Beer-lambert Law Beer-lambert Law In optics, the Beer–Lambert law, also known as Beer's law or the Lambert–Beer law or the Beer–Lambert–Bouguer law (named after August Beer, Johann Heinrich Lambert, and Pierre Bouguer) relates the absorption of light to the properties of the material through which the light is travelling. Most of the organic molecules have two electronic states. The first is singlet state in which all molecular electrons are in spin-paired state and second is triplet state in which one set of electron spins is unpaired. Distribution of energy between possible electronic and vibrational states :- The excess energy of the molecule is converted to vibrational energy which is called the internal conversion. This excess vibrational energy, lost by collision with other molecules, is known as vibrational relaxation. This conversion of energies is especially helpful for "loose and floppy" molecules as they can reorient themselves in ways which aid the internal transfer of energy. A combination of intra and inter-molecular energy redistribution :- If the molecule is excited to an excited triplet state due to reversed spin of the electron, this is called inter system crossing. As the triplet state is a state of lower electronic energy than the excited singlet state.
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The possibility of the excitation of molecule to excited triplet state is high when the vibrational levels of these two states overlap with each other. For example, the lowest singlet vibrational level can overlap with one of the highest vibrational levels of the triplet state. The excess energy of molecule in the excited triplet state is lost by collision with solvent molecules and returns to the lowest vibrational level of the triplet state. It can also undergo a second inter system crossing to a high vibrational level of the electronic ground state. Finally, the molecule comes back to the lowest vibrational level of the electronic ground state by vibrational relaxation. Molecular fluorescence spectroscopy is widely used in the studies of electronic properties of organic and inorganic molecules in all existing phases, in chemical analysis of various samples, in forensic, environmental, biological and also in industrial operations. It's also used in quantitative measurements of molecules, in solution and fluorescence detection, and in liquid chromatography. Instrumentation :- The fluorimeter is used to measure the fluorescence. The excitation source, sample cell, fluorescence detector are the main parts of Fluorimeter. Usually a deuterium or xenon lamp is used for excitation of molecules in solution. A t-broad-band excitation light passes through a monochromator which allows passing of the light of only a selected wavelength. The fluorescence is detected by a photomultiplier tube. The excitation spectrum and the fluorescence spectrum are found by scanning of the excitation monochromator and the fluorescence monochromator respectively. The pass filters are also used to select the excitation wavelength in simple instruments. Molecular Absorption Spectroscopy These spectroscopic techniques are used to measure the absorption of radiation in terms of frequency or wavelength. The sample absorbs energy in the form of photons from the radiating field.
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The intensity of the absorption varies with frequency. The variation in the intensity is known as the absorption spectrum. Absorption spectroscopy is related to the electromagnetic spectrum. Various experimental techniques are used to measure absorption spectra but the common arrangement is the one in which a beam of radiation passes through a sample and the intensity of the radiation is measured. This transmitted intensity can be used to calculate the absorption. The correct choice of the source, a sample arrangement and detection technique depends on the frequency range and the aim of the experiment. Thus, the absorption spectroscopy is used in analytical chemistry to detect the presence and amount of a particular substance in a sample, in astronomy (astronomical spectroscopy), in remote sensing, and to study molecular physics.
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