We report on an experimental approach that reveals crucial details of the composition of singlet-triplet mixed eigenstates in acetylene. Intersystem crossing in this prototypical polyatomic molecule embodies the mixing of the lowest excited singlet state (S1) with 3 triplet states (T1, T2, and T3). Using high-energy (157-nm) photons from an F2 laser to record excited-state photoelectron spectra, we have decomposed the mixed eigenstates into their S1, T3, T2, and T1 constituent parts. One example of the interpretive power that ensues from the selective sensitivity of the experiment to the individual electronic state characters is the discovery and examination of destructive interference between two doorway-mediated intersystem crossing pathways. This observation of an interference effect in nonradiative decay opens up possibilities for rational coherent control over molecular excited state dynamics.Decomposition mixed eigenstates ͉ Nonradiative decay interference ͉ Photoelectron spectroscopy S inglet-triplet intersystem crossing (ISC) is a fundamental nonradiative decay pathway for molecular electronically excited states. At its basis is the spin-orbit interaction that mixes low-lying vibrational levels of a singlet state with isoenergetic highly excited vibrational levels of lower-lying triplet electronic states (1). Being able to exert control over intersystem crossing or over any nonradiative decay pathway of molecular excited states potentially has important applications. For example, it could lead to control over photochemical reactions (2), more efficient dyes in fluorescence microscopy (3), driving of molecular motors (4), and other photonic applications. However, mixing between the ''optically bright'' state and the bath of ''optically dark'' states is generally viewed to be statistical, and as such offers no possibility for the design and application of rational coherent control schemes. Sometimes the interaction between bright and dark states is locally promoted by a ''doorway'' state* (5-8) that facilitates stronger indirect mixing than would occur in the case of direct interaction between bright and dark states. Nevertheless, even for ISC mediated by a doorway state, control schemes cannot go beyond a simple energy and temporal control based on prior knowledge of specific spectroscopic details, such as the choice to excite close to or far from the doorway state.It is only after an appropriate experimental approach has been devised to understand the mechanism of ISC that one may hope to exert full external control over ISC. Typically, studies of intersystem crossing focus on its effects in the time domain, i.e., the evolution of the relevant electronic state populations is followed after excitation of the singlet state. However, with this approach details of interaction mechanisms are very difficult to determine when vibrational levels of more than one triplet electronic state lie below the singlet state. Clearly, determining the composition of the mixed wavefunctions in terms of all of their contribut...