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  Home > JCE Print > Journal of Chemical Education > Issues > 1999  > September  >
Research: Science and Education
Advanced Chemistry Classroom and Laboratory
Spectroscopic Selection Rules: The Role of Photon States
Andrew M. Ellis
University of Leicester, Department of Chemistry, University Road, Leicester LE1 7RH, United Kingdom
Cover
September 1999
Vol. 76 No. 9
p. 1291
Abstract
Selection rules play a central role in spectroscopy. In an introductory undergraduate course it is frequently desirable to give some justification for these rules without recourse to a formal derivation. One way of doing this is to apply conservation arguments. Two of the quantities that must be unchanged by photon absorption or emission are the overall angular momentum and parity of the system. This argument can be used to justify certain selection rules if the angular momentum and the parity of the photon are known. This article begins by showing that the assumption commonly made about the angular momentum of a photon, namely that it has "unit" angular momentum, ±h/2p, fails to account correctly for angular momentum selection rules. To avoid this difficulty, a more detailed account of the properties of a photon is necessary. In quantum electrodynamics, photons may be viewed as existing in different states according to their different angular momenta and parities. The various states can be associated with specific types of photons. It is shown how this can be used, together with vector diagrams, to provide a simple justification of angular momentum selection rules that is suitable not only for electric dipole transitions, but also for magnetic dipole and electric quadrupole transitions.
More Information
*  Citation
 Ellis, Andrew M. J. Chem. Educ. 1999 76 1291.
*  Keywords
 Physical Chemistry; Atomic Properties / Structure; Quantum Chemistry; spectroscopy
*  History
 Created:
Last Updated:		 July 30, 1999
June 23, 2005
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