The intense continuous UV emission which extends from 200 nm to beyond 500 nm produced by electron impact on carbontetrafluoride, CF4, has been investigated in a crossed electron-beam - gas-beam apparatus as well as in a gas-target apparatus under single collision conditions with special emphasis on the regime of impact energies below 50 eV. Systematic measurements of excitation functions (relative emission cross sections) at various wavelengths between 225 nm and 420 nm revealed the same onset of 23.5±2 eV and the same energy dependence for all excitation functions independent of the selected wavelength indicating that the continuous emission consists of a single fluorescence contribution. Evidence of a second onset around 40 eV, roughly 16 eV above the first onset which is an energy close to the ionization energy of atomic fluorine (17.4 eV), was found - more or less prominent - in all excitation functions. The presence of the second onset lends strong support to the assignment of the excited (CF3+)* fragment ion as the emitting source of the emission continuum. We identify the lower onset at 23.5 eV with the electron-impact induced breakup of the parent CF4 molecule into an excited (CF3+)* ion and a ground state fluorine atom ("neutral" channel), whereas the second onset around 40 eV indicates the presence of an "ionic" fragmentation channel in which an excited (CF3+)* fragment ion is produced together with a fluorine ion, F+. Wavelength scans taken at impact energies above and below the second onset revealed no significant difference in the envelope of the observed emission continuum which further supports the notion that the emission continuum consists of a single fluorescence contribution.
|Original language||English (US)|
|Number of pages||9|
|Journal||Zeitschrift für Physik D Atoms, Molecules and Clusters|
|State||Published - Jun 1992|
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics