Population trapping in short-pulse laser ionization

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Abstract
The finite bandwidth of a short laser pulse often overlaps several initially unpopulated low-lying excited states of an atom. We show that when such a pulse is used to photoionize an atom, the low-lying levels may be pumped into a coherent superposition that inhibits or prevents excitation of the atom, effectively trapping population in the initial state of the atom. The existence of the effect was first noted in a numerical study of short-pulse excitation of hydrogen. Here we explore the effect through a series of simple models that demonstrate the physics of the effect in the simplest possible situations.