Intermolecular potentials for the metastable neon*-rare gas and neon*-molecule systems

The absolute value and the velocity dependence of the total cross section Q(g) has been measured in a crossed beam machine for the Ne*-Ar, Kr, Xe and Ne*-O2, N2, CH2 and CO2 systems, using a mixed beam containing Ne*(3P(0) and Ne * (3P2) fine structure states in a 1:5 ratio. The range of velocities is typically 1000 g 8000 m s-1, always including the interesting N = 1 glory oscillation. The results for the Ne* -rare gas systems are in excellent agreement with the predictions of the ion-atom Morse-Morse-spline-van der Waals potentials of Gregor and Siska, both with regard to the absolute value (1.5%), position of the N = 1 glory maximum (2.7%) and the amplitude of the N = 1 glory maximum (4.3%). The predictions of the potentials proposed by Hausamann are less satisfactory, most likely due to the specific switchover function used to connect the well area at R/RM ˜ 1.1 to the van der Waals long-range attractive branch at R/RM ˜ 2 (RM is the well position). By using a semiclassical scaling method the potential parameters e (well depth), RM (well position) and C6 (van der Waals constant) have been determined for the Ne*-molecule systems, using the Gregor and Siska IAMMSV potential for the Ne*-Xe system as a reference. The well parameters are (e (meV), RM (Å)) = (3.21, 5.43), (4.24, 5.17), (13.55, 4.74) and (7.08, 5.44) for the Ne*-N2, O2, CO2 and CH4, systems, respectively. For the C6 values we observe a fair scaling with the polarisibility a of the molecule. For the Ne*-CO2 system we observe a damping of the amplitude of the glory oscillations, which increases rapidly with decreasing velocity. This damping is interpreted in terms of the probability for ionisation along the glory trajectory, providing useful information for determining a complex potential for this system

Polarization effects in the ionization cross section of Ar, Kr, and Xe by laser-excited Ne**[(2p)5(3p);J = 3, M] atoms

In a crossed-beam experiment the total ionization cross section for the title systems has been investigated in the range 0.1¿E (eV) ¿4 of collision energies. The population of the short-lived Ne**[(3p);J=3] state is produced by saturated optical pumping of the Ne*[(3s);J=2]¿Ne**[(3p);J=3] two-level system with a polarized laser beam, resulting in a well-determined distribution of the magnetic substrates |J,M> with respect to the relative velocity g. By measuring the ion yield in the scattering center at five different orientations of the laser polarization (linear and circular) with respect to g, the data can be analyzed in terms of pure-state total ionization cross sections Q3|M| corresponding to a single asymptotic state |J,M>. The observed polarization effect at E=0.1 eV is Q3|M|=0,1/Q3|M|=3=2.5, which is in good agreement with the data of Bussert, T. Bregel, R. J. Allan, M. W. Ruf, and H. Hotop [Z. Phys. A 320, 105 (1985)] in the thermal energy range as obtained by analyzing the Penning electrons. This polarization effect decreases to a value of 1.4 for E>2 eV. The results are discussed in terms of semiclassical scattering calculations with an optical potential as input, using a model-potential approach for calculating both the real and the imaginary parts. For the autoionization width this results in a two-state Gs' and Gp' basis for the s' and p' orientations of the (2p)-1 hole, calculated in a one-electron orbital overlap approximation. The preference for the O=0,1 states at E=0.1 eV indicates the correct relative scaling of these two ionization widths, leading to Gs'=79Gp' at R=4.5a0. The observed energy dependence is due to the decrease of "locking" of the total angular momentum J to the internuclear axis R with increasing angular velocity f¿, leading to the dynamical criterion ¿prec=4f¿ for the transition of a space-fixed to a body-fixed description of J. The semiclassical precession frequency ¿prec of J around R is related to the average O splitting of the real part of the optical potential by ¿prec=/h. With these assumptions we observe a good agreement between the experimental results and the semiclassical calculations. Finally, we discuss the validity of a semiclassical locking picture, with emphasis on the difference between locking of the angular momentum versus locking of the electron orbitals involved

Polarization effects in the ionization cross section for collisions of excited Ne**{(2p)5(3p); J = 3}) with Ar : a sensitive probe for "locking" phenomena

At a collision energy E=0.1 eV we see a large polarization effect Qion*Mj*=0,1/QionMj*=3=2.5, decreasing to 1.4 for E=1 eV. A two-state basis is used for the autoionization width of the p and s orientations of the (2p)-1 core hole, resulting in a preference for the O=0,1 molecular states as seen at 0.1 eV. The energy dependence is due to the decrease of ‘‘locking’’ of J to the internuclear axis with increasing angular velocity f¿, leading to the semiclassical criterion ¿prec=4f¿ for the transition from a space-fixed to a body-fixed description of J

Polarization effects in the ionization cross section of Ar, Kr, and Xe by laser-excited Ne**[(2p)5(3p);J = 3, M] atoms

In a crossed-beam experiment the total ionization cross section for the title systems has been investigated in the range 0.1¿E (eV) ¿4 of collision energies. The population of the short-lived Ne**[(3p);J=3] state is produced by saturated optical pumping of the Ne*[(3s);J=2]¿Ne**[(3p);J=3] two-level system with a polarized laser beam, resulting in a well-determined distribution of the magnetic substrates |J,M> with respect to the relative velocity g. By measuring the ion yield in the scattering center at five different orientations of the laser polarization (linear and circular) with respect to g, the data can be analyzed in terms of pure-state total ionization cross sections Q3|M| corresponding to a single asymptotic state |J,M>. The observed polarization effect at E=0.1 eV is Q3|M|=0,1/Q3|M|=3=2.5, which is in good agreement with the data of Bussert, T. Bregel, R. J. Allan, M. W. Ruf, and H. Hotop [Z. Phys. A 320, 105 (1985)] in the thermal energy range as obtained by analyzing the Penning electrons. This polarization effect decreases to a value of 1.4 for E>2 eV. The results are discussed in terms of semiclassical scattering calculations with an optical potential as input, using a model-potential approach for calculating both the real and the imaginary parts. For the autoionization width this results in a two-state Gs' and Gp' basis for the s' and p' orientations of the (2p)-1 hole, calculated in a one-electron orbital overlap approximation. The preference for the O=0,1 states at E=0.1 eV indicates the correct relative scaling of these two ionization widths, leading to Gs'=79Gp' at R=4.5a0. The observed energy dependence is due to the decrease of "locking" of the total angular momentum J to the internuclear axis R with increasing angular velocity f¿, leading to the dynamical criterion ¿prec=4f¿ for the transition of a space-fixed to a body-fixed description of J. The semiclassical precession frequency ¿prec of J around R is related to the average O splitting of the real part of the optical potential by ¿prec=/h. With these assumptions we observe a good agreement between the experimental results and the semiclassical calculations. Finally, we discuss the validity of a semiclassical locking picture, with emphasis on the difference between locking of the angular momentum versus locking of the electron orbitals involved

Polarization effects in the ionization cross section for collisions of Ne**{(2p)5(3p);J = 3} with Ar : a sensitive probe for "locking" phenomena [Errata, Phys.Rev.Lett., 62,2369(1989)]

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