Absorption coefficient temperature effects

Thickness. The traditional definition of thermal conductivity as an intrinsic property of a material where conduction is the only mode of heat transmission is not appHcable to low density materials. Although radiation between parallel surfaces is independent of distance, the measurement of X where radiation is significant requires the introduction of an additional variable, thickness. The thickness effect is observed in materials of low density at ambient temperatures and in materials of higher density at elevated temperatures. It depends on the radiation permeance of the materials, which in turn is influenced by the absorption coefficient and the density. For a cellular plastic material having a density on the order of 10 kg/m, the difference between a 25 and 100 mm thick specimen ranges from 12—15%. This reduces to less than 4% for a density of 48 kg/m. References 23—27 discuss the issue of thickness in more detail. 

The rate of heat conduction is further complicated by the effect of sunshine onto the outside. Solar radiation reaches the earth s surface at a maximum intensity of about 0.9 kW/ m. The amount of this absorbed by a plane surface will depend on the absorption coefficient and the angle at which the radiation strikes. The angle of the sun s rays to a surface (see Figure 26.1) is always changing, so this must be estimated on an hour-to-hour basis. Various methods of reaching an estimate of heat flow are used, and the sol-air temperature (see CIBSE Guide, A5) provides a simplification of the factors involved. This, also, is subject to time lag as the heat passes through the surface. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

Consider continuous radiation with specific intensity I incident normally on a uniform slab with a source function 5 = Bv(Tex) unit volume per unit solid angle to the volume absorption coefficient Kp and is equal to the Planck function Bv of an excitation temperature Tcx obtained by force-fitting the ratio of upper to lower state atomic level populations to the Boltzmann formula, Eq. (3.4). For the interstellar medium at optical and UV wavelengths, effectively S = 0. 

It is important to note that the calculation of the initial concentrations of phenol ( 10-2 mol dm-3) and acetonitrile (possibly 1 mol dm-3) were corrected for the density of the solvent at each temperature. The temperature effect on the molar absorption coefficient (e) was also considered when relating [PhOH] to the absorbance of the O-H free band. This was empirically made by measuring the absorbances (A) of a phenol solution (in the same solvent and with a concentration similar to that used in the equilibrium study) over the experimental temperature range. For each temperature, the Lambert-Beer law [312],... 

In order to determine the molar absorption coefficient, a pure, dry sample of the compound must be available. Purity is often difficult to check in a routine analytical laboratory but dryness may be achieved by desiccation. Care must be exercised in the choice of desiccant and the temperature used, owing to the potential instability of the compound at even ambient temperatures or the effect of light. It is advisable to determine the value for the coefficient using different samples of the compound and subsequently compare the results. 

Here, L total is the depth of the etched hole per pulse and is assumed to be the sum of photochemical and photothermal contributions, Tphoto and Thermal, respectively 0Ceff is the effective photon absorption coefficient of the medium and can vary with laser emission characteristics, e g., photon density Fis the incident laser fluence Fth is the medium s threshold fluence A and F are the effective frequency factor with units of pm/pulse and the effective activation energy with units of J/cm2, respectively, for the zeroth-order thermal rate constant F0, comparable in magnitude to Fth, is important only at low fluences.64 Equation (5) is obtained after assuming that the polymer temperature T in the laser-exposed region of mass mp and the thermal rate constant k are given, respectively, as... 

The first feature of interest is the variation of the peak height of the 3630 cm "3 (monomer) band with temperature and with concentration. Fig. 3 shows the apparent absorption coefficient, em, of the monomer band in ethanol as a function of concentration at several temperatures. The general decrease of em with increasing concentration is, of course, the result of depletion of monomer by formation of H-bonded species. Tt should be noted that the intercepts at zero concentration vary with temperature, with the highest value coming at the lowrest temperature. We reported this effect previously [10] and attributed it to interaction between alcohol monomers and the 0014 solvent, probably similar to weak H-bonding. Others have also noted this effect [11]. 

The absorption of ultrasound increases the temperature of the medium. Materials that possess higher ultrasound absorption coefficients, such as bone, experience severe thermal effects as compared to muscle tissue, which has a lower absorption coefficient [5]. The increase in the temperature of the medium upon ultrasound exposure at a given frequency varies directly with the ultrasound intensity and exposure time. The absorption coefficient of a medium increases directly with ultrasound frequency resulting in temperature increase. 

As described in Volume 2IB, Hydrogenated Amorphous Silicon Optical Properties, the absorption coefficient of undoped a-Si H is strongly influenced by the deposition conditions. For example, the optical gap usually increases as the substrate temperature decreases, and this effect has been attributed to an increase in the hydrogen content (Zanzucchi et al.,... 

Surface recombination reduces the luminescence intensity when the excitation light has a very short absorption length. The effect is weak because of the low carrier mobility and diffusion length, so that although there are many defects at the surface of a-Si H, the effect on the luminescence intensity is only 10-20% (Dunstan 1981). A measure of the surface recombination is the product, aLp, of the optical absorption coefficient and the diffusion length. In most situations in a-Si H, and particularly at low temperature, aLp is less than unity, whereas in crystals it is often much greater. 

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