Primary quantum yields

After photon absorption there are different ways in which the excited state may be deactivated, so not every excited molecule will form a primary product. The quantum yield for this primary process is given by  

The species -CH3 and -CH3CO are radicals species containing unpaired electrons. Radicals are formed by homolytic fission of a covalent bond, where the electron pair constituting the bond is redistributed such that one electron is transferred to each of the two atoms originally joined by the bond. 

The overall quantum yield is the number of molecules of reactant, R, consumed per photon of light absorbed. 

According to the Stark-Einstein law, O should be equal to 1. However, if secondary reactions occur, can be greater than 1. 

O can be very large for chain reactions, with the propagation reactions acting as an amplifier of the initial absorption step. 

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The study of quantum yields. The quantum yield is the fraction of absorbed light that goes to produce a particular result. There are several types. A primary quantum yield for a particular process is the fraction of molecules absorbing light that undergo that particular process. Thus, if 10% of all the molecules that are excited to the state cross over to the T state, the primary quantum yield for that process is 0.10. However, primary quantum yields are often difficult to measure. A product quantum yield (usually designated ) for a product P that is formed from a photoreaction of an initially excited molecule A can be expressed as... 

J. Zimmerman and R. M. Noyes, The primary quantum yield of dissociation of iodine in hexane solution. J. Chem. Phys. 18(5), 658-666 (1950). 

To help clarify the situation, we generally define two types of quantum yield primary and secondary. The magnitude of the primary quantum yield refers solely to the photochemical formation of a product so, from the second law of Photochemistry, the value of 0(primary) cannot be greater than unity. 

The primary quantum yield cj) should always be cited together with the photon pathway occurring it is common for several possible pathways to coexist, with each characterized by a separate value of . 

As a natural consequence of the second law of photochemistry, the sum of the primary quantum yields cannot be greater than unity. 

Note that the overall quantum yield is

[Pg.452]

CO and sulphur are the end products of photolysis. The quantum yield of CO is 1.8, independent of COS pressure, for the photolysis at low light intensities of pure, gaseous COS at 2527 A and at 2288 A94. This value is exactly halved by the addition of a large excess of olefin, a scavenger for both ground-state and excited sulphur atoms93, suggesting that the primary quantum yield of S and CO is 0.9... 

Note that total quantum yield for the formation of freely diffusing products (in contrast with the primary quantum yield for the extremely fast formation inside the solvent cage), because that is the process that takes place during the experimental time window. 

The primary quantum yield of radical formation in nitrous acid, CHjO, and CHjC O photolyses should be better established as a function of the appropriate wavelength range of sunlight,... 

A primary quantum yield is the fraction of light-absorbing molecules that are converted in a particular process. Unfortunately, such values are often very difficult to measure. A product quantum yield is the ratio of the number of molecules of product formed per number of quanta absorbed by the reactant. That product formation is the rate-determining step is a strong possibility if the observed value does not vary with experimental conditions. 

The relative efficiencies of the various photophysical and photochemical primary processes are described in terms of quantum yields, . The primary quantum yield, , for the ith process, either photophysical or photochemical, is given by Eq. (I) ... 

The primary quantum yields for each process are defined as follows ... 

While the aim of photochemical studies is generally to measure primary quantum yields, this is not always experimentally feasible. For example, NO reacts rapidly with N03 to form N02. Thus determination of 4il or (f>4b by measuring the NO and N02 formed can be complicated by this secondary reaction of NO with N03, and the measured yields of NO and N02 may not reflect the efficiency of the primary photochemical processes. 

In some cases, then, the overall quantum yield, rather than the primary quantum yield, is reported. The overall quantum yield for a particular product A, usually denoted by A, is defined as the number of molecules of the product A formed per photon ab-... 

The primary quantum yield (A) represents the fraction of excited molecules that undergo photochemistry... 

