Electromagnetic radiation molecular absorption

Section 13 20 IR spectroscopy probes molecular structure by examining transitions between vibrational energy levels using electromagnetic radiation m the 625-4000 cm range The presence or absence of a peak at a charac tenstic frequency tells us whether a certain functional group is present Table 13 4 lists IR absorption frequencies for common structural units... 

Atomic and Molecular Energy Levels. Absorption and emission of electromagnetic radiation can occur by any of several mechanisms. Those important in spectroscopy are resonant interactions in which the photon energy matches the energy difference between discrete stationary energy states (eigenstates) of an atomic or molecular system = hv. This is known as the Bohr frequency condition. Transitions between... 

The techniques available to achieve molecular structure determinations are limited. They include structural analysis with diffraction techniques—such as electron, neutron, and x-ray diffraction—and various absorption and emission techniques of electromagnetic radiation—such as microwave spectroscopy and nuclear magnetic resonance (NMR). For molecules with unpaired spins a companion technique of electron spin resonance spectroscopy (ESR) is highly informative. 

Electromagenetic Radiation. Atomic and Molecular Energy. The Absorption and Emission of Electromagnetic Radiation. The Complexity of Spectra and the Intensity of Spectral Lines. 

Electromagnetic radiation has its origins in atomic and molecular processes. Experiments demonstrating reflection, refraction, diffraction and interference phenomena show that the radiation has wave-like characteristics, while its emission and absorption are better explained in terms of a particulate or quantum nature. Although its properties and behaviour can be expressed mathematically, the exact nature of the radiation remains unknown. 

Only arc/spark, plasma emission, plasma mass spectrometry and X-ray emission spectrometry are suitable techniques for qualitative analysis as in each case the relevant spectral ranges can be scanned and studied simply and quickly. Quantitative methods based on the emission of electromagnetic radiation rely on the direct proportionality between emitted intensity and the concentration of the analyte. The exact nature of the relation is complex and varies with the technique it will be discussed more fully in the appropriate sections. Quantitative measurements by atomic absorption spectrometry depend upon a relation which closely resembles the Beer-Lambert law relating to molecular absorption in solution (p. 357 etal.). 

The absorption of electromagnetic radiation by molecular species in solution in the UV/visible region is followed by relaxation from excited electronic states to the ground state mostly by a combination of radiationless processes. Vibrational relaxation, where the excess energy is rapidly dis-... 

When electromagnetic radiation passes through transparent matter, some of it is absorbed. Strong absorption will occur if there is a close match between the frequency of the radiation and the energy of one of the possible electronic or molecular absorption processes characteristic of the medium. A plot of absorbance (A) against wavelength (X) or frequency (v) for a particular material is termed an absorption spectrum. The complexity of the absorption spectrum depends on whether atomic (simple, with a few sharp absorption bands) or molecular (complex, with many broad bands) processes are responsible. 

Molecular absorption spectroscopy deals with measurement of the ultraviolet-visible spectrum of electromagnetic radiation transmitted or reflected by a sample as a function of the wavelength. Ordinarily, the intensity of the energy transmitted is compared to that transmitted by some other system that serves as a standard. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

Emission and absorption of electromagnetic radiation by molecular systems takes place in transitions from an initial quantum state i) to a final state /). The dipole transition matrix element associated with such a transition is obtained from the wavefunctions of these states, ipfr) and... 

Collision-induced absorption is a well developed science. It is also ubiquitous, a common spectroscopy of neutral, dense matter. It is of a supermolecular nature. Near the low-density limit, molecular pairs determine the processes that lead to the collision-induced interactions of electromagnetic radiation with matter. Collision-induced absorption by non-polar fluids is particularly striking, but induced absorption is to be expected universally, regardless of the nature of the interacting atoms or molecules. With increasing density, ternary absorption components exist which are important especially at the higher temperatures. Emission and stimulated emission by binary and higher complexes have also... 

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