Vibronic Coupling through the Continuum in the e+CO_{2} System.

We report two-dimensional electron energy-loss spectra of CO_{2}. The high-resolution experiment reveals a counterintuitive fine structure at energy losses where CO_{2} states form a vibrational pseudocontinuum. Guided by the symmetry of the system, we constructed a four-dimensional nonlocal model for the vibronic dynamics involving two shape resonances (forming a Renner-Teller Π_{u} doublet at the equilibrium geometry) coupled to a virtual Σ_{g}^{+} state. The model elucidates the extremely non-Born-Oppenheimer dynamics of the coupled nuclear motion and explains the origin of the observed structures. It is a prototype of the vibronic coupling of metastable states in continuum.