Spectroscopic signatures of states in the continuum characterized by a joint experimental and theoretical study of pyrrole

We report a combined experimental and theoretical investigation of electron-molecule interactions using pyrrole as a model system. Experimental two-dimensional electron energy loss spectra (EELS) encode information about vibrational states of the molecule as well as position and structure of electronic resonances. The calculations using non-Hermitian extensions of equation-of-motion coupled-cluster theory facilitate the assignment of all major EELS features. We confirm the two previously described π* resonances at about 2.5 and 3.5 eV (the calculations place these two states at 2.92 and 3.53 eV vertically and 2.63 and 3.27 adiabatically). The calculations also predict a low-lying resonance at 0.46 eV, which has a mixed character---of a dipole-bound state and σ* type. This resonance becomes stabilized at one quanta of the NH excitation, giving rise to the sharp feature at 0.9 eV in the corresponding EELS. Calculations of Franck-Condon factors explain the observed variations in the vibrational excitation patterns. The ability of theory to describe EELS provides concrete illustration of the utility of non-Hermitian quantum chemistry, which extends such important concepts as potential energy surfaces and molecular orbitals to states embedded in the continuum.