m where the cm are coefficients to be determined and where the probability of finding a particle in the mth stationary state is |cm|2.
Suppose now that a photon interacts with the particle to promote a change between two such stationary states. Let us assume that the transition between two energy levels is caused by a time-dependent influence, which we represent by a small additive potential u(r, t). Suppose also that at time t = 0 we know that the particle is in a certain stationary state ftn of energy En. The coefficients cm some time later (where m = n) may be shown from perturbation theory (e.g., Rae, 1985) to be cm ~
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