1 / 36

Electron-Driven Chemistry: Dissociative Attachment to H2O

Learn about the crucial role of electron-driven chemistry in various applications, including plasma technology and radiation damage. Explore the dissociative attachment of water molecules and the quantum dynamics of polyatomic collisions.

atapper
Download Presentation

Electron-Driven Chemistry: Dissociative Attachment to H2O

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dissociative Attachment to H2O C. William McCurdy, Lawrence Berkeley National Lab and The University of California Collaborators:D. Haxton (UCB), Z. Zhang, T. Rescigno (LBNL), and H-D. Meyer (University of Heidelberg)

  2. Electron-Driven Chemistry Plays a Crucial Role in a Multitude of Applications High Intensity Plasma Arc Lamp (OSRAM-Sylvania) Plasma Flat Panel Display (Fujitsu) Plasma vapor deposition and plasma etching – no chemistry without electrons. LBNL-AMO

  3. Electron-Driven Chemistry Associated with Ionizing Radiation Secondary electron cascades in mixed radioactive/ chemical waste drive much of the chemistry that determines how those materials age, change, and interact with the natural environment. Cascades of secondary electrons from ionizing radiation Low energy electrons with energies significantly below the ionization energies of DNA molecules can initiate single and double strand-breaks by attaching to components of DNA molecules or the water around them and driving bond dissociation. Most energy deposited in cells by ionizing radiation is channeled into secondary electrons between 1eV and 20eV (Research group of L. Sanche) LBNL-AMO

  4. Secondary electrons play a pivotal role in radiation damage • Dissociative attachment and resonant processes occur primarily for electron energies below 20 eV • The energy distribution of secondary electrons emphasizes those processes Figure from Thom Orlando, Ga Tech LBNL-AMO

  5. A primer on the electron-driven chemistry of water • The ground state configuration of water is • 1a122a121b223a121b12 1A1 • Electron-driven chemistry through both dissociative excitation and dissociative attachment Electron impact dissociation Dissociative Attachment LBNL-AMO

  6. Simple molecular orbital picture of the dissociation processes • Dissociative excitation proceeds by excitation of low-lying dissociative electronic states, e.g, those observed in EELS spectra: • 1a122a121b223a121b114a111,3 B1 • 1a122a121b223a111b124a11 1,3A1 • Dissociative attachment of H2O proceeds through Feshbach resonances • 1a122a121b223a121b114a122B1 (~ 6.5 eV) • 1a122a121b223a111b124a12 2A1 (~ 9 eV) • 1a122a121b213a121b124a12 2B2 (~ 12 eV) LBNL-AMO

  7. H- production is primarily through the 2B1 Resonance C. E. Melton, Journal of Chemical Physics, 57, pp.4218-25, (1972). LBNL-AMO

  8. DA products are vibrationally (and rotationally) excited • Up to v = 7 is observed for OH stretch in the H- production channel • Rotational excitation: J=7 most populated for ground state OH, j=4 or 5 for v =4 OH stretch D. S. Belic, M. Landau and R. I. Hall, Journal of Physics B 14, pp.175-90 (1981) Vibrational excitation cross sections H- energy distribution for various electron energies LBNL-AMO

  9. The problem: From first principles, solve the scattering problem including the nuclear dynamics, predict the cross sections and show how they display the polyatomic dynamics of the collision. • Breaking up the problem into two parts: • Electron scattering for fixed nuclei: Calculate the position and lifetime of the Feshbach resonances • Nuclear dynamics during the resonant collision: Calculate the quantum molecular dynamics leading to dissociative attachment LBNL-AMO

  10. O r R H H A Complete ab initio Treatment of Polyatomic Dissociative Attachment: 2B1 Resonance • Electron scattering: Calculate the energy and width of the resonance for fixed nuclei • Complex Kohn calculation of fixed-nuclei electron scattering cross sections (7 state close coupling)– produce • CI calculations -- produce • Fitting of complete resonance potential surface including the electronically bound (product) regions • Nuclear dynamics in the local complex potential model on the anion surface • Multiconfiguration Time-Dependent Hartree (MCTDH) • Flux correlation function calculation of DA cross sections LBNL-AMO

  11. A D B C Complex Kohn Variational Method Variational Functional for the T-Matrix (scattering amplitude) Trial wave function for the N+1 electron system target continuum exchange Correlation and Polarization Continuum functions are further expanded in combined basis of Gaussians and continuum functions LBNL-AMO

  12. ER and G are determined from the eigenphase sums in the Complex Kohn calculation near the resonance E.g. at equilibrium geometry G = 0.005819eV LBNL-AMO

