- Title : Handbook of Material Weathering by George Wypych
- Publish : ChemTec Publishing 38 Earswick Drive Toronto-Scarborough Ontario MlE lC6 Canada
- Type Document : pdf
- Release : December 1995
- Total Page : 19 Chapter
- Size : 7.34 Mb
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Decrypted Contents
Energy calculated from Eq 2 can be used to evaluate the probability of a photochemical reaction. Table 1.2 gives the energy of radiation for some common energy sources used in photochemical studies and shows the energy level difference between g-rays irradiation, laser etching, UV degradation by mercury lamp, and UV degradation by sun’s rays.
RADIATION INTENSITY
Table 1.2 indicates that the energy of laser light is the same as the energy of visible light or UV (depending on wavelength). But the fact that laser light is substantially more intense is central to the following discussion. Table 1.3 shows some of the quantities which characterize radiation intensity. Laser radiation can emit radiation in the range of 1 mW(lasers frequently used in optical experiments) to 10 W (moderately powerful argon laser) and more. At the same time, this power is emitted onto a very small surface area (laser light has high coherence, monochromacity and small beam width) such as 10 mm2 or 1 mm2. If one calculates the value of irradiance, it is in the range of 107-108 W/m2 respectively, for given conditions (in fact the surface area is limited by and equal to the wavelength of radiation, and power can be as large as 100Wgiving an irradiance of 1013 W/m2). If one compares these values with the mean intensity of sunlight on the Earth’s surface, which is in the range of 103 W/m2, it is easy to understand the difference between these two sources of radiation and to explain the differences in the results of their action (surface etching versus minor changes or no changes at all).
The above example illustrates the importance of the conditions under which the experiment is run and reported. We can use the laser example to elaborate further.
Table 1.2 indicates that the energy of laser light is the same as the energy of visible light or UV (depending on wavelength). But the fact that laser light is substantially more intense is central to the following discussion. Table 1.3 shows some of the quantities which characterize radiation intensity. Laser radiation can emit radiation in the range of 1 mW(lasers frequently used in optical experiments) to 10 W (moderately powerful argon laser) and more. At the same time, this power is emitted onto a very small surface area (laser light has high coherence, monochromacity and small beam width) such as 10 mm2 or 1 mm2. If one calculates the value of irradiance, it is in the range of 107-108 W/m2 respectively, for given conditions (in fact the surface area is limited by and equal to the wavelength of radiation, and power can be as large as 100Wgiving an irradiance of 1013 W/m2). If one compares these values with the mean intensity of sunlight on the Earth’s surface, which is in the range of 103 W/m2, it is easy to understand the difference between these two sources of radiation and to explain the differences in the results of their action (surface etching versus minor changes or no changes at all).
The above example illustrates the importance of the conditions under which the experiment is run and reported. We can use the laser example to elaborate further.
Laser light delivers 1012 to 1017 photons/cm3.At this intensity, several photons will react with a electric fields often as much as 100 gigavolts per meter, which inevitably creates conditions for orientation, dipole formation, ionization, etc.
The use of pulsed radiation showed that lasers, with their highly ordered (polarized) beams, can selectively excite the single isomer (in the mixture) which has the right configuration for energy absorption. This means that irradiation by chaotic radiation (e.g., sunrays) will produce totally different results from radiations of different intensities (e.g., lasers).
The use of pulsed radiation showed that lasers, with their highly ordered (polarized) beams, can selectively excite the single isomer (in the mixture) which has the right configuration for energy absorption. This means that irradiation by chaotic radiation (e.g., sunrays) will produce totally different results from radiations of different intensities (e.g., lasers).
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