Chirped excitation in I2 - reflectron.
One of the simplest forms of pulse shaping is chirping, in which the central frequency of the pulse changes with time. Consider the simplest, ease of linear chirping:
,
where
τ is the pulse duration and
c/2 is the linear chirp rate in time.
One distinguishes an upwardly chirped pulse, in which the central frequency increases with time, from a downwardly chirped pulse, in which the central frequency decreases with time. One effect of chirp is wavepacket
focussing, This effect can be described within first order perturbation theory, treating the chirped field and the anharmonic wavepacket evolution
on the excited electronic state within a single coherent framework. Following Wilson and coworkers, (Kohler, 1995; Cao, 1997), we distinguish the case where the wavepacket focussing occurs
before the classical turning point is reached
(a "cannonball") and the case where the focussing occurs
after the turning the point is reached and the wavepacket is already on its way back to the Franck-Condon region
(a "reflection"). For a wavepacket initiated on the steep portion of a Morse potential, upward chirping leads to cannonballs while downward chirping leads to reflections.
