Tellurite And Fluorotellurite Glasses For Active And Passive
Tellurite And Fluorotellurite Glasses For Active And Passive Tellurite And Fluorotellurite Glasses For Active And Passive
6. Optical properties; MDO 171 where δmax = maximum pathlength difference of the inferometer. Therefore, a resolution of 0.2 cm -1 , gives δmax = 2.5 cm. Detectors The detector in a spectrometer converts the incident radiation received from the sample into an electronic signal to be processed. Much like the source, the detector type depends on the region of the electromagnetic spectrum being investigated. However, semiconductor detectors are commonly used in the ultraviolet (UV), visible and IR. One popular type of semiconductor detector is known as a charge coupled device (CCD) [3]. In higher frequency regions, such as the visible and UV, a photomultiplier is alternatively used. This photomultiplier works by releasing an electron from a photosensitive screen each time it is struck by an incident photon. This electron is accelerated by a current, striking further screens creating a cascade effect, resulting in an amplified signal. Although semiconductor devices are becoming more frequently used, thermocouple detectors are sometimes used for the IR [3]. Intensities of absorption bands and the Beer-Lambert law An empirical equation, known as the Beer-Lambert law, can be used to calculate the intensity of absorption, I, with sample thickness, l. This is shown by equation (6.8) [3]. log I I 0 = −ε[ J ] l (6.8)
6. Optical properties; MDO 172 where I0 = incident intensity, [J] = molar concentration of absorbing species J, and ε = extinction coefficient or molar absorption coefficient. ε has units mol. -1 cm -1 , therefore the product of ε[J]l is dimensionless, and known as absorbance, γ. Transmittance, ϕ, of the sample is given by I/I0. The relation between transmittance and equation (6.8) is shown by equation (6.9) [3]. log ϕ = −γ (6.9) The absorbance of the sample can be easily obtained by determining I and I0 (and hence ϕ) experimentally. As equation (6.8) implies, sample absorption decreases exponentially with sample thickness, l, and concentration, [J] [3]. Molecular explanation of vibrational spectroscopy If a molecule possesses a permanent electric dipole moment, P, with particles of charges +e and –e separated by distance r, shown by equation (6.10) [2]. P = er (6.10) A heteronuclear diatomic molecule above absolute 0K vibrates at a particular frequency. The molecular dipole also vibrates about its equilibrium position. This dipole can only absorb energy from an electric field (i.e. incident IR radiation) if the dipole oscillates at the same frequency. A resonant energy transfer occurs, for example, between the net
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6. Optical properties; MDO 172<br />
where I0 = incident intensity, [J] = molar concentration of absorbing species J, and ε =<br />
extinction coefficient or molar absorption coefficient. ε has units mol. -1 cm -1 , therefore the<br />
product of ε[J]l is dimensionless, and known as absorbance, γ. Transmittance, ϕ, of the<br />
sample is given by I/I0. The relation between transmittance and equation (6.8) is shown<br />
by equation (6.9) [3].<br />
log ϕ = −γ<br />
(6.9)<br />
The absorbance of the sample can be easily obtained by determining I and I0 (and hence<br />
ϕ) experimentally. As equation (6.8) implies, sample absorption decreases exponentially<br />
with sample thickness, l, and concentration, [J] [3].<br />
Molecular explanation of vibrational spectroscopy<br />
If a molecule possesses a permanent electric dipole moment, P, with particles of charges<br />
+e and –e separated by distance r, shown by equation (6.10) [2].<br />
P = er<br />
(6.10)<br />
A heteronuclear diatomic molecule above absolute 0K vibrates at a particular frequency.<br />
The molecular dipole also vibrates about its equilibrium position. This dipole can only<br />
absorb energy from an electric field (i.e. incident IR radiation) if the dipole oscillates at<br />
the same frequency. A resonant energy transfer occurs, for example, between the net