LOW-ENERGY ELECTRON-IMPACT SPECTROSCOPY OF [1.1.1]PROPELLANE - ELECTRON-ATTACHMENT ENERGIES AND SINGLET AND TRIPLET EXCITED-STATES

Electron transmission spectra and energy and angular dependence of vibrational excitation by electron impact were recorded and used to characterize different states of the short-lived negative ion (resonances) of the title compound. The first attachment energy, corresponding to electron capture into the 3a2" LUMO, is 2.04 eV, exceptionally low in comparison with a typical value of approximately 6 eV for a saturated hydrocarbon. Observation of (faint) vibrational structure indicates a lifetime broadening of the order of a vibrational spacing for this resonance, much less than is typical for sigma* resonances of saturated hydrocarbons (several electron volts). The Frank-Condon width of the band, 0.6 eV (fwbh), and intense excitation of nu3 vibrational mode point to appreciable lengthening of the interbridgehead distance (R(bb)) in the negative ion. Additional, higher-lying and broad sigma* resonances with maxima around 6-7 eV are observed in vibrational excitation functions, and a core exited 2(5a1',3a2"2) resonance is observed at 7.36 eV in the excitation function of the lowest triplet state. Electron energy loss spectra in the electronic excitation energy range reveal the triplet and singlet excited states. The vertical excitation energy to the lowest triplet state is 4.70 eV, also exceptionally low for a saturated hydrocarbon and in line with the low energy of the LUMO. A long progression in the C-C stretch vibration nu3 indicates appreciable lengthening of R(bb) in the triplet state. An intense dipole allowed transition to the lowest valence singlet state is found at 7.26 eV. The band is unusually narrow and the state is proposed to have partly Rydberg character. The spectra further reveal several Rydberg states and Feshbach resonances.