Photodissociation quantum efficiency

For a wavelength interval dA, the photodissociation coefficient of molecule A is proportional to the actinic flux qvdA, the absorption cross section actinic flux q dA represents the number of photons per unit area and... 

The absorption cross section of hydrogen peroxide is shown in Figure 4.39. In the spectral domain which is important in the stratosphere, absorption cross section measurements have been made by Schurgens and Welge (1968) from 120 to 200 nm, Holt et al. (1948) and Holt and Oldenberg (1949) from 185 to 253 nm, and Urey et al. (1929) from 215 to 380 nm. More recent measurements include those of Lin et al. (1978), Molina and Molina (1981), Nicovich and Wine (1988), and Vaghjiani and Ravishankara (1989). The cross sections vary slightly with temperature. The photodissociation quantum efficiency is believed to be unity. 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

Figure 2 Variation of the quantum efficiencies of photodissociation of photoionization and of nonradiative transition to the ground state, r] r = l — — t]i, in liquid water as a function of...

In comparing these results with experimental data it has to be remembered that in contrast to ketones, azo compounds can also undergo photochemical trans-cis isomerizations. (Cf. Section 7.1.7.) In the gas phase n-> r excitation results in photodissociation with nearly unit quantum efficiency. At higher pressures, however, and especially in solution, this reaction almost completely disappears and photoisomerization dominates. The latter is observed even at liquid nitrogen temperatures. This is understandable if it is accepted that photodissociation proceeds in the gas phase as a hot ground-state reaction. According to Figure 7.18, it has to overcome a barrier in the excited state and is therefore not observed in solution. For the trans-cis isomerization, on the other hand, no excited-state barrier is to be expected from the results in Section 7.1.7. 

In an effort to assess the importance of this process, Clark and Noxon (58) searched for fluorescent emissions in the wavelength interval 100 to 800 nm upon excitation of CO2 with radiation in the 150 to 170 nm range. Their failure to detect any emissions led them to place a limit of 0.1% upon the quantum efficiency for fluorescence at a CO2 pressure of 0.1 torr. Thus it appears that photodissociation in this wavelength interval occurs as a single-step mechanism with near unit efficiency. 

Much of the evidence has been provided by Gunning and co-workers. In experiments (102) in which OCS is photodissociated in the presence of increasing amounts of olefin, they demonstrated that quantum yields of CO production are reduced to a limiting value equal to half that observed in the absence of olefin. It is expected, therefore, that the quantum yield for the production of CO should be 2 in the absence of olefin unless the quantum efficiency of the primary process is less than unity. Sldhu et al. (103) found that the CO quantum yield at 253.7 nm was pressure independent between 80 and 680 torr with a value of 1.81. This leads to a primary quantum efficiency of 0.90 for 16a +... 

An investigation of the reaction products from the photodissociation of OCS in mixtures with olefins in both liquid and solid solution (102) indicates that 16b continues to participate as a primary process but with reduced quantum efficiency in comparison to the gas phase. It is suggested (102) that inter-system crossing between the singlet excited state of OCS and a repulsive triplet state is more important in condensed than in gas phases. On the other hand, Gollnick and Leppin (105) have studied the photolysis of OCS at 253.7 nm In solution with a variety of solvents, find the quantum efficiency of CO formation to be 0.90 + 0.05 in all solvents, and suggest that process 15b is the only primary process. 

The spectroscopy of NO3 has been reviewed by Wayne et al. (1991). The absorption spectrum of this radical (see Figure 4.45) exhibits several predissociating bands first observed in 1962 by Ramsay. Laboratory studies reviewed by Johnston et al. (1996) provided quantitative values of the photodissociation parameters. The quantum efficiency for each of the two possible pathways... 

These results " taken together with those of Ausloos strongly suggest a unit quantum efficiency for the photodissociation of CO2 at all wavelengths shorter than 167 nm, the threshold for production of 0( 0). 

Unfortunately, there are no data that establish the wavelength dependence of the primary photodissociative processes in 3-pentanone in air at tropospheric pressures. However, it is likely that the quantum efficiency is somewhat greater than that seen for acetone and 2-butanone under similar conditions. 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

The emission spectra of 10-CPT in water-methanol mixtures exhibits dual fluorescence (Fig. 1 left). The appearance of the low energy emission band at 570 nm for 10-CPT in a neat methanol solution indicates an efficient PTTS process. The large fluorescence quantum yield and similarity of the emission at neutral and basic solutions is evidence of the excited anion (RO ) formation, in contrast to 6HQ, for which double PTTS leads to the tautomer [2], With the increase of water content in the mixtures, we observed a substantial decrease in the fluorescence intensity of the nondissociated form of 10-CPT at 430 nm and a concomitant increase of RO " intensity at 570 nm. This is a well-known effect in hydroxyaromatic compounds [4] and is attributed to the increase of the protolytic photodissociation rate with increasing water concentration. 

The photolysis of anthracene-benzene adducts 111 and 112 has been studied in detail [128], Photodissociation of 111 was found to give electronically excited anthracene with a quantum yield of 0.80, but the isomeric 47i + 27i adduct 112 photodissociates mainly diabatically, leading to electronically excited anthracene with a quantum yield of 0.08. The different efficiencies of adiabatic cycloreversions have been rationalized by correlation diagrams involving doubly excited states. Evidence for biradicals as intermediates in the photolyses of 111 and 112 has not been obtained. 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

The van der Waals attraction between Br and I2 is estimated to be 400 cm-1 by analogy with halogen/rare gas complexes (Bieler and Janda 1990 Bieler et al. 1991). This ensures that photodissociation of the HBr moiety cannot produce Br + I2 except via quenching of Br or the unlikely instance in which the hydrogen is trapped efficiently between the heavv particles. With the Br atom unable to escape from the I2 because of the Br-I2 van der Waals attraction, the system is ensured of an essentially unity quantum yield. 

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