Two-Photon Transitions with Time-Delayed Radiation Pulses
It is known that in order to have two-photon transitions in a system interacting with two monochromatic radiation beams the sum of the photon energies has to be equal to the energy difference between the initial and final level.1 Although all states have to be taken into account as intermediate virtual states in the calculation of the transition rate, it is not necessary that the single photon energies be in resonance with an intermediate level in order to have a two-photon transition. In the case of near-resonance on an intermediate state the two-photon transition rate increases. This resonant enhancement has been observed by Bjorkholm and Liao.2 In the case of resonance on an intermediate state another process consisting of two successive one-photon transitions (two-step transition) is expected to occur. Two-photon transitions are characterized by the fact that the two beams have to interact simultaneously (coherent absorption). On the other hand, two-step transitions are expected to occur also if the beams are separated in time. Moreover, in the latter case no enhancement in the transition rate is expected when the sum of the photon energies equals the energy difference between the initial and final level.
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