Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session O6: Electron-Molecule Collisions |
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Chair: William McCurdy, University of California, Davis Room: Nittany Lion Inn Boardroom II |
Friday, May 30, 2008 8:00AM - 8:12AM |
O6.00001: Probing the nonlocal resonance model of electron collisions with diatomic molecules Karel Houfek, Premysl Kolorenc, Michal Tarana, Thomas N. Rescigno, C. William McCurdy The nonlocal resonance model [W. Domcke, Phys. Rep. 208, 97 (1991)] to nuclear dynamics of low-energy resonant collisions of electrons with molecules was successfully applied to many diatomics. We show how underlying assumptions of this theory can be tested using a simple model of the electron-molecule collisions with one electronic and one nuclear degree of freedom, which was introduced by the authors [Phys. Rev. A 73, 032721 (2006)] and which can be solved numerically exactly without the Born-Oppenheimer approximation. The basic result of our investigation is that the ``background'' contribution to the cross sections can be significant even for inelastic collisions such as vibrational excitation of the molecule F2. Moreover the Fano-Feshbach R-matrix theory [B.M. Nestmann, J. Phys. B 31, 3929 (1998)] which was proposed to provide parameters of the nonlocal resonance model has been recently tested within our model and its drawbacks are discussed. [Preview Abstract] |
Friday, May 30, 2008 8:12AM - 8:24AM |
O6.00002: Multichannel quantum defect studies of e$^-$ + LiH$_2^+$ and e$^-$ + NO$_2^+$ Daniel Haxton, Chris Greene We present the results of calculations on electron-molecule scattering involving the molecules LiH$_2^+$ and NO$_2^+$. We focus on dissociative recombination (DR), vibrational excitation, and vibrational autoionization. Our treatment involves the \textit{ab initio} calculation of quantum defect matrices using the UK R-matrix codes of Tennyson, Morgan, and co-workers. We employ rovibrational frame-transformation techniques of multichannel quantum defect theory to calculate cross sections and rates for these processes. In the case of the LiH$_2^+$ system, we calculate the DR rate using an exact Born-Oppenheimer rovibrational Hamiltonian. We provide comparisons with recent experiment. In the case of NO$_2^+$, we analyze the effect of several Feshbach resonances upon vibrational excitation and autoionization. Our MQDT treatment of this system incorporates these doubly excited valence states of neutral NO$_2$ and the valence-rydberg coupling that is known to be significant in this system. We compare our results with prior theory and with the experimental results of Grant. [Preview Abstract] |
Friday, May 30, 2008 8:24AM - 8:36AM |
O6.00003: Two- and three-body breakup in dissociative electron attachment to water D.J. Haxton, T.N. Rescigno, C.W. McCurdy We present calculations on two- and three-body breakup in dissociative electron attachment (DEA) to water, \[ e^- + H_2O \longrightarrow H^- + OH, H_2 + O^-, ... \] This process is mediated by three metastable electronic states (Feshbach resonances) of H$_2$O$^-$ which are coupled by a conical intersection and by Renner-Teller coupling. We define complex-valued potential energy curves using \textit{ab initio} scattering and bound-state calculations. We use these coupled curves in calculations of the time-dependent nuclear dynamics using the Multi Configuration Time Dependent Hartree (MCTDH) approach. For DEA via the higher $^2$A$_1$ and $^2$B$_2$ Feshbach resonances, the three body channels are open. We discriminate between two- and three-body breakup in this system by performing calculations in both Jacobi and hyperspherical coordinate systems. We provide strong evidence that the observed O$^-$ production from the $^2$A$_1$ resonance state is exclusively due to three-body breakup. For DEA via the highest $^2$B$_2$ state, our treatment of the dynamics is necessarily imperfect, but we achieve good agreement with experiment in certain respects. We explain how the conical intersection plays a crucial role in the nuclear dynamics. [Preview Abstract] |
Friday, May 30, 2008 8:36AM - 8:48AM |
O6.00004: Elastic electron scattering from water John Muse, Helen Silva, Maria Cristina Lopes, Murtadha A. Khakoo Accurate measurements of differential cross-sections for elastic electron scattering from water will be presented. The data were taken using a modified form of the relative flow method with an aperture gas source replacing conventional tube source [1]. In addition the source was moveable [2] to accurately determine the background scattering. Data were taken at incident energies of 1eV to 100eV and scattering angles of 10 to 130 deg. The results show that present recommended cross-sections for water should be changed. [1] M. A. Khakoo et al., Journal of Physics B, \underline {40}, 3601 (2007). [2] M. Hughes et al., Meas. Sci. Technol. 14, 841 (2003). [Preview Abstract] |
