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

J.P.J. Driessen, M.P.I. Manders, F.J.M. van de Weijer, G.J. Sandker, W. Boom, H.C.W. Beijerinck, B.J. Verhaar

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For the molecular targets CH4, H2, N2, O2, CO, CO2 and N2O we have investigated the ionization cross section 3Q 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 H2). A crossed-beam apparatus with well defined beam geometry is used, which allows the investigation of polarized-atom cross sections 3Q|M|, with |M| the magnetic quantum number with respect to the initial relative velocity. Following Bussert et al., the large ionization cross section 40 3Q 80 Å2 at thermal energies are interpreted in terms of an initial state with an attractive type interaction with a deep well e/E1. An orbiting collision model predicts 3QQorb = 5.9 (C6/E)1/3 with C6 the Van der Waals parameter. At E = 100 meV we find an ionization probability 3Q/Qorb in the range 0.6 to 0.8 (0.3 for CH4) which are realistic values. In the superthermal energy range the cross section 3Q decreases to the value of 2Q for the metastable Ne*{(3s); 3P2} 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 3Q|M| = 0,1/3Q = 1.11 to 1.00 when going from CH4 to N2O, 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 (CH4, H2, N2, CO2, N2O).
Originele taal-2Engels
Pagina's (van-tot)447-464
Aantal pagina's18
TijdschriftChemical Physics
Nummer van het tijdschrift3
StatusGepubliceerd - 1991


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