J.P.J. Driessen

Pure state Ne*(*) [ J,M>]-preparation in a magnetic field : polarization effects in ionizing collisions

Abstract A compact device (lengh 175 mm) has been built for the energy resolved detection of low energy (1–5 eV) Penning electrons with a 2π solid angle collection efficiency, based on the principle of adiabatic parallelisation of electron motion in a diverging magnetic field. A retarding field analysis is then used as a high pass filter to discriminate between Penning electrons released in collisions of rare gas atoms in metastable and shortlived, laser excited states. The overall detection efficiency is 0.13. The Zeeman-splitting of the atomic levels in the scattering center (maximum B = 222 G) allows the preparation of single magnetic substates |J, M⪢B. By rotating the detector in the collision plane, well defined |J, M⪢g states can be produced with respect to the relative velocity g, the quantization axis relevant for the collisions. The system has been tested by measuring the collision energy dependence of polarized-atom cross sections JQ|M| for the Ne* [3P2]-Ar and Ne** [3D3]-Ar systems. For the Ne* [3P2] metastable atoms we find 2Q0/2Q2 = 1.55 ± 0.06 and 1.05 ± 0.06 in the thermal and superthermal energy range, respectively, which should be compared to 1.30 of Bregel et al. at thermal energies. For the Ne** [3D3] state we find 3Q0,1/3Q2,3 = 1.65 ± 0.06 and 1.00 ± 0.10 for the same energy ranges.

Calculation of two-center integrals involving a rapidly oscillating free electron wave function

Optical potentials are used in a quantum mechanical treatment of loss processes, e.g., ionization, where the loss of flux is described by the imaginary part. We present a numerical method for calculating two-center two-electron integrals necessary to construct the imaginary part of the optical potential. By introducing Slater-type orbitals with complex-valued exponents (CSTOs), we are able to represent the free electron wave with a limited number of CSTOs. For the representation of free electron wave functions with many oscillations, i.e., in a large r range or for a high kinetic energy, these new CSTOs form a more natural set of basis functions. The introduction of CSTOs is inevitable for the calculation of integrals concerning collisions in the mK energy range, where the interaction acts over large internuclear distances. Extensive numerical checks show that the final imaginary part of the optical potentials can be calculated with an accuracy better than 2%.

Excitation transfer in collisions of Kr{4p55p; 3D3} with N2 molecules

Cross sections for the excitation of the ν1 = 0 and 1 vibrational levels of the electronically excited N2(C,ν′) product state have been measured, for collision energies in the range 85 ⩽ E(meV) </ 115, using the short-lived laser-excited Kr{(4p)55p;3D3} atoms as projectile. The cross sections Qν′ = 10.0 A2 for ν′ = 0 and 7.2 A2 for ν′ = 1 for this slightly exothermic process are interpreted in terms of a Landau-Zener model, with the ionic Kr+ + N2− state as an intermediate which crosses both the initial and final state. The crossing parameters are in good agreement with the results obtained for the similar but strongly endothermic process for the Kr{(4p)55s;3P0,3P2} + N2 systems, investigated by Vredenbregt et al. [Chem. Phys. 145 (1990) 267] in the superthermal energy range.

Polarization effects in collision-induced intramultiplet mixing for Ne)(2p)/sup 5/(3p)) + He

High-quality polarized-emission cross sections for the )..cap alpha..)/sub 5/ = chemically bondJ = 1,M/sub J/>..-->..)..cap alpha..)/sub 7/ and )..cap alpha..)/sub 5/..-->..)..cap alpha..)/sub 4/ transitions in the )..cap alpha..) = )(2p)/sup 5/(3p)) multiplet (lifetime 20 ns) have been measured in a crossed-beam experiment. For the )..cap alpha..)/sub 5/..-->..)..cap alpha..)/sub 7/ transition we observe a strong preference for the chemically bondM/sub J/chemically bond = 0 orientation. The small cross section for the chemically bondM/sub J/chemically bond = 1 orientation can be understood qualitatively from the model potentials of Hennecart and Masnou-Seeuws by the strong coupling to the )..cap alpha..)/sub 4/ and )..cap alpha..)/sub 6/ states (avoided crossings), which is absent for the ..cap omega.. = chemically bondM/sub J/chemically bond = 0/sup -/ molecular potentials because of the constraint of reflection symmetry.

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

Abstract 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 A2 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 3Q⩽Qorb = 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).

Polarization Effects in Collision-Induced Intramultiplet Mixing forNe**{(2p)5(3p)}+He

High-quality polarized-emission cross sections for the )..cap alpha..)/sub 5/ = chemically bondJ = 1,M/sub J/>..-->..)..cap alpha..)/sub 7/ and )..cap alpha..)/sub 5/..-->..)..cap alpha..)/sub 4/ transitions in the )..cap alpha..) = )(2p)/sup 5/(3p)) multiplet (lifetime 20 ns) have been measured in a crossed-beam experiment. For the )..cap alpha..)/sub 5/..-->..)..cap alpha..)/sub 7/ transition we observe a strong preference for the chemically bondM/sub J/chemically bond = 0 orientation. The small cross section for the chemically bondM/sub J/chemically bond = 1 orientation can be understood qualitatively from the model potentials of Hennecart and Masnou-Seeuws by the strong coupling to the )..cap alpha..)/sub 4/ and )..cap alpha..)/sub 6/ states (avoided crossings), which is absent for the ..cap omega.. = chemically bondM/sub J/chemically bond = 0/sup -/ molecular potentials because of the constraint of reflection symmetry.