Photolysis cross sections

In the case of a metal substrate, the experimental evidence shows that metal excitation is dominated by surface photon absorption. Optical radiation excites surface charge carriers, usually free or sub-vacuum-level electrons that can efficiently couple to the adsorbate. This often leads to enhanced photolysis cross sections or altered product distributions. Excitation localized on the adsorbed molecule in close proximity to a metallic solid may efficiently couple to the electronic states of the surface, leading to excitation quenching. When light-absorbing molecules are separated from the surface by spacer molecules, the influence of the surface on molecular excitation and relaxation decreases [4,21],... 

Before discussing possible mechanisms that could explain these results, it is worthwhile comparing the time constants to results from previous electron-induced surface chemical reactions. The extracted time constant, t, equals 1/ct/, where/is the X-ray flux density and % is the photolysis cross section. Using approximate values of the beam area (3 x 10 4 cm2) and X-ray flux (3.5 x 10n ph/s), yields a flux density of 1.7 x 1015 ph/s cm2 and a a of 6 x 10-19 cm2. Our results show that it is the secondary electrons that are inducing the chemical changes. Therefore, it is more applicable to use the secondary electron flux to compute the cross section. An upper bound to this is given by the TEY flux density. This is determined from the measured sample current of 3.8 nA or 2.4 x 1010 e/s, which results in a cross section of 9 x 10 18 cm2 (9 Mb) This value compares fairly well with reported dissociative electron impact cross sections for CO production from condensed films of acetone (9.6 Mb) [124] or methanol (4.2 Mb) [125] via a DEA mechanism. In the present case a DEA mechanism, in which a temporary negative ion state is formed,... 

Mo(CO)6 adsorbed on a CO saturated Rh(lOO) surface had a higher photolysis cross section than when adsorbed on a disordered Mo(CO)6 layer. Since the CO layer allows closer approach to the surface of the Mo(CO)6 this is opposite to the result predicted from field coupling quenching. 

The electric field (polarization) vector can be divided into two orthogonal components, named s-polarized and p-polarized light (see Section 2.6). Recall that for the former the polarization vector is perpendicular to the plane of incidence (the one defined by the surface normal and the direction of incidence), and for the latter it is parallel to the plane of incidence. The key point is to investigate whether the angular-dependent photolysis cross-section, for a given wavelength and polarization, tracks the metal... 

The total dissociation cross sections of silane and disilane have been taken from Perrin et al. [201]. An uncertainty in the present knowledge of the silane chemistry is the branching ratio of the silane dissociation channels [192]. Here, the branching ratio is taken from Doyle et al. [197], who suggest using the branching ratio determined by Perkins et al. [202] for photolysis, viz., a branching of... 

As discussed above, the solution environment provides for a set of time scales different from the gas phase environment. In solution, there are typically 1013 collisions second"1 of a solute molecule with solvent molecules. Thus, if a photolytically generated species is expected to have a large cross section for reaction with solvent and it is desired to monitor that reaction, both generation and monitoring must be done on a picosecond (psecond) or even sub-psecond timescale. That monitoring this rapid is necessary has been confirmed in an experiment on Cr(CO)6 in cyclohexane solution where psecond photolysis and monitoring was not rapid enough to detect the naked Cr(CO)5 that existed before coordination with cyclohexane (55). 

Furuta, T., Wang, S. S. H., Dantzker, J. L., Dore, T. M., Bybee, W. J., Callaway, E. M., Denk, W. and Tsien, R. Y. (1999). Brominated 7-hydroxycoumarin-4-ylmethyls Photolabile protecting groups with biologically useful cross-sections for two photon photolysis. Proc. Natl. Acad. Sci. USA. 96, 1193-1200. 

The rate of photolysis, J, depends on the absorption cross-section, a, the number density, the scale height and the angle, all of which are unique properties of a planetary atmosphere. For the Earth and the Chapman mechanism for ozone the O3 concentration maximum is 5 x 1012 molecules cm-3 and this occurs at 25 km, shown in Figure 7.12, and forms the Chapman layer structure. 

A quantitative determination of two-photon absorption cross-sections from direct measurements of chemical yield was performed by Speiser and Kimel i ) who studied the two-photon-induced photolysis of iodoform with a Q-switched ruby laser. 

The direct detection of the S <- Sj absorption in organic compounds has so far been achieved by a nanosecond or picosecond laser flash photolysis method. The general features of transient absorption spectra of metalloporphyrins actually suggest the presence of strong absorption bands in visible or ultraviolet region (38-40). However, as the transient absorption of the state often overlaps with that of ground state depletion, it is usually difficult to evaluate the absolute absorption cross sections for the transition by... 

