Wavepacket Interferometry
The first, order perturbation theory formula for optical excitation reads:
The integral
dt'
indicates that 0ne must take into account all instants in time
t' at which the interaction with the field could have taken place. In general, if the field has some envelope of finite duration in time, the promotion to the excited state can take place at, any instant under this envelope, and there will be interference from portions of the amplitude that are excited at, one instant and portions that are excited at another.
The simplest case of interference is
E(t') given by two short pulses. The first pulse prepares a localized wavepacket that evolve on the excited state potentials, moving to the outer turning point and returning to the Franck Condon region. The second pulse transfers an additional portion of amplitude to the excited state potential. If the second pulse is incident when the wavepacket prepared by the first pulse is back in the Franck Condon region, the two portions of amplitude will interfere, either constructively or destructively depending upon their relative phase. The relative phase can in turn be controlled by the phase of the second pulse relative to the first.
