Time-Dependent Perturbation Theory The Sum over States Method

1 Time-Dependent Perturbation Theory The Sum over States Method. - The 

The subtraction of what is essentially the ground state dipole from the diagonal terms is the consequence of a canonical transformation that ensures that the ground state can be omitted from the perturbation sums, thereby removing a 

it is apparent from the nature of the formulae that, under the right circumstances, near resonance with one of the excitation energies may lead to one term dominating the response. Pre-resonant enhancement can be studied through SOS formulae provided all the photon energies are such that no combination produces a transition energy lying within the line width. 

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