Electron induced fluorescence and dissociative excitation of molecules

The study theme of the present work is electron induced fluorescence and dissociative excitation processes of N₂, H₂, CH₄ a C₂H₂ molecules. A cross-beam apparatus was used, where an electron and a molecular beam interacted in high vacuum. The fluorescence radiation produced in the reaction was collected by an optical system and detected by a photomultiplier. Emission spectra measured in spectral range 200-700 nm, and emission functions measured in electron energy range 0-100 eV are presented. The emission functions were calibrated to absolute values and threshold energies were estimated from them. Probable excitation and dissociative-excitation channels with their theoretical threshold values were suggested. The results were compared with works of other authors, if possible. Our interest at N₂ molecule was aimed at the radiation of vibronic transitions of the second positive system N₂ (C³Π_u(ν^') → B³Π³(ν'')) and the ionic first negative system N₂⁺ (B²Σ₂⁺(ν') → X²Σ_g⁺(ν'')). Emission functions of higher vibronic transitions, previously unpublished, were measured. For H₂ molecule measurements were done of the hydrogen continuum H₂(a³ Σ_g⁺ → b³Σ_u⁺), Balmer series of atomic hydrogen H2(n > 2 → 2), and of Fulcher α system H₂(d³Π → a³Σ_g⁺). The experiment proved that the hydrogen continuum extends to higher wavelengths than the present cutoff value 500 nm, probable all the way to IR spectral range. CH₄ molecule dissociates when excited to a higher electronic state. Our focus was thus placed on its fragments CH, CH⁺, C a H observed in the UV/VIS spectral range. Lower values of threshold energies were obtained than previous measurements, implying the activity of new dissociative channels. For C₂H₂ molecule only the radiation of fragments was observed in the UV/VIS spectral range. Transitions of the fragments CH, CH⁺, C₂, C a H were studied. Few electron induced fluorescence studies were published about this molecule, concentrating on radiation of a few fragmentary transitions only. A broader collection of emission functions, their threshold energies and supposed reaction channels is presented than in previous publications. (author)