Electromagnetic radiation interaction with molecules

The interaction of light and molecules forms the basis of IR spectroscopy. Here it will be given a short description of the Electromagnetic Radiation, the energy levels of a molecule and the way the Electromagnetic Radiation interacts with molecules and their structure [5,6]. 

Today, organic chemists rely on an array of very powerful instruments that enable them to identify compounds in much less time. With use of these instruments, it is often possible to determine the structure of an unknown compound in less than an hour. Three of the most powerful techniques are presented in this and the following chapters. They are infrared spectroscopy and two related techniques proton and carbon-13 nuclear magnetic resonance spectroscopy. Spectroscopy is the study of the interaction of electromagnetic radiation (light) with molecules. 

Spectroscopy (Chapter 13) The study of the interaction of electromagnetic radiation (light) with molecules. 

In most of the examples described in this book, the rotational angular momentum is coupled to other angular momenta within the molecule, and the selection rules for transitions are more complicated than for the simplest example described above. Spherical tensor methods, however, offer a powerftd way of determining selection rules and transition intensities. Let us consider, as an example, rotational transitions in a good case (a) molecule. The perturbation due to the oscillating electric component of the electromagnetic radiation, interacting with the permanent electric dipole moment of the molecule, is represented by the operator... 

When electromagnetic radiation interacts with a molecule, certain transitions between states can occur, and others cannot. There are selection rules which tell us which transitions are possible and which are not. The detailed theory underlying these selection rules is fairly complicated, but some important general conclusions can be stated. 

Most of our knowledge about the structure of atoms and molecules is based on spectroscopic investigations. Thus spectroscopy has made an outstanding contribution to the present state of atomic and molecular physics, to chemistry, and to molecular biology. Information on molecular structure and on the interaction of molecules with their surroundings may be derived in various ways from the absorption or emission spectra generated when electromagnetic radiation interacts with matter. 

The variation with wavelength of the refractive indices derives solely from the dependence of the polarisability on wavelength. The polarisability determines the extent to which electromagnetic radiation interacts with the charge distribution within the molecule, and the in-phase and out-of-phase... 

Up to this point, the wavefunctions considered do not evolve with time. In some cases, the Hamiltonian may have time-dqtendent terms indicating that the system changes with time. An important example is when electromagnetic radiation interacts with a system. Electromagnetic radiation consists of electric and magnetic fields that oscillate in space and time. When electromagnetic radiation interacts with a molecule (such as in spectroscopy), the oscillating fields will result in a time-dependent element in the complete Hamiltonian for the molecule. As already observed in the case of infrared spectroscopy, this interaction may result in a transition of states. 

The method by which incident radiation interacts with the medium to cause ionization may be direct or indirect. Electromagnetic radiations (x rays and gamma photons) are indirectly ionizing that is, they give up their energy in various interactions with cellular molecules, and the energy is then utilized to produce a... 

In addition to describing the conformation of the hydrocarbon chains for amphiphilic molecules at the A/W interface, external reflectance infrared spectroscopy is also capable of describing the orientation of the acyl chains in these monolayers as a function of the monolayer surface pressure. The analysis of the orientation distribution for an infrared dipole moment at the A/W interface proceeds based on classical electromagnetic theory of stratified layers (2). In particular, when parallel polarized radiation interacts with the A/W interface, the resultant standing electric field has contributions from both the z component of the p-polarized radiation normal to the interface, as well as the x component of the p-polarized radiation in the plane of the interface. The E field distribution for these two... 

The mass spectrometry does not involve an interaction between electromagnetic radiation and sample molecule. The functions of a mass spectrometer are to produce positive ions from the sample under investigation, to resolve these ions into a series of ion beams that are homogeneous with respect to their mass/charge ratio (m/e), and to measure the relative abundance of the ions in these beams. The main applications include the molecular weight and structural determinations (Bieman, 1992). Mass spectrometry has emerged as the method for rapid analyses of protein sequences and annotation of their databases (Mann and Pandey, 2001). 

Fig. 3.8 Classification of electromagnetic radiation in the wavelength range below X of 1200 nm as bands with specific names, and the interaction with molecules M (VUV vacuum-UV).

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