have not been determined. Also there is a crossover from the singlet to the triplet state so that all or nearly all of the primarily excited singlet state molecules either fluoresce or cross over to the triplet state. Nevertheless, in the vapor phase at room temperature, about 80 per cent of the molecules which absorb neither react chemically nor emit radiation100, 101. 

The calculated radiative lifetime of benzene vapor based on absorption coefficients111 is in excellent agreement with the mean lifetime measured by Donovan and Duncan112 and is about 5 x 10-7 sec. And yet this agreement must be partly fortuitous, since the emission yield is only about 0.2 and hence there must be processes which compete with radiative emission100 101. It is true that, if twenty per cent of the molecules emit, these competitive processes must have rate coefficients not greatly different from the rate coefficient of the process100... 

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