Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session TR2: Electron and Photon Interactions with Atoms and Molecules |
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Chair: Klaus Bartschat, Drake University Room: Saratoga Hilton Ballroom 2 |
Thursday, October 22, 2009 1:30PM - 2:00PM |
TR2.00001: Laser--assisted electron impact autoionization of Helium Invited Speaker: We have begun to perform the first electron impact autoionization experiments in the presence of a laser field. Laser assisted electron collisions are a poorly understood process. A theoretical discussion of electron impact ionization in the presence of a laser field was published as long ago as 1988,\footnote{C. J. Joachain et al., Phys. Rev. Lett. 61, 165 (1988)} but the first experimental results were only published in 2005.\footnote{C. H\"{o}hr {\em et al.} Phys. Rev. Lett. 94, 153201 (2005)} Distinct differences in the ionization process between the field free (laser off) and field assisted (laser on) cases were found. The results were in poor agreement with quantum calculations but could (oddly enough) be explained with a simple classical model. Our experiments have the advantage that autoionizing levels provides energy markers: the absorption or emission of a photon is expected to shift the resonance position in the ejected electron spectrum by the photon energy. It is easier to detect the presence of such sideband peaks than it is to observe quantitative shifts in the energy onset of traditional simultaneous electron-photon excitation (SEPE) experiments. We will provide details of our experimental apparatus, and provide a progress report on these experiments. [Preview Abstract] |
Thursday, October 22, 2009 2:00PM - 2:15PM |
TR2.00002: Super-elastic electron excitation of atoms in magnetic {\&} optical fields Andrew Murray, William MacGillivray, Martyn Hussey Understanding electron impact excitation of atoms usually requires coincidence techniques to fully ascertain the interactions that occur, {\&} to allow detailed comparison to collision theories. These experiments are slow, but have advantages since many targets can be studied. By contrast, the `time reversed' super-elastic scattering experiment produces equivalent information thousands of times faster with higher precision, but is limited to only a few targets that can be excited by tunable lasers. Here two new techniques invented in Manchester are discussed. The first allows the cross sections to be determined over all angles using a Magnetic Angle Changer to steer electrons to {\&} from the target [1-3]. The second adopts a resonant optical cavity around the interaction region to efficiently excite a much wider range of targets. We can then for the first time study targets of relevance to industry, including Zn (which may provide an alternative to Hg in low energy lighting), through to Au {\&} Ag. \\[4pt] [1] M Hussey et al Phys Rev Lett 99 133202 (2007)\\[0pt] [2] A J Murray et al Phys Rev A 77 013409 (2008)\\[0pt] [3] M Hussey et al J Phys B 41 055202 (2008) [Preview Abstract] |
Thursday, October 22, 2009 2:15PM - 2:30PM |
TR2.00003: (e,2e) ionization studies of diatomic {\&} triatomic molecules Kate Nixon, Andrew Murray, Christian Kaiser, Ola Al-Hagan, James Colgan, Don Madison (e,2e) studies yield the most detailed experimental data on electron impact ionization of atomic {\&} molecular targets for comparison to quantum collision theories. Coincidence techniques are here used to measure the probability of ionization as a function of the incident electron scattering angle and angle of the electron ejected from the target. In Manchester we study this process at low energies, where the ionization probability is greatest {\&} the interaction most complex. We recently considered ionization of simple molecules (eg H$_{2}$ {\&} H$_{2}$O) from a coplanar geometry to the perpendicular plane[1-4], and have discovered the interaction is far more complex than for ionization of atoms [5]. We here present comparisons between theory {\&} experiment, and discuss new methods we intend to implement to study ionization from laser-aligned atoms {\&} molecules. References. [1] J Colgan et al Phys Rev Lett 101 233201 (2008) [2] O Al-Hagan et al Nature Physics 5 59 (2009) [3] J Colgan et al Phys Rev A 79 052704 (2009) [4] C Kaiser et al J Phys B 40 2563 (2007) [5] A J Murray et al J Phys B 36 4875 (2003) {\&} references therein [Preview Abstract] |
Thursday, October 22, 2009 2:30PM - 2:45PM |
