Photolytic Quantum Yields

To which case of Figs. 2.1 or 2.2, the photo-excitation is applicable, can be judged by the appearance of an absorption spectrum whether it is a continuum or has a band structure. In the case of Fig. 2.2, since the absorption occurs to the vibrational energy levels in the bound excited state, absorption spectrum reveals band structure. As an example, absorption spectra of chlorine (CI2) (Marie et al. 1993) 

So far, the explanation was for an example of diatomic molecules. For the polyatomic molecules composed of n-pieces of atoms, the idea of the correspondence of the appearance of the absorption spectrum and the photo-excitation is the same, but the potential surfaces are in n 1 dimensions. Since photodissociation can occur with multiple processes such as ABC AB + C, A + BC, multi-dimensional potential surfaces for each dissociating inter-atomic distance must be considered. 

1 nm for other region (Adapted from Saiz-Lopez et al. 2004) 

A probability of dissociation of a molecular per absorbed photon is called the photolytic quantum yield. Thus, the photolytic quantum yield 0 is defined by 

Number of dissociated molecules Number of absorbed photons 

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The photolytic quantum yield and lifetime have been determined over a wide range of temperature in ethylene glycol/water solutions [91]. drops sharply as the temperature is decreased. is reduced from 6 x 10 at 293K to 2 X 10 at 235 K and 7 x lO s" at 225 K. The viscosity increases from 5 to > lOOcP in the same temperature interval. Since changes in k, and k d cannot account for much a large decrease in k , the evidence favors a thermally activated dissociation step. 

When a certain wavelength light excites a molecule, photodissociation can occur in energetically permissible multi-pathways, and the photolytic quantum yield for each process has to be determined experimentally. For example, the photodissociation pathways of ozone (O3) by solar ultraviolet radiation. 

The photolytic quantum yields of N2O5 is reported to be close to unity (Harwood et al. 1993, 1998). 

Fig. 4.17 Photolytic quantum yields of HCHO (a) (H + HCO), (b) OfHa -I- CO) (Adapted from...

As for the temperature dependence of the photolytic quantum yields of CH3COCH3, the total quantum yields increase with temperature in the wavelength region shorter than 295 nm, while they decrease with temperature in the wavelength region longer than 295 nm. NASA/JPL Evaluation No. 17 (Sander et al. 2011) gives the approximation formula of the quantum yields for the pressure and the temperature dependence based on Blitz et al. (2004). Table 4.17 shows the temperature dependent photolysis quantum yields and Fig. 4.21 depicts the photolytic quantum yields calculated by the approximation compared with experimental values (Blitz et al. 2004). 

The photolytic quantum yield for each process is unity, as expected from the complete continuum of the absorption spectrum (Vaghjiani and Ravishankara 1989). 

Photolytic Quantum Yields The major photolytic processes for HO2NO2 are thought to be,... 

Other reaction pathway which split into three product species, e.g., are also energetically possible within the wavelength range of tropospheric solar actinic flux. The photolytic quantum yields of HO2NO2 are unity at the wavelength longer than 200 nm measured by MacLeod et al. (1988), Roehl et al. (2001), and Jimenez et al. (2005). Since the quantum yields of HO2 and NO2 are 0.8 and those of OH and NO3 are 0.2 (Sander et al. 2011), (HO2-b NO2) = 0.8, and 4> (OH -b NO3) = 0.2, if they are ascribed to reactions (4.39) and (4.40), respectively. 

Since the absorption spectrum, absorption cross sections, and photolytic processes in the tropospheric solar actinic flux region, and photolytic quantum yields have already been described in detail in the Sects. 3.2 (Fig. 3.6) and 4.2.1 (Fig. 4.2,4.3,4.4, 4.5, 4.6, 4.7, and 4.8, Table 4.1, 4.2,4.3, and 4.4), only photolytic processes in the stratosphere will be described here. As seen in Fig. 3.6, the absorption spectrum of the Hartley bands of O3 extend over broad range of 200-300 nm that is the most important in the stratospheric solar actinic flux. The photolytic process of O3 molecules reached by the absorption of photons in the Hartley bands is thought to be... 

The quantum yields of each process are reported at several wavelengths, but the total photolytic quantum yield summing the reactions (4.69) and (4.70) is thought to be unity (Kang et al. 1996). 

The formation quantum yields of the excited bromine atoms Br( Pi/2) increases from 0.4 at 444 nm to 0.89 at 510 nm close to the dissociation limit of reaction (4.82), and then decreases (Peterson and Smith 1978). As for the photolytic processes of Br2, several other studies have also been made (Haugen et al. 1985 Cooper et al. 1998), but no absolute photolytic quantum yields has been measured. For the purpose of atmospheric chemistry total photolytic quantum yields of Br2 can be approximated to be unity for the wavelength region of 200-510 nm. 

Total photolytic quantum yields of HCl, HBr and HI in the wavelength region of tropospheric and stratospheric actinic fluxes are thought to be unity. The photolytic processes of each of them,... 

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