1. Slow Photoelectron Velocity-map Imaging Spectroscopy
Etienne Garand, Jia Zhou and Daniel M. Neumark
Department of Chemistry, UC Berkeley, Berkeley CA 94720

2. Study of energetic neutral molecules
Radicals
Hydrocarbon combustion
Interstellar clouds
Upper atmosphere
Small clusters
Transition state and pre-reactive complexes
How can we get information on the geometry and electronic structure of those molecules?

3. Anion photodetachment spectroscopy
Photoelectron Spectroscopy (PES)
Zero Kinetic energy electron(ZEKE)
<1 cm-1 resolution
Limited to s-wave scatterers
~ 50 cm-1 maximum resolution

4. Velocity-map imaging
Charged particles with the same initial velocity vector (but different positions) are
mapped onto the same point on the detector.
Field free flight tube
Electrostatic lens
Imaging detector
(MCP + P-screen)

5. Photoelectron velocity-map imaging
Chloride (Cl-)
Quadrant symmetrized SEVI image
Inverse Abel transformed image
eKEr2

6. Slow Electron Velocity-map Imaging
Advantages of the SEVI technique:
Higher resolution than PES (up to 1 cm-1)
Higher collection efficiency than TOF
Faster acquisition time than ZEKE
Larger cross-section than ZEKE
Selectable energy range/resolution
Electron angular distribution available

7. Examples of SEVI: C2H and C4H
Important in combustion chemistry, photolysis processes, and interstellar chemistry
1) C≡C-H: Prototype of complex vibronic coupling
- 2S+ ground state with low-lying 2P excited state (~3600 cm-1)
- Renner-Teller coupling on the 2P state
- Pseudo Jahn-Teller between the 2S+ and 2P(A’) for all bent geometries
2) C≡C-C≡C-H: Very close lying states
- 2S+ and 2P states within <500 cm-1

8. C2H PES: Taylor, Xu, and Neumark JCP 108 10018 Calculations:
Tarroni and Carter, JCP

9. C4H a A1 A2 B1 B2
PES:
Taylor, Xu, and Neumark JCP (1998)

10. C4H Tentative assignements: P belongs to 2S+ and S to 2P
Progression of carbon backbone bending modes for the 2 states.
2S+ ground state with 2P excited-state only 202 cm-1 apart

11. Some take home messages
Resolution up to 1 cm-1 in atomic anions
Peak width limited by unresolved rotational profile in molecular systems
Several new transitions not resolved in the PES spectra are now observable
p-wave photodetachment observed very close to threshold (~200cm-1).
Complete high-resolution spectra are obtained within few hours compared to several days for ZEKE.

12. Cl + H2  HCl + H
Controversy on the reactivity of the Cl spin-orbit states:
Experiment: Cl*>Cl
S. H. Lee, K. P. Liu, J. Chem. Phys. 111, 6253 (1999).
Theory: Cl>Cl*
M. H. Alexander, G. Capecchi, H. J. Werner, Science 296, 715 (2002).

13. Cl + H2 pre-reactive complex

14. ClH2 and ClD2 spectra SEVI Adiabatic calc. non-adiabatic calc.
Manolopoulos et Alexander. PCCP & private communication

15. Conclusions Validate the accuracy of the Capecchi-Werner surfaces
Validate the theoretical treatment of the couplings responsible for non-adiabatic coupling in the reactant valley.
Support the reactive scattering study of Alexander et al.
So what’s going on?

16. Future directions Pure p-wave scatterer: C3-, C4-, N3-
van der Waals and solvation clusters: X-(CO2)n , X-(H2O)n, X-(CH4)n
Metal and semi-conductors clusters
Transition-state spectroscopy
F-(H2) (F+ H2  HF + H)
F-(CH4) (F+CH4  HF + CH3)

17. Acknowledgements Pr. Daniel M. Neumark
Jia Zhou (SEVI labmate) & Neumark’s group
Pr. M.H. Alexander and Pr. D.E Manolopoulos
Project funded by the AFOSR
Doctoral fellowship from the National Science and Engineering Research Council of Canada (NSERC)