TR2.00004: Electron Impact Ionization Cross Sections of H$_{2}$~for Low Excess Energies Ranging from 2eV to 20eV Ola Al-Hagan, Don Madison, James Colgan, Christian Kaiser, Andrew Murray We had recently investigated the nuclear structure effect for the fully differential electron impact ionization cross sections of H$_{2}$ and He measured in the perpendicular plan where the two outgoing electrons have equal energies of 10 eV (20eV excess energy). For this case we demonstrated that He had a maximum for back-to-back scattering and H$_{2}$ had a minimum due to the different nuclear configurations. We have extended our investigation for H$_{2}$ to lower excess energies down to 2eV (both final state electrons have 1eV energy). We will show that, as the excess energy decreases, the effective impact parameter increases and molecular cross sections start to look like the atomic ones as the nuclear separation becomes less important. [Preview Abstract] |
Thursday, October 22, 2009 2:45PM - 3:00PM |
TR2.00005: Accuracy of the Gamow Factor for Approximating the Post Collision Interaction (PCI) in Electron-Impact Ionization of Atoms Adam Upshaw, Hari Saha, Don Madison Recently Kheifets et al. [1] reported a distorted wave Born (DWBA) calculation for ionization of helium, neon and argon where the post collision interaction (PCI) between the two final state electrons was approximated using the Gamow-factor (called the G-factor). For cases where there was a large difference between experiment and theory at the recoil peak, the G-factor significantly improved agreement between experiment and theory. The G-factor is an approximation for including the final state Coulomb interaction between the two continuum electrons in the final state wavefunction. The 3DW (three-body-distorted-wave) method properly includes the Coulomb interaction in the final state wavefunction without approximation. The G-factor approximation is attractive due to its computational simplicity (i.e. the effects of PCI can be determined by multiplying DWBA results by the Gamow factor). 3DW calculations, on the other hand, require a full numerical 6-dimensional integration. We will examine the accuracy of the G-factor approximation by comparing 3DW results with G-factor results for ionization of neon, argon and xenon for low, intermediate and high energy. [1] Kheifets et al., J. Phys. B, 41, 145201 (2008). [Preview Abstract] |
Thursday, October 22, 2009 3:00PM - 3:15PM |
TR2.00006: Electron scattering {\&} high resolution spectroscopy from cold atoms in an AC-MOT Andrew Murray, Matthew Harvey A new revolution in atomic physics has arisen due to the ability to precisely control the external motion of atoms using laser forces. By using high resolution continuous wave lasers it is possible to reduce {\&} compress the momentum of an atomic beam, {\&} to trap {\&} cool these atoms to produce a high density cloud of cold atoms in a magneto optical trap (MOT). The temperature of the atoms can be further reduced to sub-micro-Kelvin levels using a variety of techniques, so as to form a Bose Einstein Condensate (BEC). Here we discuss a new type of atom trap (the AC-MOT) which allows the magnetic fields produced by the trap to be switched off $>$300 times faster than with a conventional MOT [1]. Electron scattering {\&} high resolution laser experiments can then be performed from cold atoms with virtually no loss (apart from the normal processes associated with background gases). Results from potassium ionization will be presented, together with a discussion of new types of precision scattering experiments that can be performed using these cold targets. \\[4pt] [1] M Harvey and A J Murray Phys Rev Lett 101 173201 (2008) [Preview Abstract] |
Thursday, October 22, 2009 3:15PM - 3:30PM |
TR2.00007: Production of Excited Atomic Hydrogen and Deuterium from H$_{2}$, D$_{2}$ and HD Photodissociation J.R. Machacek, V.M. Andrianarijaona, J.E. Furst, T.J. Gay, A.L.D. Kilcoyne, A.L. Landers, K.W. McLaughlin We have measured the production of Ly$\alpha $ and H$\alpha $ fluorescence from atomic H and D resulting from the photodissociation of H$_{2}$, D$_{2}$ and HD by linearly-polarized photons with energies between 20 and 65 eV. In this energy range, excited photofragments result primarily from the production of doubly-excited molecular species which promptly autoionize or dissociate into two neutrals. Comparison between the relative cross sections of H$_{2}$ and D$_{2}$ and the available theory show only qualitative agreement. We will discuss the various systematic effects which affect this and other types of synchrotron-based measurements in this energy range. Support provided by the NSF (Grant PHY-0653379), DOE (LBNL/ALS) and ANSTO (Access to Major Research Facilities Programme). [Preview Abstract] |
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