A precise description of the photolysis of pTpT (16a, R = P03H2 see Glossary for nomenclature) has been given by Deering and Setlow,66 where the rate constants for the dimerization and mono-merization reactions of pTpT are described in terms of cross sections for the reactions in eq. (7). 

Separate samples of each dimer were photolyzed and the cross sections ra and r2 obtained from the initial slopes of the growth curves. The values of k3i and ki3 are obtained similarly by photolysis of separated samples of P4 and P3. A comparison of the cross sections measured for TpT with those measured by Deering and Setlow68 for pTpT is shown in Table VI. The general agreement among the cross... 

The cross sections for formation of the two dimers are similar, but not identical, with those for the formation of two of the dimers from UpU, and the wavelength dependencies are similar. Hydrate formation occurs more rapidly in d-UpU than in UpU. The marked quantitative and qualitative differences in the photochemistry of these two closely similar dinucleotides, along with the additional difference in results observed with poly U, to be discussed below, make it clear that the factors influencing the course of the photolysis of polynucleotides are not well known. It must be pointed out that the structures of the various... 

Fig. 19. Wavelength dependence of the cross sections for formation of the hydrate, d-UpU, and the two dimers in the photolysis of d-UpU (Helleiner, Pearson, and Johns73).

Most commercial spectrometers report absorbance, as defined in Eq. (Q), versus wavelength. This is very important to recognize, since as we will see later, calculations of the rate of light absorption in the atmosphere require the use of absorption coefficients to the base e rather than to the base 10. While the recent atmospheric chemistry literature reports absorption cross sections to the base e, most measurements of absorption coefficients reported in the general chemical literature are to the base 10. If these are to be used in calculating photolysis rates in the atmosphere, the factor of 2.303 must be taken into account. 

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]

It must again be stressed that the absorption cross sections, a (A), used to calculate photolysis rates are to the base e, not base 10, even though the latter is what has often been measured and reported in the literature in the past. 

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. 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.ld and A.4f in Chapter 11.). 

Gierczak et al. (1998) have also measured the temperature dependence for the absorption cross sections in addition to the quantum yields as a function of pressure and temperature. They have used these data, combined with the kinetics of the OH-acetone reaction, which is the other major removal process, to calculate the contributions of the OH reactions and of photolysis to the loss of acetone in the atmosphere as a function of altitude. Figure 4.31 shows that photolysis is a significant, but not the major, contributor at the... 

Figure 4.33 shows the absorption cross sections of HC1 and HBr at room temperature (DeMore et al., 1997 Huebert and Martin, 1968). Neither absorb above 290 nm, so their major tropospheric fates are deposition or reaction with OH. Even in the stratosphere, photolysis is sufficiently slow that these hydrogen halides act as temporary halogen reservoirs (see Chapter 12). 

Barnes et al. (1998) have measured the yield of OH from HOC1 photolysis and find, in addition to the strong absorption shown in Fig. 4.39, a weak absorption feature at 380 nm due to excitation to the lowest triplet state. Although the absorption cross section of this weak absorption is only 4 X 10 21 cm2 molecule-1, its contribution lowers the calculated stratospheric lifetime of HOC1 by 10-20%. 

Figures 4.44 and 4.45 show absorption spectra of some simple chlorofluoro-methanes and ethanes, respectively (Hubrich and Stuhl, 1980). Tables 4.37 and 4.38 give the recommended absorption cross sections for some of these compounds (DeMore et al., 1997). None of these compounds absorb in the actinic region above 290 nm, but do around 180-200 nm, wavelengths only found in the stratosphere. As discussed in Chapter 12, it is photolysis at these short wavelengths to generate atomic chlorine that is responsible, along with bromine and perhaps in some cases, iodine atoms, for the chain destruction of stratospheric ozone.

Deters, B., J. P. Burrows, and J. Orphal, UV-Visible Absorption Cross Sections of Bromine Nitrate Determined by Photolysis of Br0N02/Br2 Mixtures, . /. Geophys. Res., 103, 3563-3570 (1998). 

Laszlo, B M. J. Kurylo, and R. E. Huie, Absorption Cross Sections, Kinetics of Formation, and Self-Reaction of the IO Radical Produced via the Iraser Photolysis of N20/I2/N2 Mixtures, . /. Phys. Chem., 99, 11701-11707 (1995). 

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