• Comparison of experiment and theory for superelastic electron collision studies from laser aligned magnesium

      Pursehouse, James; Bostock, Chris; Nixon, Kate; Harvey, Matthew; Fursa, Dmitry V; Bray, Igor; Murray, Andrew James (APS, 2018-08-09)
      A combined experimental and theoretical study of superelastic electron collisions from laser-aligned magnesium atoms for a range of collision energies from 35eV to 55eV is presented. 24Mg atoms were excited from the 31S0 ground state to the 31P1 excited state using continuous-wave linearly-polarized laser radiation at ~285 nm. Electrons of well-defined energy Einc then de-excited the targets, and the superelastically scattered electrons emerging from the collision were detected as a function of scattering angle and laser polarization. Results for alignment of the target by the electron beam are presented for a range of scattering angles, for outgoing energies from Eout = 35eV to 55eV . The agreement between the measurements and the results of the convergent close-coupling theory are encouraging, but some discrepancies remain.
    • Comparison of experimental and theoretical electron-impact-ionization triple-differential cross sections for ethane

      Ali, Esam; Nixon, Kate; Murray, Andrew; Ning, Chuangang; Colgan, James; Madison, Don (American Physical Society, 2015-10-30)
      We have recently examined electron-impact ionization of molecules that have one large atom at the center, surrounded by H nuclei ( H 2 O , N H 3 , C H 4 ). All of these molecules have ten electrons; however, they vary in their molecular symmetry. We found that the triple-differential cross sections (TDCSs) for the highest occupied molecular orbitals (HOMOs) were similar, as was the character of the HOMO orbitals which had a p -type “peanut” shape. In this work, we examine ethane ( C 2 H 6 ) which is a molecule that has two large atoms surrounded by H nuclei, so that its HOMO has a double-peanut shape. The experiment was performed using a coplanar symmetric geometry (equal final-state energies and angles). We find the TDCS for ethane is similar to the single-center molecules at higher energies, and is similar to a diatomic molecule at lower energies.
    • Evidence for unnatural-parity contributions to electron-impact ionization of laser-aligned atoms

      Armstrong, G. S. J.; Colgan, J.; Pindzola, M. S.; Amami, S.; Madison, D. H.; Pursehouse, J.; Nixon, K. L.; Murray, A. J. (American Physical Society, 2015-09-11)
      Recent measurements have examined the electron-impact ionization of excited-state laser-aligned Mg atoms. In this work we show that the ionization cross section arising from the geometry where the aligned atom is perpendicular to the scattering plane directly probes the unnatural parity contributions to the ionization amplitude. The contributions from natural parity partial waves cancel exactly in this geometry. Our calculations resolve the discrepancy between the nonzero measured cross sections in this plane and the zero cross section predicted by distorted-wave approaches. We demonstrate that this is a general feature of ionization from p-state targets by additional studies of ionization from excited Ca and Na atoms.
    • Resonant excitation of acoustic waves in one-dimensional exciton-polariton systems

      Yulin, AV; Kozin, VK; Nalitov, AV; Shelykh, IA (American Physical Society, 2019-10-18)
      We analyze the interaction of exciton-polariton condensates in a one-dimensional semiconductor microcavity with acoustic phonons. We consider the case of a coherently pumped condensate and demonstrate that upon passing of a certain threshold the parametric instability in the system leads to the generation of a coherent acoustic wave and additional polariton harmonics. The process is strongly affected by exciton-exciton interactions which lead to the appearance of the effects of the bistability and hysteresis in the system.
    • Theoretical and experimental (e,2e) study of electronimpact ionization of laser-aligned Mg atoms

      Amami, Sadek; Murray, Andrew; Stauffer, Al; Nixon, Kate; Armstrong, Gregory; Colgan, James; Madison, Don (2014-12-08)
      We have performed calculations of the fully differential cross sections for electron-impact ionization of magnesium atoms. Three theoretical approximations, the time-dependent close coupling, the three-body distorted wave, and the distorted wave Born approximation, are compared with experiment in this article. Results will be shown for ionization of the 3 s ground state of Mg for both asymmetric and symmetric coplanar geometries. Results will also be shown for ionization of the 3p state which has been excited by a linearly polarized laser which produces a charge cloud aligned perpendicular to the laser beam direction and parallel to the linear polarization. Theoretical and experimental results will be compared for several different alignment angles, both in the scattering plane as well as in the plane perpendicular to the incident beam direction.