Experimental scaling of plane-Born cross sections and ab initio assignments for electron-impact excitation and dissociation of XF₄ (X = C, Si, and Ge) molecules

Electron energy loss spectra of carbon tetrafluoride, silicon tetrafluoride, and germanium tetrafluoride molecules (CF₄, SiF₄, and GeF₄) have been measured for incident electron energies of 50–360 eV at 1.5°–15.5° and for 30 eV and 30° scattering angle, while sweeping the energy loss over the range 9.0–20.0 eV. Low-lying valence excited triplet and singlet states are investigated by quantum chemical ab initio calculations. The Rydberg series converging to the (lowest) ionisation energy limits of XF₄ (X = C, Si, Ge) are also identified and classified using the systematic behaviour according to the magnitude of the quantum defects. A generalized oscillator strength analysis is employed to derive oscillator strength f₀ value and the apparent Born integral cross sections from the corresponding differential cross sections by using the Vriens formula for the optically allowed transitions. The f₀ value is compared with the optical oscillator strength of the photoabsorption, pseudo-photon measurements, and theoretical values. The binary-encounter and f-scaled Born cross sections of the most intense optically allowed transitions have been also derived from the excitation threshold to the high energy region where the Born approximation is valid. Potential energy curves were obtained along the XF₃ + F coordinate with two different basis sets to lend support on electron impact dissociation processes yielding radical formation. We found that in CF₄, the lowest-lying dissociative character is due to intramolecular conversion from Rydberg 3s to valence character (σ*(C–F)), whereas in SiF₄ and GeF₄, an antibonding behaviour prevails.