Absorptivity and emissivity

The enclosure schematically illustrated in Fig. 5.70 contains a homogeneous gas mixture with an absorbent component. The element dA on the surface of the gas volume, shown in Fig. 5.70, will be used for the definition and calculation of its directional spectral absorptivity a XG. The radiation emitted from dA, with the spectral intensity Lx, is weakened by absorption. Depending on the direction, the path through the gas is of different lengths, which according to (5.179) leads to varying reductions in Lx. 

The spectral absorptivity a x G belonging to a certain direction is the ratio of the energy absorbed in the distance s, to the energy emitted  

The directional spectral absorptivity is a property (variable of state) of the absorbing gas. Its direction dependence manifests itself in the dependence on the beam length s, through which the radiation passes in the gas. 

According to Kirchhoff s law, cf. section 5.3.2.1., the directional spectral emissivity e x G of a gas is equal to its directional spectral absorptivity  

With e x G, the radiation flow d3 bA G received by an element dA on the surface of the gas volume, in the wavelength interval dA, from the solid angle element dtp highlighted in Fig. 5.72, can be calculated. According to (5.4), the defining equation for the spectral intensity, it holds for this that 

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There are two fimdamental types of spectroscopic studies absorption and emission. In absorption spectroscopy an atom or molecule in a low-lying electronic state, usually the ground state, absorbs a photon to go to a higher state. In emission spectroscopy the atom or molecule is produced in a higher electronic state by some excitation process, and emits a photon in going to a lower state. In this section we will consider the traditional instrumentation for studying the resulting spectra. They define the quantities measured and set the standard for experimental data to be considered. 

All nonlinear (electric field) spectroscopies are to be found in all temis of equation (B 1.3.1) except for the first. The latter exclusively accounts for the standard linear spectroscopies—one-photon absorption and emission (Class I) and linear dispersion (Class II). For example, the temi at third order contains by far the majority of the modem Raman spectroscopies (table B 1.3.1 and tableBl.3.2). 

For > 0, one has absorption for < 0, emission. Multiphoton absorption and emission fall into this class. The Class I Raman spectroscopies clearly exliibit a net absorption of energy in Stokes scattering and a... 

Figure Bl.16.22. Schematic representations of CIDEP spectra for hypothetical radical pair CH + R. Part A shows the A/E and E/A RPM. Part B shows the absorptive and emissive triplet mechanism. Part C shows the spin-correlated RPM for cases where J and J a.. ...

One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

Returning to the kinetie equations that govern the time evolution of the populations of two levels eonneeted by photon absorption and emission, and adding in the term needed for spontaneous emission, one finds (with the initial level being of the lower energy) ... 

The absorption and emission eases ean be eombined into a single net expression for the rate of photon absorption by reeognizing that the latter proeess leads to photon produetion, and thus must be entered with a negative sign. The resultant expression for the net rate of decrease of photons is ... 

Figure 2.2 (a) Absorption and emission processes between states m and n. (b) Seeding... 

ABSORPTION AND EMISSION OF RADIATION For the vibrational energy level ... 

So far as rule 2 is concerned, since AJ is conventionally taken to refer to J -J", where J is the quantum number of the upper state and J" that of the lower state of the transition, AJ = — 1 has no physical meaning (although it emerges from the quantum mechanics). It is commonly, but incorrectly, thought that AJ = +1 and AJ = — 1 refer to absorption and emission, respectively in fact AJ = +1 applies to both. Transition wavenumbers or frequencies are given by... 

Fig. 1. Absorption and emission spectra in solution of a compound of stmcture (1).

Research and development in the field ate stiU continuing at a fast pace, particularly in the area of absorption and emission characteristics of the polymers. Several reasons account for this interest. First, the intractable polydimethyl silane [30107-43-8] was found to be a precursor to the important ceramic, siUcon carbide (86—89). Secondly, a number of soluble polysdanes were prepared, which allowed these polymers to be studied in detail (90—93). As a result of studies with soluble polymers it became cleat that polysdanes are unusual in their backbone CJ-conjugation, which leads to some very interesting electronic properties. 

Materials characterization techniques, ie, atomic and molecular identification and analysis, ate discussed ia articles the tides of which, for the most part, are descriptive of the analytical method. For example, both iaftared (it) and near iaftared analysis (nira) are described ia Infrared and raman SPECTROSCOPY. Nucleai magaetic resoaance (nmr) and electron spia resonance (esr) are discussed ia Magnetic spin resonance. Ultraviolet (uv) and visible (vis), absorption and emission, as well as Raman spectroscopy, circular dichroism (cd), etc are discussed ia Spectroscopy (see also Chemiluminescence Electho-analytical techniques It unoassay Mass specthot thy Microscopy Microwave technology Plasma technology and X-ray technology). 