Only reaction (7) leads to the removal of NOz via photochemistry and hence the quantum yield for reaction (7) is needed to calculate the photolysis rate. Data on both primary quantum yields and absorption cross section [Pg.80]

Because the actinic flux data are reported as averages over certain wavelength intervals, rather than integrating over Eq. (OO) in a continuous manner, in practice one calculates the sum of the product discrete wavelength intervals AA. The intervals are chosen to match the available flux data for example, in Table 3.7, actinic fluxes are reported as averages over 2-nm intervals from 290 to 320 nm, which is important for the 03 absorption, 5-nm intervals from 320 to 420 nm, 10-nm intervals from 420 to 580 nm, and 20-nm intervals from 580 to 700 nm. Since the primary quantum yield, ( A), and the absorption cross section, a(A), are not normally reported over identical intervals, representative averages of these parameters over the same intervals for which the actinic flux data are reported must be calculated from the literature data. 

The sum (or integral if Eq. (OO) is used) is carried out from the lower limit of wavelengths in the troposphere, 290 nm, to some wavelength A at which either the primary quantum yield or the absorption cross section becomes negligible. 

Experimentally, while the determination of absorption cross sections is fairly straightforward, measuring primary quantum yields is not, due to interference from rapid secondary reactions. As a result, in cases where quantum yield data are not available, calculations of maximum rates of photolysis are often carried out in which it is assumed that (A) = 1.0. It should be emphasized in such cases that this represents only a maximum rate constant for photolysis the true rate constant may be much smaller, even zero, if photophysical fates of the excited molecule such as fluorescence or quenching predominate. 

From Eq. (PP), we need the primary quantum yields for each of the processes (9a)-(9d), the absorption cross sections, base e, and the actinic flux values, F(A). Table 3.18 gives IUPAC recommendations (Atkinson et a,l., 1997) for the absorption cross sections and primary quantum yields for CH3CHO. Primary quantum... 

Figure 4.12 shows the primary quantum yields for the production of NO in reaction (12). The quantum yield is within experimental error of 1 up to 395 nm, declining slightly to 0.82 at the theoretical threshold for dissociation at 397.8 nm. This has been attributed to the formation of a nondissociative excited state of N02. In ambient air, electronically excited NOz which does not dissociate to form O P) is collisionally deactivated. When 02 is the collision partner, energy transfer may occur a fraction of the time to form 02( Ag) (Jones and Bayes, 1973a) ... 

In discussing the relations for the photocurrent and the photovoltage of p—n junctions it should not be forgotten that the sensitivity of these devices depends among other things (e.g. the primary quantum yield rj) on the diffusion length L,... 

The primary quantum yield of N02 dissociation obtained from d>0j falls off rapidly above 3980 A and is 0.005 at 4358 A [Pitts et al. (810)]. Since the extent of isotopic scrambling of02 by illuminating mixtures ofN02 and1818 02 closely follows the quantum yield of NO production, there is little doubt that O atoms are formed by direct dissociation of N02 at four wavelengths tested, 3660, 4020, 4060, and 4120 A [Jones and Bayes (550)]. 

From the product analysis, the following primary quantum yields arc derived (282) ... 

Nitrogen dioxide is one of a few simple molecules in which the primary quantum yield near the dissociation limit (3980 A) has been measured nearly continuously as a function of incident wavelength. The energetics of photodissociation is given in Table VI- 5. The thcrmochcmical threshold at O K for the reaction, N02 - NO + O( P), corresponds to the incident wavelength 3978 A, which nearly coincides with the wavelength 3979 1 A below which... 

Earlier we defined the terms primary photochemical process and secondary reaction. Another term of great significance when dealing with the chemical effects of light is primary photochemical yield. This is the number of absorbing molecules which either dissociate or react per photon absorbed. The primary quantum yield may not exceed unity and it may be zero, if, for example, all absorbing molecules fluoresce without reacting (14). 

Noyes et al. point out, If the absorbing species is reformed in any way by subsequent reactions, the primary quantums yield is not thereby reduced. For example, the process... 

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