Absorption coefficient resonant absorbers

If we express the effective thickness t for the resonant absorption in terms of the total absorber thickness t in g cm, a nuclear absorption coefficient can be defined in cm g such that = t. The signal amplitude is then given by ... 

Apparently monochromatic resonance radiation of mercury which passes through mercury vapor at the saturated pressure at 25 °C is about half absorbed in four millimeters distance. Beer s law is not obeyed at all because the incident radiation cannot be considered to be actually monochromatic, and absorption coefficients of mercury vapor vary many times between zero and very high values in the very short space of one or two hundredths of an Angstrom unit. Moreover, absorption of mercury resonance radiation by mercury vapor is sufficiently great even at room temperature to make radiation imprisonment a very important phenomenon. If the reaction vessel has any dimension greater than a few millimeters the apparent mean life of Hg(63P ) may be several fold the true radiative life of 1.1 x 10"7 sec, reaction (27), because of multiple absorption and re-emission. 

Several reductive assays have been proposed (Table 3). A test using reduction of l,l-diphenyl-2-picrazyl (DPPH) radical was introduced in the 1950s (B15) and has been used by various authors. DPPH is a stable free radical, which can be bought in substantia. It has an absorbance maximum at 515 nm (millimolar absorption coefficient e = 12.5 mM-1 cur1) (A15). A disadvantage of this chromogen lies in the fact that it can be dissolved only in organic (especially alcoholic) but not in aqueous media, which limits its use for studies of hydrophilic antioxidants. Reduction of ABTS also can be followed by electron spin resonance (ESR) (Yl). 

The emission spectrum of the cathode material includes a number of intense, sharp lines arising from transitions between excited states and the ground state, so-called resonance radiation. Generally, only a few resonance lines per element are suitable for quantitative work and there will be variation in the ranges of concentration over which absorbance measurements can be made. The maximum value of the absorption coefficient is directly proportional to the oscillator strength / and indirectly proportional to the... 

Pure MCD with no rotation occurs if the sample is optically thick and completely absorbing in one circular polarisation, but not in the other. Pure MOR will occur when both circular polarisations are equally absorbed, i.e. the absorption coefficients ot+ v) and ot-(v) are equal, but the refractive indices n+(v) and n-(v) are not equal. The latter condition is satisfied at the centre of symmetry of the rotation pattern, viz. the field-free resonance frequency vq.6 In principle, the situation seems simpler when either pure MOR or pure MCD occurs, which is why most of the effort has traditionally been expended in separating one from the other, leading to MOR and MCD spectroscopies. 

Change in the refractive index can be induced by either a resonant or a nonresonant process. For a resonant process, the frequency of the incident light overlaps with an electronic absorption band, by either a one-photon or a multiphoton process. The energy is absorbed by the sample and an excited state population is generated. This induces a transient change in the absorption spectrum of the material due to the bleaching of the ground state absorption and/or the appearance of the excited state absorption. A nonlinear absorption coefficient a2 can be defined similarly to Eq. (15) ... 

Fig. 3. Schematic of the outgoing and backscattered photoelectron wave, which illustrates the concept of interference in EXAFS. The central atom is the absorbing atom, and the photoelectron is backscattered from the surrounding atoms, (a) The backscattered wave is in phase with the outgoing wave at energy E. This leads to an increase in the absorption coefficient at ,. (b) At energy Ei the backscattered wave destructively interferes with the outgoing wave, which leads to a decrease in the cross section at Ei. (c) Attenuation in the cross section in the absorption coefficient. [Adapted from D. B. Goodin, Ph.D. Dissertation, University of California, Berkeley. Lawrence Berkeley Laboratory Report, LBL 16901 (1983) and R. A. Scott, in Structural and Resonance Techniques in Biological Research (D. L. Rousseau, ed.), p. 295. Academic Press, Orlando, Florida, 1984.]...

The effect of admitting an absorbing gaseous sample into a Fabry-Perot, or any resonant cavity, is to lower its Q by an amount proportional to the absorption coefficient a defined as the power attenuation per unit length traversed. Thus the incident power Pq is reduced to P ... 

When the absorbing sample is placed inside the resonator of a tunable laser, the Lamb dip in the absorption coefficient a((o) causes a corresponding peak of the laser output power P co) (Fig. 2.16). 

Ambient air is sucked through a cell, where the ozone concentration is measured by its UV-absorption at 253.7 nm. At this wavelength the ozone absorption coefficient is close to its maximum. The UV light is produced by a Hg resonance lamp at 253.7 nm and the intensity of the light is measured by a photomultiplier. The ozone concentration is determined fi om the absorbance making use of the law of Lambert Beer ... 

In samples prepared later on with better optical quality W. Schmid observed a different behaviour [120]. The damping became stronger with lower repetition rates, i.e. higher peak intensity. No thermo-optic effect was observed. This can be well explained by the advances in crystal preparation solvent inclusions were reduced and the crystal quality improved, resulting in a lower absorption coefficient at the same, near-resonant wavelength (e.g. 720 nm). So, the absorbed heat load and the thermo-optical coefficients were reduced concurrently. 

---