Measured Quantum Yield

Furthermore, the occurrence of a reaction could not be correlated with the energy involved in the primary process, e.g., iodine absorbed quanta higher in energy than the I-I bond but cleaved with efficiency much less than unitary and part of the radiation absorbed was given back as fluorescence. 

All of these observations were best accommodated by having an electrMiically excited (Bohr) state as the primary photoproduct, to which the Einstein equivalence law applied, and then considering which chemical processes could occur within the lifetime of the excited state, the role of collisions, and the effect of the environment [40, 41]. 

The postulated equivalence between molecules destroyed and quanta absorbed so clearly expressed in the 1912 paper seemed to offer an ideal field for verification. In the meantime, mercury arcs had become available and a much easier measurement of quantum yield was possible. An early version of the optical bench for the measurement of quantum yield is reported in Fig. 2.5. The emission ray from a quartz mercury concentrated on a cuvette trough a condenser behind which was placed the actinometric cuvette (Fig. 2.5) [42]. 

As a matter of fact, experimental attempts for the determination of molecules/ quanta relation had been carried out also before or at the same time as Einstein s paper [44, 45]. Thus, Winther remarked in the same year (1912) that a limitation of the well established Grotthuss law, generally and correctly recognized as necessary for the foundation of a theory of the photochemical phenomena is that it establishes no quantitative connection between the absorption and the rate of the reaction. This was a very important point and actually light absorbed caused reaction to a varied extent. 

Winther selected a number of known solar light-induced reactions and calculated the energy of the light absorbed (or rather its upper limit) by using a table of 

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The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

The photostationary state composition for the benzophenone-sensitized isomerization of 2,4-hexadienes is given in Table 9.2. Table 9.3 gives the measured quantum yields for benzophenone-sensitized isomerization of 2,4-hexadienes along with the calculated quantum yields based on Eqs. (9.47)-(9.49) and the pss values given in Table 9.2. 

Alkene Linkages. Using (250-350)-nm irradiation (30,31) or the unfiltered light from a high-pressure mercury lamp (32), Balandier and Decker have measured quantum yields under nitrogen... 

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

Warneck et al.38S have presented evidence that absorption in the 1800-2400 A range consists of discrete structure superimposed on a continuum which they attribute to the dissociation to ground state SO and O. The overlying bands are predissociated at X < 1900 A, but the nature and states of the resulting fragments are not established. On the basis of their measured quantum yield, relative importance of the possible primary processes. There is as yet, however, insufficient information for the evaluation of quantitative rate data for the photolysis in this region. 

Rousseau and Leroi studied the two-photon-induced chemical reaction in AgCl by 30 kW ruby-laser pulses which results in a decomposition of AgCl into collodial silver and chlorine. The resultant blue-green emission was proportional to the square of the laser intensity and the measured quantum yield was 10". ... 

Figure 18 Measured quantum yields, per incident electron, (a) for the induction of DSBs, (b) SSBs, and (c) loss of the supercoiled DNA form, in DNA solids by low-energy electron irradiation as a function of incident electron energy the curves are guides to the eye. (From Ref. 262.)...

By using the techniques mentioned before, room-temperature x s for about 50 alkanes were determined. In Fig. 2, we show the fluorescence quantum yields as a function of lifetimes. The 4>[ values were generally taken from the work of Rothman et al. [25] most of the fluorescence quantum yields were measured using 165-nm photons for excitation. This wavelength is close to the absorption onset of most alkanes and (with the exception of the smaller molecules) the measured quantum yield is close to the fluorescence quantum yield of the relaxed Si molecules [26]. The plot in Fig. 2 is similar to the plot we published in Ref. 59 using only our measurements. Here we use practically all the data that are available in the literature. For most of the alkanes, several lifetime measurements were published. When... 

The wavelength thresholds are 338 nm for (31a) and 299 nm for (31b). Thus, (31a) is expected to predominate at the earth s surface. As might be expected, once excited, acetone can be collisionally quenched in competition with decomposition, and hence the quantum yields decrease with increasing total pressure. Figure 4.30, for example, shows the measured quantum yields for the decomposition of acetone at 760 Torr total pressure and the values extrapolated to zero pressure (Gierczak et al., 1998). In the tropospherically important wavelength region, the yields are small beyond about 330 nm at 1 atm pressure. 

FIGURE 4.30 Measured quantum yields for acetone photodissociation as a function of wavelength at f atm total pressure and extrapolated to zero total pressure (adapted from Gierczak et al., f998). 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

The quantum yield measured at 313 nm for CO elimination from [Ru-ClH(CO)(PPh3)3] is 0.06 0.02. Because of the air sensitivity of [RuClH(PPh3)3], the yield was determined by irradiating a CH2CI2 solution of [RuClH(CO)-(PPh3)3] in a degassed and sealed uv cell. Since the reaction vessel was sealed, reverse reaction with CO was not prevented, and the measured quantum yield should be considered a lower limit. 

Alternatively, since r is the reciprocal of the sum of rate constants for all the processes undergone by the excited state, the reaction rate constant may be estimated if the others (e.g. for phosphorescence and intersystem crossing in the case of a triplet) are known from other studies. The most convenient way, however, of measuring t for a reaction that can be quenched is to carry out a quantitative quenching study at different quencher concentrations. In the most straightforward systems the results can be fitted to a straight-line plot expressed as < 1.16, where is the quantum yield in the absence of quencher, < > is the measured quantum yield at quencher concentration [Q], and is the rate constant for quenching. 

