Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session OR4: Electron-Impact Ionization |
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Chair: Allison Harris, Illinois State University Room: 303 AB |
Thursday, October 15, 2015 10:00AM - 10:30AM |
OR4.00001: Progress in (e, 2e) electron momentum spectroscopy: from the static to the time-resolved regime Invited Speaker: Masahiko Takahashi Electron momentum spectroscopy (EMS) is a kinematically-complete electron-impact ionization experiment performed under the high-energy Bethe ridge conditions, where the collision kinematics can be described by electron Compton scattering that most nearly corresponds to the collision of two free electrons with the residual ion acting as a spectator. The remarkable feature of this technique is its ability to measure momentum distributions of each electron bound in matter or to look at molecular orbitals in momentum space. We have been exploring atomic and molecular science using EMS, such as 3D orbital imaging for a stable gaseous molecule [Takahashi et al., PRL 2005], observation of the giant resonance phenomenon in the 2nd order projectile-target interactions [Takahashi et al., PRL 2007], and determination of spatial orientation of the constituent atomic orbitals in molecular orbitals [Watanabe et al., PRL 2012]. Recently, we have started to direct our efforts also towards expanding frontiers of EMS, through development of time-resolved EMS (TR-EMS) that employs ultrashort laser (120 fs) and electron (1 ps) pulses in a pump-probe scheme [Yamazaki et al., RSI 2013]. In spite of the low data statistics as well as of the limited time-resolution due to velocity mismatch, our experimental results on the deuterated acetone molecule in its second excited singlet state with a lifetime of 13.5 ps [Yamazaki et al., PRL 2015] have represented the first time that EMS measurements of short lived transient species are feasible, opening the door to time-resolved orbital imaging in momentum space. With further technical development, TR-EMS could eventually enable one to take a series of snapshots of molecular orbitals changing rapidly during chemical reaction, thereby making it possible to exploit a new area for studies of ultrafast molecular dynamics as well as the nature of molecular excited states; it is electrons that bind atoms into molecules, and chemical reactions are all about the rearrangement of these electrons or the change in spatial patterns of the corresponding molecular orbitals. In this contribution, some results of our recent studies will be presented, which may examine the current status and future prospects of EMS. [Preview Abstract] |
Thursday, October 15, 2015 10:30AM - 11:00AM |
OR4.00002: Accuracy of Theoretical Calculations for Electron-Impact Ionization of atoms and Molecules Invited Speaker: Don Madison In the last two decades, there have been several close-coupling approaches developed which can accurately calculate the triply differential cross sections for electron impact ionization of effective one and two electron atoms. The agreement between experiment and theory is not particularly good for more complicated atoms and molecules. Very recently, a B-spline R-matrix with pseudostates (BSRPS) approach was used to investigate low energy electron impact ionization of neon and very good agreement with experiment was found. The perturbative 3-body distorted wave (3DW) approach which includes the exact final state electron-electron interaction (post collision interaction - PCI) gave comparably good agreement with experiment. For ionization of molecules, there have been numerous studies of high-energy electron impact. These studies are called EMS (Electron Momentum Spectroscopy) and they were very valuable in determining the accuracy of molecular wavefunctions since the measured cross sections were proportional to the momentum space molecular wavefunction. More recently, lower energy collisions have started to be measured and these cross sections are much more difficult for theory since the detailed kinematics of the experiment become important. So far, the only close coupling calculation reported for ionization of molecules is the time-dependent close-coupling calculation (TDCC) which has been developed for ionization of H2 and it yields relative good agreement with experiment. Again the molecular 3-body distorted wave (M3DW) gave equally good agreement with experiment. For polyatomic molecules, the only theory available is the M3DW. In this talk, I will show the current status of agreement between experiment and theory for low and intermediate energy single ionization of atoms and molecules. [Preview Abstract] |
Thursday, October 15, 2015 11:00AM - 11:15AM |
