1. Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado, Boulder, CO 80309-0440 OSU International Symposium on Molecular Spectroscopy June 19, 2008

2. Low Energy Photoelectron Imaging of Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado, Boulder, CO 80309-0440 OSU International Symposium on Molecular Spectroscopy June 19, 2008

3. Nitromethane - a model system Binding energy of the excess electron in the anion is less than energies of the fundamental CH stretching vibrational transitions.  interaction of vibrational excitation and electron emission Vibrational autodetachment mediated internal vibrational relaxation (IVR) upon excitation of CH stretching fundamental observable in photoelectron spectrum? Small molecule, tractable by theory Relatively large error bars for current electron affinity (EA = 260 ± 80 meV) Compton et al., J. Chem. Phys. 105 (1996) 3472

4. So what’s already known?

5. Photoelectron Spectrum of bare Nitromethane

6. Compton et al., J. Chem. Phys. 105 (1996) 3472

7. EA = 260 ± 80 meV Compton et al., J. Chem. Phys. 105 (1996) 3472 Photoelectron Spectrum of bare Nitromethane

8. Vibrational progression with ~ 645 cm -1 (80meV) Matches the NO 2 wagging mode of the neutral molecule EA = 260 ± 80 meV Compton et al., J. Chem. Phys. 105 (1996) 3472 Photoelectron Spectrum of bare Nitromethane

9. Assignment of vibrational features:  compare geometry of the neutral and the anion

10. 1.459 Å Ө = 18° Ө = 0° 1.484 Å Anion Ө = 109.5° Ө = 106.9° 1.097 Å 1.085 Å What is the Geometry of the Anion and the Neutral? Neutral

11. Predominant Active Modes The wagging vibration of the neutral should give the most prominent vibrational progression in a PES. Other modes, such as the stretching or bending motion of the NO 2, could also contribute to the PES. NO 2 Wag ~ 655 cm -1 (81 meV )

12. IR Spectrum of MeNO 2 - CH Stretching Vibrations 3 = 2775 cm -1 2 = 2922 cm -1  = 2965 cm -1 Compare to EA ≤ 2100 cm -1 2 1 3

13. Vibrational Autodetachment ZOBS Dark States

14. ZOBS Dark States Vibrational Autodetachment

15. ZOBS Dark States Vibrational Autodetachment

16. e-e- ZOBS Dark States e-e- Vibrational Autodetachment

17. Experimental Setup MCP detector

18. Ion Beam Laser Beam Direction Velocity Map Imaging Photoelectron Spectroscopy (VMIPES)

19. Ion Beam Laser Beam Direction

20. Example: VMIPES of S - (532 nm) Raw ImageTransformed Image BASEX Transformed Image Integration over emission angles Photoelectron Spectrum

21. …and what can we learn from the experiment? determine electron affinity assign vibrational features in PES

22. 1 MeNO 2 - 3200 cm -1

23. Peak Assignments – EA determination MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1

24. Peak Assignments – EA determination MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1 peaks are spaced by ~ 645 cm -1 (80 meV), corresponding to the wagging motion of the neutral.

25. MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1 Peak Assignments – EA determination peaks are spaced by ~ 645 cm -1 (80 meV), corresponding to the wagging motion of the neutral. The first prominent peak, located at 170 meV, is identified as the origin of the vibrational progression (v anion =0, v neutral =0).

26. peaks are spaced by ~ 645 cm -1 (80 meV), corresponding to the wagging motion of the neutral. The first prominent peak, located at 170 meV, is identified as the origin of the vibrational progression (v anion =0, v neutral =0). Argon solvation shifts the vibrational progression by ~63 meV (508 cm -1 ). Peak Assignments – EA determination MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1

27. MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1 Hot band Peaks observed at binding energies less than 170 meV are identified as hot bands. Peak Assignments – EA determination

28. MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1 Hot band Peaks observed at binding energies less than 170 meV are identified as hot bands. The hot bands are suppressed in the Ar solvated spectrum. Peak Assignments – EA determination

29. MeNO 2 - ·Ar 3200 cm-1 MeNO 2 - 3200 cm -1 MeNO 2 - 2500 cm -1 Peaks observed at binding energies less than 170 meV are identified as hot bands. The hot bands are suppressed in the Ar solvated spectrum. The difference in binding energies of the hot band and origin of the vibrational progression matches the energy of the anionic wag. Peak Assignments – EA determination

30. New value for EA: 170 ± 4 meV 1370 ± 30 cm -1 Old value: 260 ± 80 meV New Assignments

31. Binding Energy [meV] Frank Condon Simulation for MeNO 2 - CCSD(T)/aug-cc-pVDZ (Kent Ervin) Experimental Theory

32. …and what can we learn from the experiment? determine electron affinity assign vibrational features in PES

33. …and what can we learn from the experiment? determine electron affinity assign vibrational features in PES effects of vibrational excitation on photoelectron spectra

34. IR Spectrum of MeNO 2 - CH Stretching Vibrations 3 = 2775 cm -1  = 2922 cm -1  = 2965 cm -1 Compare to EA = 1370 cm -1 2 1 3

35. IR Spectrum of MeNO 2 - CH Stretching Vibrations 3 = 2775 cm -1 2 = 2922 cm -1 1 = 2965 cm -1 2 1 3 vibrational autodetachment spectrum CH 3 NO 2 - + h  CH 3 NO 2 + e -

36. IR Spectrum of MeNO 2 - 3

37. Comparison of Off and On Resonance Spectra 2775 cm -1 (on resonance) 3200 cm -1 (off resonance) Kinetic Energy

38. Comparison of Off and On Resonance Spectra Kinetic Energy 2775 cm -1 (on resonance) 3200 cm -1 (off resonance)

39. Summary The adiabatic electron affinity is found to be (170  4) meV (1370  32) cm -1 A progression belonging to the NO 2 wagging mode is the most prominent feature in the PES Considerable contrast is observed between the electron kinetic energy distribution in vibrational autodetachment and direct photodetachment. The data suggest redistribution of vibrational energy before electron emission and retention of vibrational energy in the molecule, leading to emission of low-energy electrons

40. Acknowledgements Weber group: Mathias Weber Holger Schneider Jesse Marcum Lineberger Lab Carl Lineberger Lenny Sheps Elisa Miller Kent M. Ervin, University of Nevada, Reno

42. Coupling of the HOMO of the Anion to the Symmetric CH Stretch Vibration (2775 cm -1 )

43. Comparison of On and Off Resonance Images On Resonance ImageOff Resonance Image

44. Averaging in the Lab Frame along the Transition Dipole of the CH Stretch Vibration (2775 cm -1 )

45. Potential Energy Surfaces for Neutral MeNO 2 and the Valence Anion J. M. Weber et al., JCP 115 (2001) 10718

46. Velocity Map Imaging Photoelectron Spectroscopy (VMIPES) Ion Beam Laser Beam Direction

47. Velocity Map Imaging Photoelectron Spectroscopy (VMIPES) Ion Beam Laser Beam Direction