Amount of light absorbed

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

The West-Gaeke manual method is the basis for the U.S. Environmental Protection Agency reference method for measurement of SO2 (6). The method uses the colorimetric principle i.e., the amount of SO2 collected is proportional to the amount of light absorbed by a solution. The collection medium is an aqueous solution of sodium or potassium tetrachloromercu-... 

Absorption Cross Section the amount of light absorbed by a particle divided by its physical cross section. 

Absorption spectrum (Section 12.5) A plot of wavelength of incident light versus amount of light absorbed. Organic molecules show absorption spectra in both the infrared and the ultraviolet regions of the electromagnetic spectrum. 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

As we saw in Chapter 1, the quantum yield for a photochemical reaction is simply the amount of product produced per amount of light absorbed ... 

The physical separation of PS II and PS I permits the chloroplasts to respond to changes in illumination. The relative amount of light absorbed by these two systems varies with the distribution of light harvesting complexes (LHCs) between the stacked and unstacked portion of the thylakoid membrane. 

An optical measuring system is used in sedimentation analysis, whereby a concentrated beam of light is deflected horizontally through the lower section of a measuring vessel onto a photoelectric cell. The amount of light absorbed by the sedimenting particles decreases with time as the... 

Some of the AP recommended experiments require the use of a spectrophotometer. A spectrophotometer is an instrument that is used to measure the amount of light absorbed (or percentage transmitted) by a particular solute in a solution. In order to determine the absorbance (A) of a sample, the instrument is set to a particular wavelength a solution, contained in a holder called a cuvette, is placed in a sample chamber and an absorbance reading is taken. This procedure may be repeated for other solutions or wavelengths. The cuvette is a standard size to ensure a given path length (b). 

The total amount of light absorbed may be provided by the following mathematical expression ... 

In order to calculate the amount of oxy-Hb or deoxy-Hb present from the amount of light absorbance, the absorbance spectra for these compounds must be known. 

Because the energies of covalent (single, double, or triple) bonds throughout a molecule are quite specific to each pair of atoms, the resultant spectrum may be used to give a quite specific qualitative picture of the species present. The amount of light absorbed is also proportional (Beer s law) to the amount of material present and may be used for quantitative analyses. 

FAAS Flame atomic absorption spectroscopy the flame atomizes metals in solutions. Once in the gas phase, the atoms absorb UV-vis light, exciting electrons to higher energy levels. The amount of light absorbed is used to determine the metal concentration. 

Two-photon excitation results in an apparent violation of Beer s law. In the one-photon technique the amount of light absorbed is proportional to the intensity of the incident light. This results in emission intensity being directly proportional to the intensity of the excitation. The well-known property of two-photon excitation is that emission intensity depends on the square of the incident light intensity. 

Equation (10) allows one to calculate a by determining the quantum yield of reaction as a function of sensitizer concentration. However, it should also be remembered that if the substrate absorbs part of the light, m may need to be corrected for changes in the amount of light absorbed by the sensitizer. 

The amount of light absorbed is a function of the so-called absorption coefficient (A ) and of the optical pathlength in the atomiser cell (ft) k depends on the frequency of the selected analytical line and on the concentration of the analyte absorbing atoms. The general absorbance law (Lambert Beer Bouguer law) relates transmittance (and so measured intensities I and If) to k and b through the following equation ... 

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