The role of a molecular phase in two-pathway excitation schemes

The origin and potential implications of a molecular phase in coherent two-pathway excitation are analyzed by means of a time-independent, quantum mechanical theory. A unified model is developed describing unimolecular decay into several continua of general nature, where each breakup reaction takes place both directly and via a manifold of resonances. It is found that the molecular phase, observable in two pathway excitation experiments, correlates with the single-pathway fragmentation line shape, in agreement with recent measurements. We propose the measurement of molecular phases as a route to molecular properties which are silent in conventional observables.