Laser induced fluorescence spectra of symmetrically substituted trihalobenzene radical cations

Laser induced fluorescence excitation and resolved emission spectra are obtained in a Ne matrix for the radicals cations of C₆H ₃F₃, C₆H₃Cl³ and all their deuterated analogs. The excitation spectra show that the excited state in each of the ions is characterized by extremely sharp, extremely regular vibrational structure. The majority of the a′₁, and e′ species vibrational modes are assigned. The resolved emission spectra of the ions, however, show irregular structure for the e′ modes, which is entirely consistent with a Jahn-Teller distortion of the ground state. Gas phase hot band spectra confirm that the Ne matrix does not perturb the vibrational structure. Although spectral assignments are more difficult in the ground state, it is shown that γ″₁₃, the C-C-C bond angle deformation mode, is principally affected by this distortion. Jahn-Teller distortion parameters are derived for the four species: C₆H₃F ₃⁺, C₆D₃F₃⁺, C₆H₃Cl₃⁺ and C₆D ₃Cl₃⁺.