Total ionization cross sections for Ne**{(2p) 5(3p); J = 3}-molecule systems: Absolute values and polarization effects

Abstract

For the molecular targets CH 4, H 2, N 2, O 2, CO, CO 2 and N 2O we have investigated the ionization cross section 3 Q for collisions with laser excited Ne**{(2p) 5(3p); J = 3} atoms in the range 100 ⩽ E ⩽ 4000 meV of collision energies (80 ⩽ E ⩽ 800 meV for H 2). A crossed-beam apparatus with well defined beam geometry is used, which allows the investigation of polarized-atom cross sections 3 Q | M| , with | M| the magnetic quantum number with respect to the initial relative velocity. Following Bussert et al., the large ionization cross section 40 ⩽ 3 Q ⩽ 80 Å 2 at thermal energies are interpreted in terms of an initial state with an attractive type interaction with a deep well ϵ/ E⪢1. An orbiting collision model predicts 3 Q⩽ Q orb = 5.9 ( C 6/ E) 1 3 with C 6 the Van der Waals parameter. At E = 100 meV we find an ionization probability 3 Q/ Q orb in the range 0.6 to 0.8 (0.3 for CH 4) which are realistic values. In the superthermal energy range the cross section 3 Q decreases to the value of 2 Q for the metastable Ne*{(3s); 3P 2} state, indicating that the repulsive branch is determined by the (2p) 5 core and the (3s) or (3p) valence electron no longer plays an important role. In the thermal energy range the polarization effect decreases from 3 Q | M| = 0,1 / 3 Q = 1.11 to 1.00 when going from CH 4 to N 2O, which should be compared to 1.22 for Ar. This decrease corresponds to an increase of the anisotropy of the long-range polarizability of the molecule. At superthermal energies the polarization effect is equal to unity for all systems including Ar. For comparison we also present experimental data on the polarization effect for intramultiplet mixing by molecular targets at thermal energies. Again we observe a strong decrease when going from He (no ionization) to ionizing targets (Ar) with an increasing anisotropy (CH 4, H 2, N 2, CO 2, N 2O).