Multiphoton Atomic and Molecular Coherent Excitations with Ultrashort Pulse Sequences
With the recent progress in short pulse generation, an increasing number of experiments involving energy exchange between light and matter occur in conditions of coherent interactions. The time scale of the experiment becomes shorter than any collision time between particles of the ensemble. The radiation is thus interacting directly with isolated atoms. The condition of coherent interactions can be defined easily in the Bohr atom picture. Let us consider a rotating frame of reference in which the ground state electron appears at rest. A periodic electric field at ω = ω1/n (n integer) can induce transitions (transition rate proportional to En) to a higher orbit (of energy hω1 above the ground state). In this rotating frame of reference, the periodic motion of the electron is an atomic clock that will remember the phase of the excitation. The electron returns to the original position where it was created at every cycle. Hence, if the atom is truly isolated, it can be returned to the ground state by applying the opposite excitation after an integer number of periods. In conditions of coherent resonant interaction, the transfer of energy between radiation and matter is reversible, but also pulse shape dependent.
KeywordsPulse Sequence Harmonic Generation Stark Shift Rotational Quantum Number Dipole Matrix Element
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