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Lecture

Selection Rules for Atomic Spectroscopy

2 Pages
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Department
Chemistry
Course Code
CHEM 121
Professor
Jackie Stewart

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Description
Chemistry 121 2007 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 and m . lhe firstsconsid eration in deciding which transitions are allowed is that of conservation of energy. This requires that the energy of the photon (E photon= hν photon) is equal to the energy difference (∆E) between the two orbitals is the same as the energy of the photon. Therefore, th e principal quantum number can change by any amount consistent with the energy change of the tr ansition. This princip le leads us to our first selection rule. A selection rule is a statem ent about th e transitions that ar e allowed. W e can write a selection rule for any transition as follows, ∆n = n -fn =ianything consistent with energy conservation where n and n are the principal quantum numbers for the initial and final states respectively. i f We now have to consider any possible constrai nts on the other quantum numbers. The angular momentum quantum number, l, describes the type of orbital th at th e elec tron is in. In an excitation or de-excitation the angular momentum can change as follows, ∆l = lf– li The question is, by how much can ∆l change? The answer lies in the fact that a photon has an intrinsic spin angular mom entum of one (1) unit and that angular m omentum has to be conserved. This requires that ∆l = l – l = angular momentum of the photon= ± 1 f i where the positive sign corresponds to the abso rption of a photon and the negative sign to the emission of a photon. In summary the change in an
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