Friday, May 30, 2008 8:48AM - 9:00AM |
O6.00005: Dissociative Electron Attachment of Water Molecules Ali Belkacem, Hidehito Adaniya, Timur Osipov, Thorsten Weber, Sun Y. Lee, Marcus Hertlein, Benedikt Rudek A Coltrims method is developed to measure the kinetic energy and angular distribution of fragment negative ions arising from dissociative electron attachment of molecules. A low energy pulsed electron gun is used in combination with pulsing the extraction plates of the Coltrims spectrometer. This technique is applied to study the negative oxygen anion channel for the three resonances, $^{2}$B$_{1}$, $^{2}$A$_{1}$, and $^{2}$B$_{2}$ resonances of water. The measured kinetic energy of the O- fragment gives a good measure of the two-body channel versus three-body channel for each resonance. The angular distribution of the O- fragments with respect tom the electron beam direction is found reflect well the symmetry of the resonance state. The experimental results are compared to the theoretical predictions. [Preview Abstract] |
Friday, May 30, 2008 9:00AM - 9:12AM |
O6.00006: Theoretical description of dissociative recombination of HCO$^{+}$ Nicolas Douguet, Viatcheslav Kokoouline, Chris H. Greene Theoretical description of dissociative recombination (DR) of triatomic molecular ions is a difficult problem because several different (electronic and vibrational) degrees of freedom have to be taken into account at once. Our recent theoretical calculations of the DR cross-section of simple polyatomic ions like H$_{3}^{+}$, exhibited good agreement with experimental measurements. On the other hand, the previous theoretical cross-section for more sophisticated molecular ion, HCO$^{+}$, still diverges significantly from the experimental data. In this presentation, we describe our improved theoretical approach for calculation of the DR cross-section in HCO$^{+}$ and DCO$^{+}$ ions. Comparing with the previous theoretical study (Phys. Rev. A 74, 032707 (2006)) we took into account all vibrationally degrees of freedom. The Renner-Teller coupling between electronic and vibrational motion is also accounted for. The obtained theoretical cross-section is in better agreement with experiment, especially in the region of relatively high ($>$0.1 eV) collision energy. [Preview Abstract] |
Friday, May 30, 2008 9:12AM - 9:24AM |
O6.00007: Time-dependent quantum defect theory for dissociative recombination of diatomic molecules Samantha Santos, Viatcheslav Kokoouline, Chris Greene Dissociative recombination (DR) of molecular ions with electrons takes place on atomic quantum dynamics level with the incoming electron being captured by the field of a molecular cation transferring its energy to the nuclear motion. In its descending route, the electron goes through a series of Rydberg states until the vibration of the nuclei couples one of these resonances to a pre-dissociative state of the molecule. The DR process ends with the fragmentation of the molecule. Most studies on DR have been conducted with different diatomic or triatomic molecular ions in the time-independent framework of multi-channel quantum defect theory (MQDT). The present work uses MQDT to monitor time-dependent evolution of the DR process. The obtained results provides time-dependent picture of how a localized wave packet of incoming electrons transfers its energy to the vibration of the molecule through the Rydberg states. The wave packet is partially reflected back to the ionization channel. A model diatomic system has been used so far, and vibrational dynamics is described by Siegert pseudo-states. [Preview Abstract] |
Friday, May 30, 2008 9:24AM - 9:36AM |
O6.00008: Resonant Dissociative Recombination of ArH$^+$ V. Ngassam, A.E. Orel The rate coefficient for the ArH$^+$ dissociative recombination with electrons has been measured at the ASTRID storage ring in Denmark. The rate coefficient, as a function of the electron energy, displays several broad peaks between 5 and 35 eV. The first peak is due to dissociative recombination via the capture of the incoming electron by doubly excited Rydberg states converging to the excited state of the ion (Ar$^+$ $^2$P + H $^1$S). We will present the results of electron scattering calculations using the Complex Kohn variational method which were carried out to determine the positions and autoionization widths of the resonant states. These parameters were used in a time-dependent wave packet calculation to describe the dissociation dynamics. We also explore the low energy region where the non-adiabatic couplings between the ion, the Rydberg and the valence states drive dissociative recombination. The cross sections and dissociation rates will be compared to the available experimental data. [Preview Abstract] |
