Angle- and energy-resolved electron energy-loss spectroscopy of singlet and triplet valence and Rydberg states in 3,4-dihydro-2H-pyran: experiment and theory.

Optical spectroscopy predominantly probes dipole-allowed singlet excitations, limiting access to spin-forbidden and non-dipole transitions. In polyatomic molecules, mixing between valence and Rydberg excitations further complicates spectral interpretation. Here, we combine angle- and energy-resolved electron energy-loss spectroscopy (EELS) with high-level quantum-chemical calculations to investigate singlet and triplet electronic states and the valence and Rydberg character of excitations in gas-phase 3,4-dihydro-2H-pyran (C5H8O, DHP). By varying the incident energy and scattering angle, the relative contributions of different excitation channels are modified. Small scattering angles yield spectra dominated by singlet excitations that share the principal features of the optical absorption spectrum, whereas larger scattering angles increase the relative contribution of spin-forbidden excitations. The combined experimental and theoretical analysis suggests that the broad spectral feature around 6-7 eV contains several overlapping singlet and triplet excitations whose relative contributions vary with the scattering conditions, providing information that was not accessible from the previously reported near-threshold EELS measurement. This interpretation provides new insight into the origin of the broad spectral structure observed in this energy region and contributes to a more complete description of the low-lying excited states of DHP.