Turning off Decay: Population Trapping in Bound-Continuum Excitation
A discrete state which is coupled to a continuum of states usually finds its time-development damped by an irreversible loss of a Markovian or near-Markovian kind. The decay is described by a close-coupling, of which the Weisskopf-Wigner model is the best known, and laser-induced photcionisation and autoionisation more recently-studied examples. Attention has recently focussed on the exploitation of atomic coherence effects to disengage such decay channels by the creation of superposition states stable against photoionisation or decay.1 Such superposition states are modelled as dressed states of the atom-field system2 whose decay rates can be reduced to zero by a combination of width-subtracting linenarrowing3 and off-diagonal damping.4 Such effects have been studied in Raman excitation,5 quantum beat fluorescence,6 laser-excited autoionisation,7 predissociation8 and induced continuum structure.2 We review the theory of such effects and discuss their importance in multiphoton excitation. Two examples will be described in detail. First, the Raman lambda system of a common excited state from which decay to either of two ground states can occur exhibits the asymptotic trapping of population in a superposition of ground states from which it cannot be excited. This is reflected in the Raman absorption lineshape by deep coherence minima or “dark resonances”.5 The second example to be discussed concerns the excitation of population from a discrete state to a structured continuum. Again, population can remain in the initial discrete state provided the discrete-continuum excitation rate approaches the structured continuum width. One of the atom-field dressed states then becomes stable, at the expense of an increased decay rate of the other. Laser fluctuations and bandwidths may not entirely dephase population trapping.9 These effects demonstrate that close-coupling to a continuum does not always lead to an incoherent irreversible decay.
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