Absorption Spectroscopy Theory

With reference to absorption spectroscopy, we deal here with photon absorption by electrons distributed within specific orbitals in a population of molecules. Upon absorption, one electron reaches an upper vacant orbital of higher energy. Thus, light absorption would induce the molecule excitation. Transition from ground to excited state is accompanied by a redistribution of an electronic cloud within the molecular orbitals. This condition is implicit for transitions to occur. According to the Franck-Condon principle, electronic transitions are so fast that they occur without any change in nuclei position, that is, nuclei have no time to move during electronic transition. For this reason, electronic transitions are always drawn as vertical lines. 

The energy within a molecule is the sum of several distinct energies  

The rotational energy concerns molecular rotation around its gravity center, vibrational energy is the result of periodic displacement of atoms of the molecule away from the equilibrium position, and electronic energy is generated by electron movement within the molecular bonds. Rotational levels have lower energy than vibrational ones and thus are lower in energy. Upon photon absorption, electronic, rotational, and vibrational levels 

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S. J. Haswell, ed. Atomic Absorption Spectroscopy Theory, Design, and Application, Elsevier, New York, 1991. 

McCoustra, M. R. S. (1990). Electronic absorption spectroscopy theory and practice. In Perspectives in Modem Chemical Spectroscopy, ed. Andrews, D. L., Springer-Verlag, Berlin, 88-101. 

Pollard W T, Dexhelmer S L, Wang Q, Peteanu L A, Shank C V and Mathles R A 1992 Theory of dynamic absorption spectroscopy of nonstatlonary states. 4. Application to 12 fs resonant Raman spectroscopy of bacterlorhodopsin J. Phys. Chem. 96 6147-58... 

By comparison, the only variable measured in absorption spectroscopy is transmission as a function of incident wavelength. Once quantum theory had revealed the potential... 

P.E. Spectroscopy, Electronic Absorption Spectroscopy and Ligand Field Theory.124... 

Further details of the theory and application of Raman spectroscopy in polymer studies can be found elsewhere (1. 9). However, vibrational frequencies of functional groups in polymers can be characterized from the spacing of the Raman lines and thus information complementary to IR absorption spectroscopy can be obtained. In addition, since visible radiation is used the technique can be applied to aqueous media in contrast to IR spectroscopy, allowing studies of synthetic polyelectrolytes and biopolymers to be undertaken. Conformation and crystallinity of polymers have also been shown to influence the Raman spectra Q.) while the possibility of studying scattering from small sample volumes in the focussed laser beam (-100 pm diameter) can provide information on localized changes in chemical structure. 

Volume 4 Sample Introduction in Atomic Spectroscopy, edited by J. Sneddon Volume 5 Atomic Absorption Spectrometry. Theory, Design and Applications, edited by S.J. Haswell... 

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