| Abstract
| - It is shown both theoretically and experimentally that the phase of a transition dipole matrix element may bedetermined from the interference between competing quantum mechanical paths. Specifically, we considersimultaneous m- and n-photon excitation of a molecule to a degenerate continuum state, leading to multiplereaction products. Variation of the relative phase of the electromagnetic fields produces a modulation of theproduct signals in the various channels. A relation between the phase lag between a pair of product signalsand properties of the continuum is established. Information is revealed about coupling mechanisms, theproperties of weak spectroscopic transitions, and the phases of quasi-bound wave functions.
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