Pressure effect quantum yield

Some observations have been made on the effect of pressure on quantum yields, using pressures in the 1-2 kbar range.43-45 Apparent volumes of activation range from about -10 cm3 mol-1 to small positive values for Crni and Rh111 complexes. 

The maximum quantum yield of ozone decomposition, 0, has been measured to be 4 (546), 5.5 (640), and 6 (998). Norrish and Wayne (748) found that pressure effect on <1> o, lias been found by others (640, 998). This discrepancy is probably due to experimental conditions such as the presence of impurities and the walls, which would deactivate excited species. 

Ultraviolet light without hydrogen peroxide is not very effective for the degradation of organics. When UV light and H202 are combined, the overall oxidative reaction potential is greatly enhanced even under ambient temperature and pressure. The efficiency of direct photolysis depends on (1) UV absorbance by substrates (2) quantum yield of photolysis (3) presence of other competitive UV absorbents and (4) intensity of UV sources. 

In Fig. 30 it is seen that the effect with carbon dioxide is nearly proportional to the pressure. The reduction in quantum yield is due to the removal by collision of the excess energy of the excited nitrogen dioxide molecules before they can collide with other molecules of nitrogen dioxide and effect a chemical reaction. The energy is removed in the form of extra energy given to the colliding molecules, and thus converted into heat. 

Hoare68 using 3130 A. at 120 and 200°C showed that the quantum yield of carbon monoxide decreased and that of carbon dioxide increased with the oxygen pressure. The presence of 130 mm. of inert gas had little effect on the photooxidation. 

A few studies have been carried out at other wavelengths. Wijnen and Taylor107 found no hydrazine in the products and they report that the quantum yield of ammonia decomposition at 1470 A. varied from 2 to 11 in the pressure range from 4.4 to 68.8 mm. These yields are considerably higher than the value of 0.17 obtained by Groth11 and they ascribed this discrepancy to the thermal effect of the lamp. 

The wavelength required for effective absorption of UV light by azides depends on the substituents. The azides absorb normally below 290 nm, with the exception of arylazides, which show also transitions between 300 and 350 nm. Therefore as light sources low-pressure mercury-lamps are most commonly used. In Table 2 the range of absorption, the e-values and the quantum yields of some azide photolyses are compiled. 

Phosphorescence of s-triazine has been observed by Ohta et al. following excitation of the 6o band of the Si — So transition. Values for the phosphorescence lifetime and quantum yield were reported. The effects of rotational excitation on the yields and decays of the fast and slow components of Si state s-triazine fluorescence have been studied. Excitation along the rotational contours of the 6j and 6o bands revealed that the fast component showed little rotational level dependence in contrast to the slow component. This behaviour was interpreted in terms of an increase in the number of triplet levels coupled to the optically prepared singlet levels with increasing angular momentum quantum number, J. A broad emission feature present in addition to narrowline fluorescence from rovibronic levels of 6 or 6 in S, s-triazine has been observed and the rotational level dependence of its quantum yield and decay over a range of pressures reported... 

The quantum yield is too insensitive to k y to determine the pressure dependence of that rate coefficient. Nearly the same results are obtained if reaction (45 ) is second or third order. However, [M] is in actuality of the form [H2]+ai[HCl]+ fl2[Cl2]+U3[02], so that ai[C 2]+a [02 must have been approximately 140 in all the relevant experiments. A re-examination of the data shows, in fact, that for most of the pertinent experiments [O2 ] -I- [CI2] varied between 93 and 175 torr. The best estimate for 3 is 0.33. If we set 02 = 2.0, then we can re-calculate the predicted quantum yields using eqn. (ee) for those conditions where the effects of Cl 2 and/or O2 should be most pronounced. The results are shown in Table 4. The re-computed values are not unreasonable, but they do not fit the data nearly as well as those computed from eqn. (cc). Nevertheless, our current understanding of the H-I-O2 recombination requires that we reject eqn. (cc) in favor of (ee). Perhaps other estimates of 2 and would give somewhat better agreement. By equating the constants in eqn. (ee) to the parameters in eqn. (cc), we find that 67/ 68 = 9.4x10 torr, = 0.93x10" , and = 1-7. Norrish... 

At high pressures of Bf2 where termolecular gas-phase recombination of Br atoms is expected to predominate over wall recombination, Ritchie observed a decrease in the quantum yield of HBr formation as the pressure of non-reactive gas increased, with CO2 being most effective and H2 being least effective in reducing the quantum yield. The observed decrease is attributed to three-body gas-phase recombination of Br atoms which is pressure-dependent and serves to reduce the chain length of the reactions leading to HBr formation. The relative efficiencies of three-body recombination were... 

The photolysis of mixtures of NO, NO2, and O2 by Ford (1) shows that the quantum yields for NO2 decomposition are inversely proportional to the O2 pressure, and the effect is mainly attributed to the formation of O3 when O atoms react with O2. 

Although the rate of light absorption was lower in BF s experiments than in JF s, there seems every reason to suppose that reaction (17) is again a crucial one in the N02-sensitized reaction. BF observed a lowering in the quantum yield at high total pressures when NO2 was present and deduced from this effect the rate which was quoted earlier for OH+NO2 + M. In a revised scheme this pressure effect can be explained by the inclusion of the reactions... 

The energy of the 1236 A light is greater than the ionization potential of benzene, and hence additional reaction paths are available. All the above products were observed for the photolysis of benzene at 1236 A, with all except hydrogen having somewhat lower quantum yields. No pressure effect was observed in the photolysis at 1236 A. For the photolysis with either lamp, the addition of 3 torr NO to 1 torr of benzene eliminated allene, cyclohexadienes, biphenyl and dihydrobiphenyl as products, while the other products remained unaffected. Hentz and Rzad conclude that the former are formed by free-radical reactions, while the latter are formed by molecular elimination. In view of the curious pressure effects observed by Shindo and Lipsky at 1849 A, conclusions regarding the effect of added gases must be made with caution. 

More detailed information concerning the mode of decomposition of the triplet and the upper singlet states was expected from the results of flash photolytic investigations However, the difficulties encountered in determining quantum yields and in obtaining monochromatic light under such conditions limit the usefulness of these results. Here, the discussion will be restricted to the pressure effect observed at high intensities. 

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