Quantum yield or efficiency

UV intensity measurements were made with an International Light 700A Research Radiometer. The measuring head was tightly covered with aluminum foil for zeroing, and then exposed to the lamp output under exactly the same conditions as the actual samples (i.e., same distance, angle, elevation, etc.). The results of these experiments were used to evaluate the quantum yield or efficiency of the photochemical process. Specifically, photolysis of AETSAPPE... 

More important is the quantum yield or efficiency of fluorescence, which may affect the intensity of fluorescence over about four orders of magnitude and may determine whether fluorescence is at all observable. The quantum yield of fluorescence is dependent upon the rates of processes competing with fluorescence for the deactivation of the lowest excited singlet state. 

Several processes may compete with fluorescence for deactivation of the lowest excited singlet state. As a result only a fraction of the molecules formed in the lowest excited singlet state, < )/, actually fluoresce. <()/ is called the quantum yield or fluorescence efficiency. It is usually a fraction but may be unity in some exceptional cases and is related to the probabilities (rate constants) of fluorescence (kf) and competitive processes (kd) by... 

Before looking at the effect of the polymeric matrix on quantum yields and efficiencies of photochemical processes it is important to look first at variations which are due to the structure of the ketone chromophore itself which are observable regardless of whether the chromophore is in the solid, liquid, or gaseous state. The first of these is illustrated in Table II which illustrates the quantum yields for esters of dimethyl keto azelate (3). 

A fluorescence spectrum is characteristic of a given compound. It is observed as a result of radiative emission of the energy absorbed by the molecule. The observed spectrum does not depend on the wavelength of the exciting light, except that the spectrum will be more intense if irradiation occurs at the absorption maximum. The spectral intensity is called the quantum efficiency and is usually abbreviated as . The quantum yield or quantum efficiency, d>, which is solvent dependent, is the ratio Approximate values of quantum efficiencies are as follows naphthalene, 0.1 anthracene, 0.3 indole, 0.5 and fluorescein, 0.9. 

Ultrafast injection of carriers into the UO2 substrate suggests that overall cell efficiency is not limited by this process but by intervening transfer mechanisms (e.g., trapped state populations reducing quantum yield) or longer time scale electron-dye recombination rates (typically taking microseconds). For example, it was found that the absorbed photon to current efficiency (APCE) is considerably reduced for V compared to IV under identical cell conditions (27,55). While V has a 350 mV lower electrochemical reduction potential than IV (and hence a red-shifted absorption spectrum), it is unclear why the redder absorbing dye (V) does not inject as efficiently (56). Recent visible excitation with broadband... 

The quantum yield or photo efficiency of a photochemical process of interest is defined as... 

The difficulty of quantum yield determination in heterogeneous photocatalysis is related to reflection and scattering of UV/VlS radiation by suspended catalyst grains. Consequently, Serpone and Salinaro (1999) proposed a detailed protocol for the determination of photonic efficiencies (quantum yields) or relative photonic efficiencies, suggesting the photocatalyzed degradation of phenol as a standard reaction (using Ti02 as photocatalyst). 

A quantum yield (or quantum efficiency), O, is often used to describe the deexcitation processes following absorption of light. Here represents the fraction of molecules in some excited state that will decay by the ith deexcitation reaction out of all the possible competing pathways (each molecule must use some pathway) ... 

In the laboratory, DCMU can be added to chloroplasts, which stops electron transfer at the level of QB and thereby causes /Cphotochem to become zero. We can also raise the photosynthetic photon flux (PPF) so high that the photochemistry becomes overwhelmed by other decay processes, such as by exciting all photosynthetic pigments essentially simultaneously, as was done by Emerson and Arnold (see Section 5.4A) in this case /Cphotochem becomes small relative to /fcothei.. Either situation leads to the maximum chlorophyll fluorescence, Fm (as either a relative quantum yield or a relative photon flux). If a low PPF (e.g., <20 pmol m-2 s-1) that can be efficiently processed photochemically is used, the chlorophyll fluorescence F0 is minimal and equals p/( photochem kp + Mother)- When ifcphotochem is negligible, Fm is kp (kp + Mother) - Fm— F0 is known as the variable fluorescence, Fv, that is caused by these extreme conditions. We then have... 

The fraction of the decay rate that results in emission of a photon is the fluorescence quantum yield (or quantum efficiency) 4y ... 

Despite these important examples of the commercialization of photochemistry and photophysics, synthetic photoreactions still seem to remain underexploited in chemicals manufacture. We know that photons are relatively expensive, so that may be one reason but another factor, I believe, is that so many photochemical reactions seem to have disappointingly poor chemical yields or quantum yields, or give rather too many by-products. I was pleased therefore to see a selection of efficient photoreactions which - by virtue of high chemical yields or interesting selectivity - could compete with the best thermal processes in the... 

Chemical actinometer systems have been widely used in basic photochemical studies to enable the determination of the quantum yield of a photochemical reaction. The quantum yield or quantum efficiency cp is defined as ... 

In photochemical reactions, it is important to measure the efficiency of the photolysis, which is done by the parameter known as quantum yield (or quantum efficiency), defined as the number of molecules reacted or formed per light quanta absorbed. Ol The quantum yield for formation of a product (form) is... 

As discussed in Section ll.lO.B, the efficiency of the photolysis is measured by the quantum yield (or quantum efficiency) the number of reacted or formed molecules per absorbed light quantum." Quantum yield is measured for formation of a product (Oform) just defined. It can also be defined in terms of the number of molecules of starting material (SM), which disappear per quantum of light (Odis) ... 

The measurable parameters are the quantum yield (< )f), and the intensity and the position of peak emission (Xmax). The quantum yield or fluorescence efficiency is the fraction of molecnles that becomes de-excited by fluorescence and is defined as... 

The quantum yield (or quantum efficiency) of a photoreaction is generally wavelength-dependent and differs from the above in that the reaction obtained... 

Fluorescence resonance energy transfer (FRET) experiments commonly use the fluorescent spectrum and relaxation times of the Forster donor and acceptor chromophores to find the distances between fluorescent dyes at labeled sites in protein, DNA, RNA, etc. FRET is a type of spectroscopic ruler . The computation uses either experimental quantum yields or relaxation lifetimes to calculate the efficiency of resonance energy transfer Ej. 

The inverse of Q.r. is the quantum yield (or quantum efficiency). The quantum yield therefore represents the number of molecules transformed (i. e. COj molecules liberated) per quantum of light absorbed. 

Solar energy conversion efficiency, represents the number of electron/hole pairs generated per an absorbed photon, called the quantum yield or the internal... 

The efficiency of fluorescence (j)p, which is also called the fluorescence quantum yield, or, simply, quantum yield, can be calculated based on the above two equations ... 

The law of photochemical equivalence is restricted to primary photochemical process, i.e., each reacting species excited by the absorption of one radiation get chemicd transformation and formed products produce no further reaction. In such cases, these will be 1 1 relationship between the number of quantas absorbed and the number of reacting molecules. But in practice, most of photochemical reactions undergoes secondary photochemical reactions, i.e., photochemically activated species or product molecule initiates a series of chemical transformations while in some cases, photochemically activated species undergoes deactivation, they lose their energy in the form of heat or radiation. Under such conditions, there will be no more 1 1 relationship between the number of quanta absorbed and the product molecules. The deviation from photochemical equivalence (1 1 relationship) is described by the idea of quantum yield or quantum efficiency (< )). It is defined as... 

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