Lifetime near surfaces

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

As an excitation system, TIRF does not specifically refer to the pattern, intensity, or lifetime of the fluorescence emitted from the near-surface molecules which become excited. However, these emission characteristics are somewhat different from those far from a surface, and some of these differences may become experimentally useful. In Section 7.3, the emission pattern of a fluorophore near a dielectric surface (particularly the interface of water with either bare glass or metal-coated glass) is discussed. 

Orientation, Rotation, and Fluorescence Lifetime of Molecules near Surfaces... 

Establishing an acceptable risk or dose. There also are a number of precedents for establishing an acceptable (barely tolerable) risk or dose of substances that cause stochastic responses for the purpose of classifying waste as low-hazard or high-hazard. For radionuclides, the annual dose limit for the public of 1 mSv currently recommended by ICRP (1991) and NCRP (1993a) and contained in current radiation protection standards (DOE, 1990 NRC, 1991) could be applied to hypothetical inadvertent intruders at licensed near-surface disposal facilities for low-hazard waste. This dose corresponds to an estimated lifetime fatal cancer risk of about 4 X 10 3. Alternatively, the limits on concentrations of radionuclides in radioactive waste that is generally acceptable for near-surface disposal,... 

Suzuki, R., Kobayashi, Y., Mikado, T., Ohgaki, H. et al. (1992) Investigation of near surface defects by variable-energy positron lifetime spectroscopy", Mater. Sci. Forum 105-110,1459. 

Since both positrons and Ps could, be localized in free-volume holes, the data of positron lifetime (t2) and o-Ps bulk lifetime (t3) provide information about the size and distribution of free-volume size as a function of the depth near the surface. Figure 11.6 shows the variation of positron lifetime and o-Ps bulk lifetime vs the depth. A significant increase of lifetimes near the surface shows a larger size of free volumes near the surface than in the bulk. Similar variations vs the positron energy indicates that both positrons and Ps are localized in free volumes and holes. Figure 11.7 shows the distributions of hole size in the polymer from the data of o-Ps lifetime distribution. Near the surface, not only the size is larger than the bulk, the distribution is significantly wider [10]. 

A number of positron annihilation studies with carbon materials [10-12] have shown the existence of three lifetime components the longest-lived component with a mean lifetime from 1000 to 5000 ps resulted from pick off annihilation of the orthopositronium atoms formed in the samples the intermediate component having a mean lifetime between 350 and 400 ps has been assigned to annihilation of positrons by interaction with the electron density at the surface and near-surface regions, and the shortest-lived component, with mean lifetime from 140 to 225 ps, comes from positron annihilation with 7t-electrons in the bulk of the graphite structure. 

These studies indicated that the photoproduction rates in solutions of varying composition were approximately proportional to the dissolved organic carbon (DOC) content. Assuming that the lifetime of the solvated electron in air-saturated water is 0.2 ps and that halocarbon concentrations are much lower than that of oxygen (and thus have little effect on the electron lifetime), the photoproduction rate observed in the Greifensee (DOC = 4 mg of C/L) corresponded to estimated near-surface pseudo-first-order photoreduction rate constants of about lOVh for several halocarbons known to be present in natural waters (Table V). These estimates were derived by using previously measured rate constants for reaction of solvated electrons with the halocarbons (48). 

From the standpoint of regional tropospheric chemistry—which involves near-surface abundances of ozone, wet and dry deposition of acidic species, and transport and lifetimes of trace atmospheric constituents—the climate variables of interest include the variability of distributions of temperature, precipitation, clouds, and boundary-layer meteorology. In the global sense, these variables are controlled by surface and atmospheric temperature and water content. The distributions of temperature and water vapor are in turn controlled by solar and longwave radiation transfer involving the surface and the atmosphere. 

In the previous sections, stagnant films were assumed to exist on each side of the interface, and the normal mass transfer coefficients were assumed proportional to the first power of the molecular diffusivity. In many mass transfer operations, the rate of transfer varies with only a fractional power of the diffusivity because of flow in the boundary layer or because of the short lifetime of surface elements. The penetration theory is a model for short contact times that has often been applied to mass transfer from bubbles, drops, or moving liquid films. The equations for unsteady-state diffusion show that the concentration profile near a newly created interface becomes less steep with time, and the average coefficient varies with the square root of (D/t) [4] ... 

Irradiation of adsorbate-covered surfaces with higher energy photons (typically up to 6.4 eV) with lower intensities opens the possibility of direct valence excitation. Since the lifetimes of electronic excitations at metal surfaces are much shorter than those for nuclear motion, photochemical reactions appear rather improbable. Surprisingly, however, the cross sections determined for photodesorption were found to be comparable to those found for reactions with free molecules, mainly because the short lifetime of the excited state is compensated by a much larger cross section for absorption of the light [32,62-64]. This process takes place in the near-surface region of the metal (within about 10 nm), where relaxation of the photoexcited electrons leads to rapid establishment of a transient energy distribution. As depicted in Fig. 4.11, these hot electrons may scatter at the surface or are resonantly attached to an empty level of the adsorbate. 

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