Penning ionization electron spectra of pyrene, chrysene, and coronene in collision with metastable He(2 3S) atoms in the gas phase

Penning ionization electron spectra (PIES) of pyrene (C(16)H(10)), chrysene (C(18)H(12)), and coronene (C(24)H(12)) in the gas phase are recorded using metastable He(*)(2 (3)S) atoms. The assignments of PIES are presented based on the outer valence Green's function calculations with the correlation consistent polarized valence triple-zeta basis sets and the exterior electron density calculations of contributing molecular orbitals. The definite positions of all of the pi bands in the PIES are identified making use of the large PIES cross sections. Broad bands are observed in low-electron-energy regions for chrysene and coronene and are ascribed to ionization processes of non-Koopmans types from sigma orbitals. The anisotropic interaction potential energy surfaces for the colliding systems are obtained from ab initio model potentials for the related systems with similar outer valences Li(2 (2)S) + C(16)H(10), C(18)H(12), and C(24)H(12), respectively. The attractive well depths in the out-of-plane directions are found to be similar between these molecules, and the repulsive walls embrace the in-plane perimeters uniformly. Collision energy dependencies for partial Penning ionization cross sections and negative peak shifts in PIES for chrysene support these anisotropic interactions. Effects from thermal populations in low-frequency vibrational modes are estimated to be minor in one-electron ionization processes.