Emission Spectra of Aromatic Halides by Controlled Electron Impact

Fluorobenzene, chlorobenzene, and bromobenzene were excited at a very low pressure in the vapor phase by a controlled electron beam of 60–300 V. Their emission spectra were then measured and compared with those excited by the photon. Fluorobenzene showed a characteristic photoemission of the parent molecule in the 260–320 nm region. The band was assigned to the transition from the lowest excited state to the ground state, and its shape did not vary appreciably with the energy of the electron beam. Though the spectrum had some features of “resonance fluorescence,” the vibrational structure of the band was not identical with that of any of the spectra obtained by photon impact between 4.7–4.9 eV. This seems to indicate that the molecule gave different spectra according to the excitation energy. However, the excitation by 60 V electrons and that by 300 V electrons did not produce any remarkable difference in the spectra under the resolution of the spectrometer used. Other aromatic halides failed to show any characteristic band of the parent molecules; instead, they showed weak lines due to Cl+ or Br+ and pronounced bands due to HCl+ or HBr+. This result was ascribed to the spin-orbit coupling effect and the predissociation of the excited parent molecules.