  13. Real part of resonance energy, ER(r,R,g), nearly parallels the “parent” 3B1 state When the resonance becomes bound, the CI calculation of ER(r,R,g) must dissociate properly Dominant configuration is 1a122a121b223a121b114a12 A robust basis set is necessary: au-cc-pvTZ (augmented, correlation consistent, polarization and valence triple zeta) CI (898,075 configurations): Singles and doubles excitation from the CAS reference space (1b2, 3a1, 1b1, 4a1, 5a1, 2b2)7 4a1 is the “resonance” orbital, 5a1 and 2b2are important for correct dissociation and correlation Configuration Interaction calculation for real part of the 2B1 (2A’’) resonance surface LBNL-AMO

  14. r1 O r2 H H Entire 2B1 (2A’’) potential surface is fit with combination analytic fit and 3-D spline. OH +H- Break up OH +H- LBNL-AMO

  15. r1 O r2 H H Q = 00 O-+ H2 150 350 OH +H- 700 104.50 1250 OH +H- 1500 1800 q LBNL-AMO

  16. O H H A view of the O- channel H2 +O- LBNL-AMO

  17. O r R g H H As a function of angle in Jacobi coordinates LBNL-AMO

  18. Width of 2B1 (2A’’) resonance J. D.Gorfinkiel, L. A. Morgan, and J.Tennyson, J. Phys. B 35 543 (2002). Function of r1 with r2 and q fixed at equilibrium: (at equilibrium geometry G = 0.005819eV) LBNL-AMO

  19. Width of 2B1 (2A’’) resonance q = 104.5 r2 = 1.81 C2v symmetry LBNL-AMO

  20. O r R H H A Complete ab initio Treatment of Polyatomic Dissociative Attachment: 2B1 Resonance • Electron scattering: Calculate the energy and width of the resonance for fixed nuclei • Complex Kohn calculation of fixed-nuclei electron scattering cross sections – produce • CI calculations -- produce • Fitting of complete resonance potential surface including the electronically bound (product) regions • Nuclear dynamics in the local complex potential model on the anion surface • Multiconfiguration Time-Dependent Hartree (MCTDH) • Flux correlation function calculation of DA cross sections LBNL-AMO

  21. Local Complex Potential Model W(R) = ER(r,R,g)-i G(r,R,g) Propagate on complex resonance potential surface Solution, z(R), is Fourier Transform Cross section from the asymptotic behavior of z(R) LBNL-AMO

  22. r R g H H Hamiltonian for Nuclear Motion O • For J = 0 the wave function is a function of only internal coordinates • For arbitrary J: LBNL-AMO

  23. Solving the multidimensional Time-Dependent Schrödinger Equation • Multi-configuration time-dependent Hartree (MCTDH) method  in collaboration with Prof. H. D. Meyer, University of Heidelberg • Can be converged to the exact solution -- efficient “adaptive” method. • Start with a time-independent orthonormal product basis (DVR) • The MCTDH wave function is a time-dependent linear combination of configurations (products of single-particle functions) • The time-dependent single-particle functions are represented as • Derive equations of motion for both the coefficients and the single-particle functions LBNL-AMO

  24. Local complex potential wave packet starting from ground state H2O LBNL-AMO

  25. H- production from the ground state of H2O LBNL-AMO

  26. H- from single excitation of bend LBNL-AMO

  27. H- from single excitation of asymmetric stretch LBNL-AMO

  28. H- from single excitation of symmetric stretch LBNL-AMO

  29. H- from double excitation of symmetric stretch LBNL-AMO

  30. Cross Sections for OH vibrational states compared with experiment Total 0 1 2 3 4 5 6 7 D. S. Belic, M. Landau and R. I. Hall, Journal of Physics B 14, pp.175-90 (1981) LBNL-AMO

  31. Cross Sections compared with experiment – experiment scaled to match peak of total cross section 0 1 2 3 4 5 6 7 LBNL-AMO

  32. Rotational State Distributions for OH + H- OHv = 0 Integrated over incident electron energy OHv = 3 LBNL-AMO

  33. O-+H2 channel Experimental peak at about 6.5 eV is LBNL-AMO

  34. Experimental Isotope Effect D/H H- from H2O D- from D2O LBNL-AMO

  35. Theoretical Isotope Effect D/H D- H- LBNL-AMO

  36. It is now possible to treat dissociative attachment to a triatomic in full dimensionality from first principles. Dynamics of the 2B1 (2A’’) resonance leads almost exclusively to H - +OH An ab initio treatment reproduces the cross sections for producing OH in excited vibrational states to within a factor of 2 Interesting energy dependence is predicted for cross sections from H2O excited in symmetric stretch The dynamics of the other two resonances (2A1 and2B2) remains unknown. This calculation does not account for all of the observed O – production. Is the surface not accurate enough or is there production of O - from Renner-Teller or other nonadiabatic coupling between resonances? Isotope effect still fails to agree with experiment – Is the shape of the width “surface” the cause? Conclusions and Open Questions LBNL-AMO

More Related