The absorption of radiation

Potential-energy curves for a molecule in its ground state and its excited state, showing vibrational levels. The arrow represents a transition from the ground state at u = 0 to the excited state at v 3. 

When electromagnetic radiation interacts with a molecule, certain transitions between states can occur, and others cannot. There are selection rules which tell us which transitions are possible and which are not. The detailed theory underlying these selection rules is fairly complicated, but some important general conclusions can be stated. 

Pure rotational transitions, which give spectra in the far infrared and radio regions, will be considered first. A molecule can only absorb electromagnetic radiation if it can interact with the oscillating electric field associated with the radiation. If a molecule has a permanent dipole moment, this dipole oscillates as rotation occurs, and a pure rotational spectrum is obtained. This is the case with molecules such as carbon monoxide (CO) and hydrogen chloride (HCl), which have permanent dipole moments. Molecules such as H2 and N2, which do not have permanent dipole moments, do not have pure rotational spectra. 

Another selection rule which applies to both rotational and vibrational transitions is that the quantum number can only change by unity in other words, there can be a transition only to a neighboring rotational or vibrational level. 

The selection rule for vibration is in other respects different from that for rotation. For a transition to be possible, the dipole moment must oscillate as vibration occurs. This rule at once excludes diatomic molecules with zero dipole moments, which remain zero as the molecule vibrates (e.g., O2). However, any molecule with a dipole 

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An important question to consider when using a flame as an atomization source, is how to correct for the absorption of radiation by the flame. The products of combustion consist of molecular species that may exhibit broad-band absorption, as well as particulate material that may scatter radiation from the source. If this spectral interference is not corrected, then the intensity of the transmitted radiation decreases. The result is an apparent increase in the sam-... 

The electromagnetic spectrum measures the absorption of radiation energy as a function of the frequency of the radiation. The loss spectrum measures the absorption of mechanical energy as a function of the frequency of the stress-strain oscillation. 

Chemistry students are familiar with spectrophotometry, the qualitative and quantitative uses of which are widespread in contemporary chemistry. The various features of absorption spectra are due to the absorption of radiation to promote a particle from one quantized energy state to another. The scattering phenomena we discuss in this chapter are of totally different origin classical not quantum physics. However, because of the relatively greater familiarity of absorption spectra, a comparison between absorption and scattering is an appropriate place to begin our discussion. 

The next step is to consider tire cross-sections of the absorption of radiation by the diatomic halogen molecules in order to decide if the relative effects result from the efficiency of the radiation photon-molecule interactions. These are reflected in the dissociation cross-sections of tlrese interactions. 

Atomic absorption spectroscopy involves atomising the specimen, often by spraying a solution of the sample into a flame, and then studying the absorption of radiation from an electric lamp producing the spectrum of the element to be determined. 

To investigate the absorption of radiation by a given solution, the solution must be placed in a suitable container called a cell (or cuvette) which can be accurately located in the beam of radiation. The instrument is provided with a cell-carrier which serves to site the cells correctly. Standard cells are of rectangular form with a 1 cm light path, but larger cells are available when solutions of low... 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

Self-Absorption—Absorption of radiation (emitted by radioactive atoms) by the material in which the atoms are located in particular, the absorption of radiation within a sample being assayed. 

The most important situation occurs when a film of different optical properties is formed at the electrode surface. In this case, theory predicts that the R value can be changed, even for non-absorbing films, as a result of existence of a third phase with different refractive index interspaced between the electrode and electrolyte. Therefore, the entire observed decrease in reflectivity R is not necessarily caused by the absorption of radiation in the film. This approximation, is, however, reasonably acceptable when the film is supported by a highly reflective phase, such as smooth metal electrode. 

The radiation emitted by a real surface is less than the radiation emitted by a blackbody, and the absorption of radiation by a real surface is incomplete. Many surfaces are excellent approximations to a blackbody, but some are not. Of the radiation incident upon a real surface, I(k, 9), a portion is reflected, some is... 

