It is proposed to use the coherence properties of a laser beam to control the product distribution in the photodissociation of small molecules. The approach relies on the theory of Shapiro and Brumer that product state distribution is determined entirely by the phase of the degenerate superposition state excited by the laser, so that controlling the phase controls the product branching ratio. It is proposed to achieve phase control of the excited state by using two optical paths to excite the same state, with a well defined phase difference between the two optical pulses, corresponding to absorption of a single photo or three photos. The approach will be applied to the competition between autoionization and photo dissociation, and also to the photo dissociation of an isotopically labelled molecule to produce isotopically distinguishable neutral fragments. Oscillations is product branching ratios as the relative phases of the two laser beams is scanned will provide proof of coherent laser control.
