Energy, light absorption, spectroscopy

Absorption spectroscopy records depletion by the sample of radiant energy from a continuous or frequency-tunable source, at resonance frequencies that are characteristic of various energy levels ia atoms or molecules. The basic law of absorption, credited to Bouguer-Lambert-Beer, states that ia terms of the iacident, Jq, and transmitted, light iatensities, the absorbance, M (or transmittance, T), is given by equation 1 ... 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

A number of cyano-bridged complexes are included here even though they strictly do not fall in the general family-type defined for the section. The syntheses and photophysical properties of [(NC)(bpy)2Ru(/r-NC)Cr(CN)5] and [(NC)5Cr(Ai-CI Ru(bpy)2(M-NC)Cr(CN)5] have been described. Absorption of visible light by the Ru(bpy)2 unit results in phosphorescence from the Cr(CN)g luminophore, and the results evidence fast intramolecular exchange energy transfer from the MLCT state of the Ru(bpy)2 chromophore to the doublet state of the Cr -based unit. Time-resolved resonance Raman and transient UV-vis absorption spectroscopies have been employed to investigate the MLCT excited states of [(NC)(bpy)2Ru(//-CN)Ru (bpy)2(CN)], [(NC)(bpy)2Ru(//-CN)Ru(phen)2(CN)]+, [(NC)(phen)2Ru(//-CN)Ru (bpy)2(CN)]+, [(NC)(bpy)2... 

An electronic transition in which an electron in a nonbonding (e.g., lone pair) orbital (called an n-orbital) is promoted to a w-antibonding orbital. The excited state arising from such a promotion is often referred to as an n-CT state. An electron in an n-orbital typically interacts strongly with a polar solvent this is less hkely to be the case for an electron in a n- orbital. Therefore, the energy difference between n and n orbital electrons will increase when a substance is placed in a more polar solvent this is manifested as a shift to shorter wavelength (often called a blue shift) for light absorption. See Absorption Spectroscopy... 

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. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

The ultra-violet light induces electromagnetic excitation in molecules absorbing the laser energy, and by subsequently applying the principle of absorption spectroscopy, kinetic and spectroscopic information relating to the electronically excited states of various energetic molecules have been derived. The systems studied to-date include s-TNB, s-TNT, triphenyl-amine, and mono- as well as di-nitronaphthalenes (Ref 13, 14, 20, 21 28)... 

As the title implies, the book emphasizes the fundamental aspects of photochemistry. The first section introduces the subject by enumerating the relevance of photochemistry. Since the vocabulary of photochemistry is that of spectroscopy, the second section in which is discussed energy level schemes and symmetry properties, is like a refresher course. In the third section the actual mechanism of light absorption is taken up in detail because the probability of absorption forms the basis of photochemistry. A proper understanding of the process is essential before one can appreciate photochemistry. The next three sections present the... 

There is another optical method which studies these energy modifications and produces a spectrum that contains almost the same information as that obtained in the mid IR Raman. In this technique, a solution of the sample in a solvent such as water is irradiated by intense, monochromatic laser light in the visible region. The composition of the beam diffused by species present in the sample is analysed at 90c to the incident beam. In this process, bands called Stokes fines are observed beside the incident beam, at greater wavelengths. If the differences between these bands and the wavelength of the incident beam are expressed as wavenumbers, the values obtained correspond to the difference in rotational and vibrational energy levels obtained by absorption spectroscopy (Fig. 10.24). 

Absorption of light is fundamental to all aspects of photochemistry and provides the basis for absorption spectroscopy.3 5-11 Light absorption is always quantized. It can take place only when the energy hv of a quantum is equal to the difference in energy between two energy levels of the absorbing molecule (Eq. 23-4). 

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