In spectroscopy, oscillator strength is a dimensionless quantity that expresses the probability of absorption or emission of electromagnetic radiation in transitions between energy levels of an atom or molecule. The oscillator strength can be thought of as the ratio between the quantum mechanical transition rate and the classical absorption/emission rate of a single electron oscillator with the same frequency as the transition.
An atom or a molecule can absorb light and undergo a transition from one quantum state to another.
The oscillator strength of a transition from a lower state to an upper state may be defined by
where is the mass of an electron and is the reduced Planck constant. The quantum states 1,2, are assumed to have several degenerate sub-states, which are labeled by . "Degenerate" means that they all have the same energy . The operator is the sum of the x-coordinates of all electrons in the system, etc.:
The oscillator strength is the same for each sub-state .
Thomas–Reiche–Kuhn sum rule
To make equations of the previous section applicable to the states belonging to the continuum spectrum, they should be rewritten in terms of matrix elements of the momentum . In absence of magnetic field, the Hamiltonian can be written as , and calculating a commutator in the basis of eigenfunctions of results in the relation between matrix elements
Next, calculating matrix elements of a commutator in the same basis and eliminating matrix elements of , we arrive at
Because , the above expression results in a sum rule
where are oscillator strengths for quantum transitions between the states and . This is the Thomas-Reiche-Kuhn sum rule, and the term with has been omitted because in confined systems such as atoms or molecules the diagonal matrix element due to the time inversion symmetry of the Hamiltonian . Excluding this term eliminates divergency because of the vanishing denominator.
Sum rule and electron effective mass in crystals
In crystals, energy spectrum of electrons has a band structure . Near the minimum of an isotropic energy band, electron energy can be expanded in powers of as where is the electron effective mass. It can be shown that it satisfies the equation
Here the sum runs over all bands with . Therefore, the ratio of the free electron mass to its effective mass in a crystal can be considered as the oscillator strength for the transition of an electron from the quantum state at the bottom of the band into the same state.
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