Interaction of Electromagnetic Radiation with Molecules

Chapter 3 discussed the orbitals that an electron may occupy in an atom. The energies of these orbitals are quantized—that is, only certain energies are allowed. For example, there is no orbital with an energy intermediate between that of a Is orbital and that of a 2s orbital. Not only are the energies of the electron orbitals quantized, but all of the energy states of a molecule are quantized. It is this fact that makes spectroscopy possible. 

Ground state (molecule has lowest amount of energy) 

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Molecular spectroscopy. This spectroscopy deals with the interaction of electromagnetic radiation with molecules. This results in transition between rotational and vibrational energy levels besides electronic transitions. 

As with UV-vis spectroscopy, IR and NMR spectroscopy are based on the interaction of electromagnetic radiation with molecules, whereas MS is different in that it relies on high-energy particles (electrons or ions) to break up the molecules. The relationship between the various types of spectroscopy and the electromagnetic spectrum is shown in Table 28.1. 

The essentials of a quantum mechanical treatment of the interaction of electromagnetic radiation with molecules is described in the first chapter, and the second one deals with supramolecular photochemistry, with particular emphasis on energy and electron transfer with a description of the Marcus theory. The following chapters are devoted to the different photochemical and photophysical techniques spectrophotometry and spectrofluorimetry, actinometry, absorption and luminescence techniques with polarized light excitation, time-resolved absorption and luminescence spectroscopy, down to femtosecond resolution. Each... 

To see how this result is used, consider the integral that arises in formulating the interaction of electromagnetic radiation with a molecule within the electric-dipole approximation ... 

Most of what we know about the structure of atoms and molecules has been obtained by studying the interaction of electromagnetic radiation with matter. Line spectra reveal the existence of shells of different energy where electrons are held in atoms. From the study of molecules by means of infrared spectroscopy we obtain information about vibrational and rotational states of molecules. The types of bonds present, the geometry of the molecule, and even bond lengths may be determined in specific cases. The spectroscopic technique known as photoelectron spectroscopy (PES) has been of enormous importance in determining how electrons are bound in molecules. This technique provides direct information on the energies of molecular orbitals in molecules. 

This chapter introduces the interaction of electromagnetic radiation with organic molecules. By the end of the chapter you should be able to ... 

The interaction of electromagnetic radiation with matter in the domain ranging from the close ultraviolet to the close infrared, between 180 and 1,100 nm, has been extensively studied. This portion of the electromagnetic spectrum, called UV/Visible because it contains radiation that can be seen by the human eye, provides little structural information except the presence of unsaturation sites in molecules. However, it has great importance in quantitative analysis. Absorbance calculations for compounds absorbing radiation in the UV/Visible using Beer-Lambert s Law is the basis of the method known as colorimetry. This method is the workhorse in any analytical laboratory. It applies not only to compounds that possess absorption spectra in that spectral region, but to all compounds that lead to absorption measurements. 

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... 

Luminescence processes may be categorized by the excitation method used with any particular luminescent molecule. Photoluminescence is the excitation process that involves the interaction of electromagnetic radiation with photons. The process is termed chemiluminescence when the exciting energy results from a chemical reaction. Any luminescence arising from an organism is referred to as bioluminescence. 

In the previous section we presented a general Hamiltonian for the interaction of electromagnetic radiation with atoms and molecules that can be put in the form ... 

Mass spectrometry is used to measure the molecular mass of a compound and provides a method to obtain the molecular formula. It differs from the other instrumental techniques presented thus far because it does not involve the interaction of electromagnetic radiation with the compound. Instead, molecules of the compound being studied are bombarded with a high-energy beam of electrons in the vapor phase. When an electron from the beam impacts on a molecule of the sample, it knocks an electron out of the molecule. The product, called the molecular ion (represented as A/f), has the same mass as the original molecule but has one less electron. It has both an odd number of... 

Simple molecular photoionization is the interaction of electromagnetic radiation with a molecule (in its ground state) to generate an ion (in a variety of different possible energy, and symmetry states) and a free electron Equation (17.2). In this process, mass, momentum and energy are conserved, according to Equation (17.3). 

The interactions of electromagnetic radiation with the vibrations of a molecule, either by absorption in the infrared region or by the inelastic scattering of visible light (Raman effect), occur with the classical normal vibrations of the system (Pauling and Wilson, 1935). The goal of our spectroscopic analysis is to show how the frequencies of these normal modes depend upon the three-dimensional structure of the molecule. We will therefore review briefly in this section the nature of the normalmode calculation more detailed treatments can be found in a number of references (Herzberg, 1945 Wilson etal., 1955 Woodward, 1972 Cali-fano, 1976). We will then discuss the component parts that go into such calculations. 

In addition to the spontaneous emission of excited molecules, fluorescence and phosphorescence (Section 2.1.1), the interaction of electromagnetic radiation with excited molecules gives rise to stimulated emission, the microscopic counterpart of (stimulated) absorption. Albert Einstein derived the existence of a close relationship between the rates of absorption and emission in 1917, before the advent of quantum mechanics (see Special Topic 2.1). 

In clinical diagnostics, one of the most commonly used analytical techniques for chemical analysis of body fluid samples is spectrophotometric analysis (the study of the interaction of electromagnetic radiation with chemical compounds). Biological molecules interact in some way with many different parts of the electromagnetic spectrum. Spectrophotometry is a very popular technique and can be used to determine the concentration and/or amount of a particular analyte, determine the structure of a new compound, identify a specific compound, and determine the activity of a specific enzyme, among others. The spectrophotometric detection methods are briefly reviewed in the following paragraphs. More detailed information can be found in books such as [3]. 

It is well known that p.e. spectroscopy can be considered as the output of a particular interaction of electromagnetic radiation with matter in whatever aggregation state. Electromagnetic radiation of suitable energy, when impinging with a molecule in the gas phase, can be either simply absorbed, thus producing an excited state ... 

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