Measuring and modeling absolute data for electron-induced processes

1. Measuring and modeling absolute data for electron-induced processes Chemistry and Spectroscopy with Free Electrons A personal retrospective Michael Allan Department of Chemistry University of Fribourg, Switzerland

2. Contents • A very personal retrospective • H2 : a short or long-lived resonance? • The peculiar story of threshold peaks : HF, HCl, HBr • CO2 : threshold peaks are commonplace • H-C≡C-H : the necessity of many dimensions • HCOOH : the hybrid case • Higher energy : CH3OH, C4H9-O-C4H9 etc. • Exotic molecules: Pt(PF3)4 • Many excellent laboratories • Where do we find electron collisions ? • Conclusions

3. Pardubice Electron Tubes gloooooooow in the dark magic eye

4. Basel Energy of incident electron Energy of emitted photon M. Allan and J. P. Maier 1976

5. Yale M. J. W. Boness and G. J. Schulz 1976 A. Stamatovic and G. J. Schulz 1970

6. short – lived radical anions = resonances q = 72° background scattering coherent superposition resonant scattering

7. E (eV) DEA and VE in H2 Resonances: Feshbach (sg)13s2 valence core-excited (sg)1(su)2 shape (sg)2(su)1 “s* shape resonance” Ethreshold H-/H2 >200 D-/D2

8. Frustration over instruments • Background • Low energy not accessible • Only narrow energy range • Spectrum distorted by instrument’s response function • Only relative units • Limited angular range • ... M. J. W. Boness and G. J. Schulz 1973

9. Fribourg 1989 1981 • Very low background • Low energy OK • Wide energy range • but • Only relative units • scattering angle only 0° and 180° • no elastic scattering

10. Magnetic Angle Changer Magnetic Angle Changer (Frank H. Read) see also Andrew J. Murray, Wednesday lecture

11. Time-of-Flight mass spectrometer for absolute DEA cross section Juraj Fedor, Olivier May, Dušan Kubala, Fribourg 2008

14. full-range spectrum in N2 core excited Feshbach resonance shape resonances

15. H2 : a short or long-lived resonance?

16. H2 : a short or long-lived resonance? E (eV) 1985 calculations: Čížek, Horáček, Domcke

17. 1993 looking at large R (high final v) permits time resolution

18. H2- lifetime : going to the extreme D2 : t = 2 ms Experiment : Golser et al., 2005 (Wienna)

19. Threshold phenomena Vibrational excitation in HF – naive expectation s* - resonance

20. Threshold phenomena • threshold peaks • Vibrational Feshbach Resonances • dipole – bound resonances s* shape resonance valence dipole - bound m = 1.8 D Čížek, Horáček, Allan, Fabrikant, Domcke 2003 Original discovery: G. Knoth, M. Gote, M. Rädle, K. Jung and H. Ehrhardt, PRL 1989

21. HF – theory and experiment Čížek, Horáček, Allan, Fabrikant, Domcke, J. Phys. B (2003) review: Hotop, Ruf, Allan, Fabrikant, Adv. At. Mol. Opt. Phys. 49 (2003) pp 85-216.

22. structures everywhere

23. NO – vibrational excitation boomerang oscillations strongly influenced by existence of quasi-bound vibrational state of NO- K. Houfek, M. Čížek, J. Horáček, Chem. Phys. (2008) Allan, J. Phys. B (2005)

24. Chemistry: Dissociative electron attachment to diatomic hydrides e- + HBr  H + Br-

25. Interchannel Coupling in Dissociative Atachment COMPARISON OF ABSOLUTE CROSS SECTIONS ! blue: nonlocal resonance theory red: absolute experiment dissociative attachment cross section drops when a new vibrational excitation channel opens Fedor May Allan (2008) Čížek Horáček Sergenton Popović Allan Domcke Leininger Gadea Phys. Rev. A 63 (2000) 062710