Friday, May 30, 2008 9:36AM - 9:48AM |
O6.00009: Dissociative Electron Attachment to HCCCN S.T. Chourou, A.E. Orel Experiments on dissociative electron attachment (DEA) to HCCCN below 12 eV have led predominantly to formation of CCCN$^-$, CN$^-$, HCC$^-$ and CC$^-$ negative ions. It has been concluded that these fragments result mainly from the decay of $\pi$*- shape resonant state upon electron attachment that involves distortion of the symmetry of the linear neutral molecule. In order to study the dynamics of dissociation in these channels, we subdivided the molecule into three fragments (H), (CC) and (CN); therefore, three internal coordinates consisting in the distances between the center of masses (H) and (CC) fragments, (CC) and (CN) fragments and the (H)-(CC)-(CN) angle are included in the calculation. We have performed electron scattering calculations using Complex Kohn Variational method to determine the resonance energies and autoionization width for various geometries of the system and construct the complex potential energy surfaces relevant to the metastable HCCCN$^{-*} $ ion. The nuclear dynamics is treated using the Multiconfiguration Time-Dependent Hartree (MCTDH) formalism and the flux of the propagating wavepacket is used to compute the DEA cross section relevant to 4 channels in question. The results are then compared to the available findings. [Preview Abstract] |
Friday, May 30, 2008 9:48AM - 10:00AM |
O6.00010: Investigating asymmetries in the HD$^{+}$(1$s\sigma )$ branching ratio following electron impact at energies from threshold to 100 J.B. Williams, A.L. Landers, I. Ben-Itzhak, E. Wells We present preliminary results where momentum-imaging is used to investigate e$^{-}$ + HD collisions at energies from near threshold to 100 eV. Single ionization proceeding to the ground (1$s\sigma )$ electronic state usually results in the production of HD$^{+}$, but a small fraction of the ionization events reach the vibrational continuum, leading to H$^{+}$+D(1$s)$ or, via charge transfer to the 2$p\sigma $ state, H(1$s)$ + D$^{+}$. The H(1$s)$ + D$^{+}$ final state is 3.7 meV higher than the H$^{+}$ + D(1s) state at the separate atom limit. When initiated by fast ion impact, this energy difference leads to a measurable asymmetry in the ground state dissociation process [1]. Fast electron impact is expected to produce a similar result, but as the electron energy is reduced toward threshold, this asymmetry may increase and have significant astrophysical implications. [1] E. Wells, B.D. Esry, K.D. Carnes, and I. Ben-Itzhak, Phys. Rev. A \textbf{62}, 062707 (1999). [Preview Abstract] |
Friday, May 30, 2008 10:00AM - 10:12AM |
O6.00011: Stokes Parameter Measurements of Fluorescence from N$_2$, N$_2^+$, and NI Resulting from Polarized-Electron Impact on N$_2$ T.J. Gay, J.W. Maseberg We have measured the Stokes parameters of the N$_2^+$ B $^2\Sigma_u^+$ $\rightarrow$ X $^2\Sigma_g^+$ 391.4 nm transition, the N$_2$ C $^3\Pi_u$ $\rightarrow$ B $^3\Pi_g$ 380.5 nm transition, and the NI ($^3$P)3p $\rightarrow$ ($^3$P)3s 824.2 nm transition, all resulting from spin-polarized electron impact on N$_2$ gas. Incident electron energies ranged from threshold to 100 eV; the scattered electrons were not detected. We find the circular polarization fraction P$_3$ for light emitted in the direction of the incident electron spin polarization is consistent with zero in the region within 10 eV above threshold for these transitions. This stands in contrast to the linear polarization fraction P$_1$, which is $\sim$0.05 or greater over a broad range of energies in all three cases. These results are considered in light of the fact that H$_2$ molecular triplet transitions [1] exhibit large values of P$_3$ as do the excited atomic fragments resulting from electron-impact induced dissociation [1,2]. [1] A.S. Green, G.A. Gallup, M.A. Rosenberry, and T.J. Gay, Phys. Rev. Lett. \textbf{92}, 093201 (2004). [2] J.F. Williams and D.H. Yu, Phys. Rev. Lett. \textbf{93}, 073201 (2004). [Preview Abstract] |
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