In Figure 4 we display experimental values of r/rB and fluorescence quantum yields for a number of aromatic and heterocyclic molecules. On the whole the linear relation (3-2) is confirmed. The appreciable, though apparently nonsystematic, deviations from eq. (3-2) are not unexpected because of difficulties in measuring quantum yields, because of inaccuracies in the experimental oscillator strengths, and because of deviations expected when the ground and excited-state vibrational frequencies are different.31... 

The only type of triplet reaction for which rate constants have been measured for various kinds of ketones is type II photoelimination. The rate constants for 2-pentanone and 2-hexanone have been found to be 2 x I08 and 1 x 10 sec-1, respectively.215 The corresponding rates for triplet butyrophenone and valerophenone are 8 x 10 and 1.4 x 10s sec-1, appreciably slower. Neither p-phenylbutyrophenone366 nor a-valeronaphthone273 undergo photoelimination in a readily measurable quantum yield. These results can be rationalized by a reasonable set of hypotheses such as follow. 

The electron carriers that participate in these first few steps are all fixed in position in the reaction center so that the initial steps in electron transfer involve electron movements only. One indication of this is that the electron-transfer reactions, in addition to being phenomenally fast, are almost independent of temperature. The reactions also are amazingly efficient. This can be expressed in terms of the quantum yield of P870tQA 4, which is the number of moles of P870iQA7 formed per einstein of light absorbed. The measured quantum yield in purified reaction centers is 1.02 0.04. Essentially every time the reaction center is excited, an electron moves from P870 to QA. 

These methods, used in conjunction with suitable phosphoroscopes, can also be used to measure quantum yields of phosphorescence (process 14) (32). Data are very scanty, due to experimental difficulties, so that estimates of the relative importance of processes 2, 3, 4, 14, and 15 remain very imperfect (48,64). Emission from fluid solutions is only by process 2, although with thorough de-oxygenation to eliminate process 11 process 14 might be detectable (34). Otherwise, process 14 is observed only in rigid glassy solvents, as with naphthalene, phenanthrene, or coronene in boric acid glass at room temperatures. 

The rate of diffusive separation, k, was determined from separate experimental measurements of iodine radical diffusion rates in the high pressure diffusion limited regime (19). The rate of excited state deactivation, k i, was calculated from the measured quantum yields at high densities where G> = kd/k i (18). It was assumed that k i is proportional to the inverse diffusion coefficient, D 1 (19,23) as both properties are related to the collision frequency. 

Triplet-triplet (T2 — 7,) fluorescence has been reported for anthracene and a variety of substituted anthracenes in solution at room temperature [25,26]. This work came about as a result of the initial observation of a long wavelength emission band superimposed on the tail of the 5, —> S0 emission of 9-bromoanthra-cene (54) and 9,10-dibromoanthracene (55). Table 3 gives the wavelength of the (0,0) band for the emission as well as the measured quantum yields (see p. 259). 

In order to measure quantum yields of an extrinsic fluorophore bound to a protein and which emits at longer wavelengths than in the UV, standards such as 3,3 -diethylthiacarbocyanine iodide (DTC) in methanol (Op = 0.048) and rhodamine 101... 

As the wavepacket develops in the moat around the l)3h symmetry, the Cr(CO)5 fragment can recombine with the ejected CO. This process is likely to be more efficient in the condensed phase where the measured quantum yield for CO loss from Cr(CO)6 shows solvent dependency [22, 33], In the gas phase the excess vibrational energy is available for the expulsion of a further CO ligand. These processes are summarized in Fig. 16. 

The absorption cross sections for O3 at 273 K as recommended by the 2004 JPL/NASA evaluation are given in Table 3 while measured quantum yields (Fig. 1) for ozone photolysis are given in Table 4. In addition, the JPL/NASA evaluation committee has developed an empirical equation for the estimation of the quantum yield for 0( D) as a function of wavelength and temperature (Table 5). 

Figure 19-4. Open and solid symbols are the measured quantum yields (events per incident electron) for the induction of single strand breaks (SSB) (a) and double strand breaks (DSB) (b) in DNA films by 4-100 eV electron impact. The solid curves through the data are guides to the eye. The dotted curves symbolize general electron energy dependence of the cross sections for various nonresonant damage mechanisms, such as ionization cross sections, normalized here to the measured strand break yields at lOOeV...

The quantum yield was originally used in the photochemical sense exclusively, for the number of molecules of reactant consumed per photon absorbed. However, in this form, the quantum yield does not convey any information as to the relative contribution of different excited-state reactions or of secondary thermal reactions, which may lead to consumption of reactant. Thus, in the photochemical literature, it has become common to use two distinct types of quantum yield (a) the experimentally measured quantum yield (often called the overall quantum yield) and (b) the primary quantum yield. The quantum yield (a) accounts for reactant disappearance (or product formation) whether it occurs directly in a primary process or in a secondary thermal reaction or in both. 

Photolysis ty, = 3.5 d, based on measured quantum yields (Howard 1989). 

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

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