Absorption of ultraviolet radiation

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

In Section 17.13 reference has been made to the influence of various substituents in the benzene ring on the absorption of ultraviolet radiation, and the purpose of this exercise is to examine the effect in the case of benzoic acid by comparing the absorption spectrum of benzoic acid with those given by 4-hydroxybenzoic acid and 4-aminobenzoic acid. 

It is important to note fluorescence seldom results from absorption of ultraviolet radiation of wavelengths lower than 250 nm because such radiation is sufficiently energetic to cause deactivation of the excited states by predissociation or dissociation Fluorescence due to a a transitions is seldom observed instead, such... 

Methods of measuring the components of photochemical smog are reviewed in Chapter 6. There have been significant advances in the calibration of instruments for monitoring ozone in ambient air. A method based on the absorption of ultraviolet radiation at 254 nm has been adopted by California for the calibration of air monitoring instruments. The method is based on the use of a commercially available instrument that measures ultraviolet absorption as a transfer standard in the calibration process. 

These iodometric calibration methods are based on the assumption that there is a stoichiometric reaction between ozone and the iodine in the various potassium iodide procedures. Three essentially independent methods have been used to test the accuracy of this assumption measuring the absorption of ultraviolet radiation at 254 nm by ozone in air, measuring the absorption of infrared radiation at 9,480 nm by ozone in air, and determining the ozone concentration in air by titration with nitric oxide. 

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

Electromagnetic radiation can be absorbed or emitted. The absorption of ultraviolet radiation by our skin may cause sunburn. When we cook food in a microwave oven, the absorption of microwave radiation by the water in the food causes the water molecules to vibrate, generating heat that cooks the food. However, when electromagnetic radiation is absorbed or emitted by matter, it behaves more like a stream of particles than as a wave motion. These particles are called photons and so electromagnetic radiation can be considered both as a stream of photons and as waves with characteristic properties, such as wavelength (1) and frequency (/). Therefore we say that electromagnetic radiation has a dual nature wave motion and streams of photons. 

Spectrum of ozone, showing maximum absorption of ultraviolet radiation at a wavelength near 260 nm. [Adapted from R. P Wayne, Chemistry of Atmospheres (Oxford Clarendon Press, 1991).]... 

Due to these characteristics, these pigments are useful for the photostabilization of some white compounds, e.g. corticosteroids, whose photodegradation is only caused by absorption of ultraviolet radiation. As shown in Figure 25, the addition of 0.5% of titanium dioxide or zinc oxide leads to the effective photoprotection of... 

Absorption of ultraviolet radiation by O3 causes it to decompose to O2. In the upper atmosphere, therefore, a steady-state concentration of ozone is achieved, a concentration ordinarily sufficient to provide significant ultraviolet protection of the Earth s surface. However, pollutants in the upper atmosphere such as nitrogen oxides (some of which occur in trace amounts naturally) from high-flying aircraft and chlorine atoms from photolytic decomposition of chlorofluorocarbons (from aerosols, refrigerants, and other sources) catalyze the decomposition of ozone. The overall processes governing the concentration of ozone in the atmosphere are extremely complex. The following reactions can be studied in the laboratory and are examples of the processes believed to be involved in the atmosphere ... 

Fluorescence is the prompt emission of light from molecules excited by radiation of higher frequency. Normally, absorption of ultraviolet radiation by a molecule leads to emission in the visible region higher frequency absorption leads to lower frequency emission. 

Figure 13.33 illustrates the transition between electronic energy states responsible for the 230-nm UV band of c/i -tra i -l,3-cyclooctadiene. Absorption of ultraviolet radiation excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). In alkenes and polyenes, both the HOMO... 

As was mentioned in Chapter 2, the major source of heat in the middle atmosphere is provided by absorption of ultraviolet radiation, particularly by ozone and to a lesser extent by molecular oxygen... 

Figure 3.3. Net radiative heating rate associated with (1) absorption of ultraviolet radiation by molecular oxygen in the upper mesosphere and thermosphere, and by ozone in the stratosphere and mesosphere, and (2) emission of infrared radiation by atmospheric CO2, O3, and H2O. Values given in K/day and positive in the summer hemisphere (net diabatic heating) and negative (net diabatic cooling) in the winter hemisphere. Prom London (1980).

The theoretical framework just presented was first suggested by Chapman (1931). This theory provides an explanation for the behavior of the layers of ionization in the thermosphere or of photodissociation in the middle atmosphere. The production of ozone through photodissociation of molecular oxygen exhibits a maximum near 45 km dependent on the insolation. The rate of heating through absorption of ultraviolet radiation by ozone similarly leads to a maximum near the stratopause. Numerous examples of such layers can be found in the neutral and ionized atmosphere. 

The absorption of ultraviolet radiation by ozone in the Huggins and Hartley bands constitutes the principal source of heat in the stratosphere and mesosphere. The heating rate reaches 10 K/day near the stratopause on the average, with a maximum of about 15 K/day near the summer pole. The effect of the Huggins bands in the visible region becomes... 

Ozone decomposes following the absorption of ultraviolet radiation with wavelengths between 220 and 320 nm [4]. 

The dissociation of hydrogen has been studied over a period of 12 years by shock tube workers using the analytical methods of X-ray absorption [34, 35] and interferometry [36] to measure gas density, absorption of ultraviolet radiation by molecular hydrogen [37], spectrum-line reversal to measure vibrational temperature changes [38, 39], computer analysis... 

It is interesting to note that the principles outlined above are also relevant to the resonance absorption of ultraviolet radiation by atoms. However, for the typical values of = 6-2 eV (50,000 cm ) and M = 100 a.m.u. r is only 2-1 X 10 ° eV and d is 3 X 10 eV. In this case strong resonance absorption is expected because the emission and absorption profiles overlap strongly. The problems associated with recoil phenomena are thus only important for very energetic transitions. 

The other major compound for which the lifetime and the adjustment time differ is N20. N20 is the source of stratospheric NO. The stratospheric NO derived from N20 controls, to some extent, stratospheric 03 and hence the ultraviolet radiation field in the stratosphere, which, in turn, controls the rate of destruction of N20. Increases in N20 lead to more NO, less 03, and therefore less absorption of ultraviolet radiation. This enhanced radiation flux, in turn, increases the photolytic removal of N20. As a result, the adjustment time for N20 is about 10% shorter than the N20 lifetime. 

After the absorption of ultraviolet radiation by an oxygen molecule, it is split as follows ... 

Transition Types in Fluorescence It is iinporiani to note that fluorescence seldom results from absorption of ultraviolet radiation of wavelengths shorter than 25( nni because such radiation is sufli-cicntly energetic to cause deaclivaiion of the excited states by piedissociation or dissociation. Tor example, 2(K)-nm radiation corresponds to about 140 kcal/mol. Most organic molecules have at least some bonds that can be ruptured by energies of this magnitude. A a consequence, fluorescence duo torr — ir transitions is seldom observed Instead, such emission is conlined to Ihc less energetic 77 — - and 17 -.n processes. 

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