Knowledge Bank

The gas-phase multiphoton ionization spectra of triethylenediamine (DAECO) and quinuclidine (ABCO) have been measured. These rigid, symmetric $(D3h$ and $C3v$, respectively) caged amines provide excellent models for intramolecular through-space and through-bond orbital interactions. A wide spectral range of 20 000 $cm-1$ has been studied, and three lowest energy molecular Rydberg states (DABCO) have been found as two-photon resonances in the three and four-photon ionization spectrum. Two of these resonances $(S1$ and $S3)$ have not been observed in conventional one-photon spectra, while the third $(S2)$ resonance appears in exactly the same spectral position, with the same vibronic structure, as a previously reported one-photon resonance. Symmetry selection rules along with published photoelectron and fluorescence results are utilized to propose new assignments for these low-lying states.