OR4.00003: B-spline R-matrix with pseudo-states calculations for electron-impact excitation and ionization of beryllium Oleg Zatsarinny, Klaus Bartschat The \hbox{B-spline} R-matrix with Pseudo-States (BSRMPS) method [1,2] is employed to treat electron collisions with beryllium atoms. Results for elastic scattering, excitation, and ionization were obtained for all transitions between the lowest 19 states of beryllium in the energy range from threshold to 150~eV. The sensitivity of the predictions is checked by comparing results obtained in different approximations with increasing number of coupled states. The dataset generated from the largest model, \hbox{coupling} over 600 physical and pseudo-states, is believed to be accurate to within a few percent for the cross sections of relevance for plasma modelling.\\[4pt] [1] O.\ Zatsarinny, Comp. Phys. Commun.~{\bf 174} (2006) 273.\\[0pt] [2] O.\ Zatsarinny and K.\ Bartschat, J.\ Phys.\ B {\bf 47} (2014) 061001. [Preview Abstract] |
Thursday, October 15, 2015 11:15AM - 11:30AM |
OR4.00004: Non-perturbative B-spline R-matrix with pseudo-states calculations for electron-impact excitation-ionization of helium to the n = 3 states of He$^+$ Klaus Bartschat, Oleg Zatsarinny We present fully-differential cross sections for electron-impact ionization plus simultaneous excitation of helium obtained from a non-perturbative close-coupling formalism with our B-spline R-matrix approach [1,2]. Using a large number of pseudo-states we obtain excellent agreement with directly measured cross-section ratios [3,4] for ionization leaving the residual He$^+$ ion in either the $1s$ ground state, the $n = 2$ ($2s + 2p$) excited states, or the $n = 3$ \hbox{($3s + 3p + 3d$)} excited states.\\[4pt] [1]~O.\ Zatsarinny and \hbox{K.\ Bartschat}, \hbox{Phys.\ Rev.\ Lett.\ {\bf 107}} (2011) 023203.\\[0pt] [2]~\hbox{O.\ Zatsarinny} and \hbox{K.\ Bartschat}, \hbox{J.\ Phys.\ B {\bf 47}} (2014) 061001.\\[0pt] [3]~S.~Bellm {\it et al.}, \hbox{Phys.\ Rev.\ A {\bf 75}} (2007) 042704.\\[0pt] [4]~S.~Bellm {\it et al.}, \hbox{Phys.\ Rev.\ A {\bf 78}} (2008) 032710. [Preview Abstract] |
Thursday, October 15, 2015 11:30AM - 11:45AM |
OR4.00005: Experimental and Theoretical Fully differential cross sections for electron impact ionization of furfuryl molecules Esam Ali, Darryl Jones, Kate Nixon, Chuangang Ning, Michael Brunger, Andrew Murray, Don Madison Experimental and theoretical Fully Differential Cross Sections (FDCS) are presented for 250 eV electron impact ionization of the highest and next highest occupied molecular orbitals (HOMO and NHOMO). Theoretical results are compared with experiment for in plane scattering with projectile scattering angles of $5^{\circ}$, $10^{\circ}$, and $15^{\circ}$. Different theoretical models are examined - the molecular 3 body distorted wave (M3DW), and the distorted wave Born approximation (DWBA), with the effects of the post collision interaction (PCI) treated either exactly or with the Ward-Macek approximations. These approximations show good agreement with experimental data for binary peaks. However, for the recoil peak region, experiment finds a noticeable peak while theory predicts no peak. No recoil peak suggests no (or very weak) nuclear scattering, so we have investigated the importance of nuclear scattering by moving the nuclei closer to the center of mass. [Preview Abstract] |
Thursday, October 15, 2015 11:45AM - 12:00PM |
OR4.00006: Two-center interference effects in (e, 2e) ionization of H$_{2}$ and CO$_{2}$ at large momentum transfer Masakazu Yamazaki, Isao Nakajima, Hironori Satoh, Noboru Watanabe, Darryl Jones, Masahiko Takahashi In recent years, there has been considerable interest in understanding quantum mechanical interference effects in molecular ionization. Since this interference appears as a consequence of coherent electron emission from the different molecular centers, it should depend strongly on the nature of the ionized molecular orbital. Such molecular orbital patterns can be investigated by means of binary (e, 2e) spectroscopy, which is a kinematically-complete electron-impact ionization experiment performed under the high-energy Bethe ridge conditions [1]. In this study, two-center interference effects in the (e, 2e) cross sections of H$_{2}$ [2] and CO$_{2}$ at large momentum transfer are demonstrated with a high-statistics experiment, in order to elucidate the relationship between molecular orbital patterns and the interference structure. It is shown that the two-center interference is highly sensitive to the phase, spatial pattern, symmetry of constituent atomic orbital, and chemical bonding nature of the molecular orbital. \\[4pt] [1] M. Takahashi, Bull. Chem. Soc. Jpn. \textbf{82}, 751 (2009).\\[0pt] [2] M. Yamazaki \textit{et al}., Phys. Rev. A \textbf{90}, 052711 (2014). [Preview Abstract] |
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