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Studies of Transient Neutral Molecules by Dissociative Photodetachment of Cooled Molecular Anions Christopher Johnson Continetti Research Lab University.

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Presentation on theme: "Studies of Transient Neutral Molecules by Dissociative Photodetachment of Cooled Molecular Anions Christopher Johnson Continetti Research Lab University."— Presentation transcript:

1 Studies of Transient Neutral Molecules by Dissociative Photodetachment of Cooled Molecular Anions Christopher Johnson Continetti Research Lab University of California, San Diego June 22, 2010

2 PPC Experiments Create precursor anion of interest Detach a single electron, collect it and resulting neutral fragments in coincidence (Dissociative Photodetachment) Full kinematic description of dissociation event DirectIndirect KE tot KE max KE tot KE max

3 Internal Excitation Effects Product Internal Excitation – Cold anions ABC - -> A + BC(v=0,1) + e - Gain internal energy, lose kinetic energy v=0 v=1 KE max ABC - A+BC

4 Internal Excitation Effects Anion Internal Excitation – Hot anions ABC - (v=0,1) -> A + BC(v=0) + e - Gain kinetic energy for given internal energy v=0 v=1 KE max ABC - A+BC

5 PPC Spectrometer with Trapping Electron and multiparticle neutral time and position detectors High duty cycle, trapping for many seconds at 10 -11 torr Ions Bunched, phase locked with external laser

6 HOCO Background Combustion intermediate Benchmark 4-atom complex forming system Complex PES QCT and Quantum Dynamics struggle with H + CO 2 Difficult to isolate HOCO - + hν ->HOCO* + e - OH + CO + e - H + CO 2 + e -

7 Channel-Resolved PE Spectra Black – “cold” data Gray – “hot” data Solid – trans- limit Dashed – cis-limit HOCO H + CO 2 OH + CO E hν = 3.20 eV (388 nm) Calculated Potential Energy Surface Photoelectron Spectra ZPE corrected Yu, Muckerman & Sears, 2001 Clements, Continetti & Francisco, 2002

8 OH + CO Channel eKE max Etot max Internal excitation! HotCold Hot anions => Significant enhancement of OH + CO channel!

9 H + CO 2 Channel eKE max Etot max eKE max -C 2v HotCold Possibly 0.2 – 0.5 eV in CO 2 product

10 Energy-Resolved Branching Ratios Processes occurring over > 6 orders of magnitude of time Possible to extract tunneling info using rate calculations? OH + CO H + CO 2 HOCO Transition regions

11 Conclusions Dynamical processes on HOCO PES revealed and well-separated Unambiguous dissociation below the calculated barriers to H + CO 2 Suggestion of a different route to H + CO 2 Need for better PES in the H + CO 2 region! See also: Johnson & Continetti, JPC Lett 1, 1895 (2010) Acknowledgements: D. Zajfman, M. Rappaport, Weizmann Inst. R. Thomas, R. Vane, M. Fogle Current and past members of the Continetti Lab US Department of Energy

12 Cold HOCO - Near-threshold photoelectron spectrum (hν = 1.60 eV) Calculated Adiabatic Electron Affinities (1) CCSD(T)/6-311++G(3df,3pd) (1) Clements, Continetti and Francisco 2002

13 Linear Electrostatic Ion Beam Trap Beam Environment: ~ 20 K Background Pressure: < 10 -11 torr Beam Lifetimes: Many seconds Beam Energies: 4 – 16 keV Ion Frequency: 50 – 500 kHz Zajfman & co-workers 1998 GND + V - V Laser e-e-

14 Synchronization Clock source for experiment is laser fiber oscillator Phase lock function generator to laser fiber frequency Bunch ions with small RF voltage from generator Simple phase control

15 O 4 - Coincidence 0-100 ms trapping time500-900 ms trapping time Dissociative Photodetachment Ionic Photodissociation

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