Sequences for controlling laser excitation with femtosecond three-pulse four-wave mixing

Three-pulse four-wave mixing (FWM) is used here to study and control laser excitation processes. For general laser excitation processes, after a molec ule interacts resonantly with a laser pulse, the molecule has a probability of being in the ground or in the excited state. Control over this process depends on the phase and amplitude of the electric fields that interact wit h the molecular system. Here we show how three-pulse FWM can be used to con trol the excitation of iodine molecules. Depending on the time delay betwee n the first two pulses, the observed signal reflects the dynamics of the gr ound or excited state. A theoretical formalism based on the density matrix formulation is presented and solved for a four-level system. Experiments ar e found to be in excellent agreement with the theory. The influence of line ar chirp on three-pulse FWM experiments is explored. Spectrally dispersed t hree-pulse FWM is found to be extremely useful for studying the effect of c hirp on laser excitation of molecular systems. Experimental demonstrations of these effects are included.