Two-Dimensional Penning Ionization Electron Spectroscopy of Monohalogenobenzenes by He*(2³S): C₆H₅X (X = F, Cl, Br, I)

Penning ionization of C₆H₅X (X = F, Cl, Br, I) upon collision with metastable He*(2³S) atoms was studied by two-dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy. Partial ionization cross sections are found to be larger for ionization from orbitals having n and π characters. Interaction potentials between the target molecule and the He* atom were found to be highly anisotropic. Attractive interaction was dominated around the collinear access of He*(2³S) to C - F axis in C₆H₅F. On the other hand, attractive interaction was localized around the out-of-plane perpendicular approach of He* atom to C - X bond (X = Cl, Br, I). Attractive interaction for these compounds increases on going from C₆H₅Cl to C₆H₅I. Furthermore, the electronic factor due to the size of the halogen p orbitals and the conjugation between the benzene ring and the halogen atoms and also the steric factor due to the shielding effect were found to be important.