26. to remember: long range (dipole) attraction  „nonlocal phenomena“ Vibrational Feshbach Resonances threshold peaks in VE large CS and steps in DEA

27. CO2 has no dipole moment – is it like H2 ? Fermi Resonance the (1000) and (0200) vibrations mix true states: {(1000) + (0200)} (Fermi dyad) {(1000) - (0200)} two Raman lines

28. Exciting the Fermi-dyad in CO2 Excitation of the Fermi – split states is highly selective! p* shape resonance virtual state Allan, Phys. Rev. Lett. 87 (2001)

29. Cross section for exciting the topmost member of the tetrad {(3000), (2200), ... } Allan, (2011, in print)

30. Similarity of vibrational cross sections in CO2 and HF m = 0 D m = 1.8 D

31. Potential curves of CO2 and HF bending Physica Scripta (2004)

32. Allan, J. Phys. B (2002)

33. Understanding the selectivity within the dyad FIG. 3. Contour plots of the wave functions for the two components of the Fermi dyad in O-C-O angle. The thick line marks the seam where the anion and neutral surfaces cross. Top panel: upper member of dyad; bottom panel: lower member of dyad. Vanroose et al. PRL 2004

34. Until now: effects due to long range electron binding: • threshold peaks in VE • sharp structures in VE cross sections • Vibrational Feshbach resonances • large cross sections and threshold peaks in DEA • steps in DEA cross section • theory: nonlocal theory essential • existing theory: one dimension (diatomic or pseudodiatomic) • Next: effects due several dimensions of nuclear motion: • symmetry-lowering due to vibronic coupling • anion needs to distort in order to dissociate • theory: several dimensions of nuclear motion essential

35. isotope ratio: experiment : 14.4 theory at 0 K : 28.9 theory at 333 K : 17.9 but : theoretical cross section nearly 2× too large theory: S. T. Chourou and A. E. Orel 2009 experiment: O. May, J. Fedor, B. C. Ibanescu and M. Allan 2009

36. Dissociative Electron Attachment to Acetylene S. T. Chourou and A. E. Orel PRA 2008

37. Dissociative Electron Attachment to Acetylene S. T. Chourou and A. E. Orel

38. Chlorobenzene Skalický,Chollet, Pasquier, Allan, Phys. Chem. Chem. Phys. 2002

39. Chlorobenzene C-Cl stretch ring breathing - the p* resonances act as doorway states into the s* resonance - no activation barrier ← symmetry lowering ← vibronic coupling Skalický,Chollet, Pasquier, Allan, Phys. Chem. Chem. Phys. 2002

40. Two families of DEA: • HBr • no shape resonance • peak at threshold • steps • nonlocal theory required • H-C≡C-H • p* shape resonance • peak at resonance • LCP sufficient • inherently multidimensional • Puzzle: mechanism in formic acid ? • both p* shape resonance and polar O-H bond HCOOH + e- HCOO- + H

41. Vibrational excitation of formic acid

42. Vibrational excitation of formic acid - cusps, like HCl, HBr, HF

43. HCOOH + e- HCOO- + H : approach I theory: R-matrix G. A. Gallup, P. D. Burrow and I. I. Fabrikant PRA 2009 experiment A. Pelc, W. Seiler, P. Scheier, N. J. Mason, E. Illenberger and T. Märk 2003 & 2005

44. approach II p* anion s* anion neutral

45. Dissociation of formic acid anion on the valence p* shape resonance potential surface Isotope effect expected for D substitution on C-H DFT B3-LYP 6-31G*

46. Isotope effect D. Kubala, O. May, M. Allan, 2011

47. Formic acid is a prototype for biomolecules : forms hydrogen bonds ! M Allan, Phys. Rev. Lett. (2007)

48. Similar situation in other biomolecules : uracil

49. Family III: higher energies On the complexity of dissociation via core-excited Feshbach resonances in polyatomic molecules

50. Feshbach resonances