Atomic and Molecular Energy Levels. Absorption and emission of electromagnetic radiation can occur by any of several mechanisms. Those important in spectroscopy are resonant interactions in which the photon energy matches the energy difference between discrete stationary energy states (eigenstates) of an atomic or molecular system = hv. This is known as the Bohr frequency condition. Transitions between... 

X-rays provide an important suite of methods for nondestmctive quantitative spectrochemical analysis for elements of atomic number Z > 12. Spectroscopy iavolving x-ray absorption and emission (269—273) is discussed hereia. X-ray diffraction and electron spectroscopies such as Auger and electron spectroscopy for chemical analysis (esca) or x-ray photoelectron spectroscopy are discussed elsewhere (see X-raytechnology). 

L. H. J. Lajunen, SpectrochemicalAnalysis by Atomic Absorption and Emission, Royal Society of Chemistry, Cambridge, U.K., 1992. 

Since 1970, new analytical techniques, eg, ion chromatography, have been developed, and others, eg, atomic absorption and emission, have been improved (1—5). Detection limits for many chemicals have been dramatically lowered. Many wet chemical methods have been automated and are controlled by microprocessors which allow greater data output in a shorter time. Perhaps the best known continuous-flow analy2er for water analysis is the Autoanaly2er system manufactured by Technicon Instmments Corp. (Tarrytown, N.Y.) (6). Isolation of samples is maintained by pumping air bubbles into the flow line. Recently, flow-injection analysis has also become popular, and a theoretical comparison of it with the segmented flow analy2er has been made (7—9). 

Instmmental methods such as atomic absorption and emission spectrometry, and gamma activation ate employed in most beryUium determinations however, gravimetric and tritrimetric methods remain useful when high accuracy is required. 

During heat dissipation by radiation the colour and condition of the surface plays a similar role. Dark-coloured bodies dissipate more heat than the light-coloured ones. The amount of heat absorption and emission for the same body may therefore be assumed to be almost the same. Accordingly, Table 31.1, for selected colours, may be considered for the coefficients of absorption and emission of heat due to solar radiation and natural radiation respectively. 

In our sample calculations (Example 3 1.1) we have chosen the colour of the outdoors surface as light grey and taking the vveathering effect into account, have considered the coefficient of both absorption and emission as 0.65. The manufacturer, depending on the colour and site conditions, may choose a suitable coefficient. It is, however, advisable to be conservative when deciding the temperature rise due to solar radiation to be on the safe side. 

In comparing the radiative properties of materials to those of a blackbody, fhe terms absorptivity and emissivity are used. Absorptivity is the amount of radiant energy absorbed as a fraction of the total amount that falls on the object. Absorptivity depends on both frequency and temperature for a blackbody if is 1. Emissivity is the ratio of the energy emitted by an object to that of a blackbody at the same temperature. It depends on both the properties of fhe subsfance and the frequency. Kirchhoff s law states that for any substance, its emissivity at a given wavelength and temperature equals its absorptivity. Note that the absorptivity and emissivity of a given substance may be quite variable for different frequencies. 

The characteristic lines observed in the absorption (and emission) spectra of nearly isolated atoms and ions due to transitions between quantum levels are extremely sharp. As a result, their wavelengths (photon energies) can be determined with great accuracy. The lines are characteristic of a particular atom or ion and can be used for identification purposes. Molecular spectra, while usually less sharp than atomic spectra, are also relatively sharp. Positions of spectral lines can be determined with sufficient accuracy to verify the electronic structure of the molecules. 

The net radiation power falling the surface is the difference between the incoming and t)utgoing radiation and the difference between absorption and emission ... 

In the case of substances whose structures are pH-dependent (e.g. phenols, carboxylic and sulfonic acids, amines etc.) it is possible to produce fluorescences or make them disappear by the deliberate manipulation of the pH [213] (Table 20). Shifts of the positions of the absorption and emission bands have also been reported. This is particularly to be observed in the case of modified silica gels, some of which are markedly acidic or basic in reaction (Table 25). 

One of the most important and exciting advances in modern biochemistry has been the application of spectroscopic methods, which measure the absorption and emission of energy of different frequencies by molecules and atoms. Spectroscopic studies of proteins, nucleic acids, and other biomolecules are providing many new insights into the structure and dynamic processes in these molecules. 

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

The nature of the light emissions is influenced by the way in which the absorbed energy is transferred through the polymer matrix. In crystalline polymers, exciton migration is possible as all molecules lose their energetic individuality and all electronic and oscillation levels are coupled [20]. Thus, new exciton absorption and emission bands are formed and the excitation energy can move along the chain ... 

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

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