Experimental and theoretical electron energy spectra due to ionizing collisions of metastable Ar*(3P2), Ar*(3P0) and Kr*(3P0) atoms with ground-state Hg atoms

Abstract

Using a crossed-beams set-up, we have studied the electron energy spectra due to Penning and associative ionization in thermal energy collisions of state-selected metastable Ar*(4s 3P2, 3P0) and Kr*(5s 3P0) atoms with ground-state Hg atoms at an electron energy resolution of 30 meV. In all three cases the energy spectra exhibit a prominent peak located close to the energy difference between the metastable excitation energies and the Hg ionization energy; they show a sharp drop to lower and an extended tail towards higher electron energy with shapes which differ for the three systems. The energy-integrated ionization cross section for Ar*(3P0) exceeds that for Ar*(3P2) by about 30%. Excitation transfer to autoionizing Hg** states, resulting in a sharp additional structure, is only observed—as a weak channel—in Ar*(3P0) + Hg collisions. For the three investigated systems calculations of the electron energy spectra have been carried out within the local optical potential approximation. As input we use ab initio potential curves for the ionic exit channels as well as for the complex entrance channel potentials. Using these potentials (involving a scaled width for Kr* + Hg), impressive agreement with the experimental electron spectra is observed. In contrast, earlier empirical model potentials for the entrance channel are found to be inadequate. The calculated ionization rate coefficients at T = 300 K amount to (3.72, 5.26) × 10−10 cm3 s−1 for Ar*(3P2, 3P0) + Hg, respectively.