Spectroscopy : (atomic and Molecular)

## Concepts in Spectroscopy where 6.E is the energy difference between two quantum levels and v is the frequency of photon which can result in the electronic excitation. Applications of atomic spectroscopy in the field of chemistry are few. However, its importance has increased by the development of lasers. (b) Molecular Spectroscopy : It deals with the interaction of electromagnetic radiation with molecules. This results in transitions between rotational and vibrational energy levels in addition to electronic transitions. As a result, the spectra of molecules are much more complicated than those of atoms. In contrast to atomic spectra which arise from the transitions of an electron between the atomic energy levels, the molecular spectra arise from three types of transitions, viz., rotational, vibrational and electronic transitions. Molecular spectra extend from the visible through infrared into the microwave region. Current interest in molecular spectroscopy is very great because the number of known molecules are extremely large as compared with free atoms. From molecular spectra, information may be obtained about molecular vibrations and rotations that reveal a great deal about molecular structure. The detailed information regarding molecular structure (molecular symmetry, bond distances and bond angles) and chemicals properties (electronic distribution, bond strength and intra-and inter-molecular processes) c.an be obtained from the atomic and molecular spectra. The spectroscopic investigations were first carried out by Newton in the seventeenth century, the quantitative treatment of the subject was possible only after the introduction of quantum mechanics. Initially, it was atomic spectra that provided the first direct experimental evidence for the various quantum mechanical postulates. Concepts in ~pectroscopy 1.9 Different electronic states may differ appreciably in their associated momentum of inertia and vibrational frequencies so that the permitted values of Era! and EVlb are considerably altered by electronic excitation. There is no general formula for electronic energies in terms of simple quantum numbers, except for hydrogen like atom. ### 0.5 Interaction of Electromagnetic Radiation with Matter Absorption of Radiation : When electromagnetic radiation passes through matter, a variety of phe~omena riiay occur. Some of these are as follows : (1) If the photons of radiation possess the appropriate energies, they may be absorbed by the matter and result in electronic transitions, vibrational changes or rotational changes or combination of these. After absorption, atoms and molecules become excited. They give out energy quickly either by losing energy in the form of heat or by re-emitting electromagnetic radiation. (2) It is not necessary that the radiation passing through the matter may be absorbed completely. The portion of electromagnetic radiation which passes into matter, instead of being absorbed, may undergo scattering or reflection or may be re-emitted at the same wavelength or a different wavelength. (3) When electromagnetic radiation is neither absorbed nor scattered, it may undergo changes in orientation or polarisation. (4) In some cases, the molecules after absorbing radiation become excited but they do not lose energy very quickly but with some delay. In such cases the energy is re-emitted as radiation, usually of longer wavelength than was originally absorbed. This phenomenon is termed as fluorescence. If in some cases, there is a detectable time delay in re-emission, the phenomenon is termed as phosphorescence. Absorption and Emission Spectra : Spectroscopy is mainly concerned with the interaction of electromagnetic radiation with matter. After interaction, there may occur variation in intensity of electromagnetic radiation with frequency (or wavelength). The instrument which records this variation in intensity of radiation is known as spectrometer. There are two ways in which the interactions are observed, firstly that in which the sample itself emits radiation (Fig. 0.4), and secondly, where the sample absorbs radiations from a continuous source (Fig. 0.4). The spectrum obtained in the former case in known as emission spectrum whereas in the latter case it is termed as absorption spectrum. ## 1. 10 Concepts in Spectroscopy 5s --4p ~== . . . . . In the recent past, there has occurred rapid revolution in spectroscopic techniques. At the same time, many new spectroscopic techniques have been introduced and also the classical spectroscopic techniques have been modified to suit the modern analytical laboratory. In this short book, all these changes have been incorporated to suit B. Sc and M. Sc. students of chemistry, physics, biochemistry, environmental science, pharmacy, engineering sciences, microbiology, biotechnology, materials science and related them more suitable for students. Line diagrams have been redrawn to make the book more illustrated. The new standardised format of this book makes it the ideal reference work for the students of all the esteems. The book is also useful for anyone who uses spectroscopy in his research and class work