All intermediate species produced by the absorption of radiation (electrons, ions, excited states, free radicals, etc.) may be potentially useful for synthesis. However, the most frequently used intermediates are the free radicals. Their yield is high and relatively insensitive to temperature or state of aggregation (Wagner, 1969). 

A large variety of aqueous and a few nonaqueous solutions have been used or proposed as chemical dosimeters with respective dose ranges for use (Spinks and Woods, 1990 Draganic and Draganic, 1971). Of these, a special mention may be made of the hydrated electron dosimeter for pulse radiolytic use (l(h2 to 10+2 Gy per pulse). It is composed of an aqueous solution of 10 mM ethanol (or 0.7 mM H2) with 0.1 to 10 mM NaOH. Concentration of hydrated electrons formed in the solution by the absorption of radiation is monitored by fast spectrophotometry, which is then used for dosimetry with the known G value of the hydrated electron. 

Spectroscopy The absorption of radiation between two energy levels within an atom or molecule... 

Microwave spectroscopy The study of the rotational motion of molecules by the absorption of radiation - the technique of choice for the identification of molecules... 

Scintillation counters, which constitute an extremely important group, depend upon the absorption of radiation by a scintillator to produce UV light scintillations, which are detected and converted into amplified voltage pulses by a photomultiplier (Figure 10.10). Solid scintillators are used extensively for the detection and analysis ofy-rays and X-rays, while liquid scintillators find widespread employment in the measurement of pure negatron emitters, especially where the particle energy is low (< 1 MeV). 

In ultraviolet and visible region, electronic transition of atoms and molecules are observed. This is why it is called electronic spectroscopy. In infrared region the absorption of radiation by an organic compound causes molecular vibrations and so it is called vibrational spectroscopy. 

RADIATION-SENSITIVE GROUPS. Although the absorption of radiation energy is dependent only on the electron density of the substrate and therefore occurs spatially at random on a molecular scale, the subsequent chemical changes are not random. Some chemical bonds and groups are particularly sensitive to radiation-induced reactions. They include COOH, C-Hal, -SO2-, NHz, C=C. Spatial specificity of chemical reaction may result from intramolecular or intermolecular migration of energy or of reactive species -free radicals or ions. 

Dose" as defined here refers to the absorption of radiation energy by human tissue. Higher doses correspond to higher potential for adverse health effects, as described in Chapter 3. The terms dose and exposure are often used interchangeably. 

On the other hand, Perrin made confused remarks from the standpoint of quantum physics "We need not believe that the absorption of radiation is discon-... 

Proteins are the major components by bulk in many biological samples and hence the weighing of a dried sample should give an estimate of the amount of protein present. Similarly, solutions that contain protein show values for specific gravity and surface tension which are in some way related to protein content. Measurements of the turbid ity resulting from the precipitation of protein and the absorption of radiation at specif ic wavelengths have all been used quantitatively... 

Spectroscopy is the study of the absorption and emission of radiation by matter. The most easily appreciated aspect of the absorption of radiation is the colour shown by substances that absorb radiation from the visible region of the spectrum. If radiation is absorbed from the red region of the spectrum, the transmitted or unabsorbed radiation will be from the blue region and the substance will show a blue colour. Similarly substances that emit radiation show a particular colour if the radiation is in the visible region of the spectrum. Sodium lamps, for instance, owe their characteristic orange-yellow light to the specific emission of sodium atoms at a wavelength of 589 nm. 

Which of the following statements are true about the absorption of radiation ... 

The materials and design of the various photoelectric detectors available are such that the absorption of radiation results in the displacement of electrons and hence in the development of a potential difference between two electrodes. The main types of photoelectric detectors may be classified as either photovoltaic or photoconductive (Figure 2.24). 

Figure 2.24 Photoelectric detectors. Photovoltaic detectors measure the flow of electrons displaced by the absorption of radiation. Photoconductive detectors measure the changes in conductivity caused by the absorption of radiation.

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