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Unformatted text preview: Chemistry 121 2007 Department of Chemistry University of British Columbia Selection Rules for Atomic Spectroscopy When an atom or ion is excited or de-excited in an absorption or emission experiment an electron undergoes a change of state from one orbital to another. The two orbitals involved are described by a set of four quantum numbers, n , l , m l and m s . The first consideration in deciding which transitions are allowed is that of conservation of energy. This requires that the energy of the photon (E photon = h photon ) is equal to the energy difference ( E ) between the two orbitals is the same as the energy of the photon. Therefore, the principal quantum number can change by any amount consistent with the energy change of the transition. This principle leads us to our first selection rule . A selection rule is a statement about the transitions that are allowed. We can write a selection rule for any transition as follows, n = n f - n i = anything consistent with energy conservation...
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- Spring '09