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Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado,

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Presentation on theme: "Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado,"— Presentation transcript:

1 Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado, Boulder, CO OSU International Symposium on Molecular Spectroscopy June 23, 2009

2 Novel Approach to studying intramolecular vibrational relaxation (IVR). Motivation

3 What happens when a photon of h interacts with an anion with E eBE < h ? Motivation

4 What happens when a photon of h interacts with an anion with E eBE < h ? 1.Direct photoemission of the excess electron. A - + h → A + e - Motivation

5 What happens when a photon of h interacts with an anion with E eBE < h ? 1.Direct photoemission of the excess electron. A - + h → A + e - 2.Vibrational excitation followed by vibrational autodetachment (VAD) of the excess electron. A - + h → [A - ] * → A + e - First example: NH - (Lineberger and coworkers, 1985) Motivation

6 The excess electron is largely localized on the nitro group. Nitroalkane Anions: A Model System

7 The excess electron is largely localized on the nitro group.

8 Nitroalkane Anions: A Model System The excess electron is largely localized on the nitro group. The fundamental CH vibrational transitions have energies in excess of the adiabatic electronic affinity (AEA) ~200 meV (1600 cm -1 ).

9 ZOBS Dark States Intramolecular Vibrational Relaxation (IVR) e-e-

10 MCP detector Experimental Setup

11 IR Spectrum of MeNO 2 - Autodetachment spectrum CH 3 NO h  CH 3 NO 2 + e -

12 MCP detector Experimental Setup

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

14 Raw ImageTransformed Image BASEX Transformed Image Integration over emission angles Photoelectron Spectrum Example: VMIPES of S - (532 nm) V. Dribinski et al., RSI 73,

15 IR Spectrum of MeNO 2 - Autodetachment spectrum CH 3 NO h  CH 3 NO 2 + e -

16 What do we expect from the direct photodetachment PES?

17 Ө = 14° Ө = 0° AnionNeutral What is the Geometry of the Anion and the Neutral?

18 The wagging vibration of the neutral should give the most prominent vibrational progression in the PES. Dominant FCF Active Modes

19 The wagging vibration of the neutral should give the most prominent vibrational progression in the PES. NO 2 Wag ~ 655 cm -1 (81 meV ) Dominant FCF Active Modes

20 The wagging vibration of the neutral should give the most prominent vibrational progression in the PES. Upon emission the methyl rotor goes from being hindered to a free rotor. NO 2 Wag ~ 655 cm -1 (81 meV ) Dominant FCF Active Modes

21 The wagging vibration of the neutral should give the most prominent vibrational progression in the PES. Upon emission the methyl rotor goes from being hindered to a free rotor. NO 2 Wag ~ 655 cm -1 (81 meV ) Dominant FCF Active Modes

22 1 MeNO 2 - at 3200 cm -1

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

24 The first prominent peak, located at (172±6) meV, is identified as the origin of the vibrational progression (v anion =0, v neutral =0). Peak Assignments – AEA determination MeNO 2 - ·Ar 3200 cm -1 MeNO cm -1

25 Peaks are spaced by ~ 645 cm -1 (80 meV), corresponding to the wagging motion of the neutral. The first prominent peak, located at (172±6) 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 – AEA determination MeNO 2 - ·Ar 3200 cm -1 MeNO cm -1

26 Hot band Peak Assignments – AEA determination Peaks observed at binding energies less than 172 meV are identified as hot bands. MeNO 2 - ·Ar 3200 cm -1 MeNO cm -1

27 Peak Assignments – AEA determination Peaks observed at binding energies less than 172 meV are identified as hot bands. The difference in binding energies of the hot band and origin of the vibrational progression matches the energy of the anionic wag. MeNO 2 - ·Ar 3200 cm -1 MeNO cm -1

28 Peaks observed at binding energies less than 172 meV are identified as hot bands. The difference in binding energies of the hot band and origin of the vibrational progression matches the energy of the anionic wag. The hot bands are suppressed upon Ar solvation. Peak Assignments – AEA determination MeNO 2 - ·Ar 3200 cm -1 MeNO cm -1

29 Comparison of Experiment and Theory Franck-Condon Simulation (PESCAL) by Kent M. Ervin B3LYP/ G(2df,2p) for anion and neutral geometries Independent Harmonic Oscillator Approximation with Duschinsky rotation 14 vibrational modes treated in simulation CH 3 torsion treated separately

30 There exists a pronounced shoulder on all of the dominant features of the PES regardless of Ar solvation. Contribution of Torsion to the PES

31 There exists a pronounced shoulder on all of the dominant features of the PES regardless of Ar solvation. The direct photodetachment involves a transition from hindered-to-free methyl rotor. Contribution of Torsion to the PES

32 There exists a pronounced shoulder on all of the dominant features of the PES regardless of Ar solvation. The direct photodetachment involves a transition from hindered-to-free methyl rotor. This leads to progressions of the free internal rotor states superimposed on all transitions Contribution of Torsion to the PES

33 CH Stretching Vibrations ν 13 = 2775 cm -1 ν 14 = 2922 cm -1 ν 15 = 2965 cm Autodetachment spectrum CH 3 NO h  CH 3 NO 2 + e - IR Spectrum of MeNO 2 -

34 Vibrational Autodetachement Direct Photoelectron Emission Comparison of Off and On Resonance Images

35 Vibrational Autodetachement Direct Photoelectron emission Comparison of Off and On Resonance Images

36 Vibrational Autodetachement Direct Photoelectron emission Comparison of Off and On Resonance Images

37 On-Resonance Interpretation Both on-resonant and direct detachment contributions  subtract contribution of direct photodetachment

38 Compare with vibrational states of the neutral, neglecting torsion Without Torsion On-Resonance Interpretation

39 With Torsion On-Resonance Interpretation Compare with vibrational states of the neutral, including torsion

40 Inconsistencies with purely statistical argument. Some states preferentially occupied Nonstatistical population With Torsion On-Resonance Interpretation

41 Considerable differences between direct detachment and vibrational autodetachment Summary

42 Considerable differences between direct detachment and vibrational autodetachment Redistribution of vibrational energy before electron emission Summary

43 Considerable differences between direct detachment and vibrational autodetachment Redistribution of vibrational energy before electron emission Retention of vibrational energy in the molecule, leading to emission of low-energy electrons. Summary

44 Considerable differences between direct detachment and vibrational autodetachment Redistribution of vibrational energy before electron emission Retention of vibrational energy in the molecule, leading to emission of low-energy electrons. Methyl torsion very important for IVR Summary

45 Continue the study with the larger nitroalkane chains: Summary

46 Continue the study with the larger nitroalkane chains: Determine AEA and assign the vibrational features in the direct photodetachment spectra Summary

47 Continue the study with the larger nitroalkane chains: Determine AEA and assign the vibrational features in the direct photodetachment spectra Monitor the evolution of the VAD PES as the site of initial excitation is moved further away from the nitro group Summary

48 Acknowledgements Mathias Weber Holger Schneider Jesse Marcum Kent Ervin (UN Reno) Carl Lineberger and the Lineberger Lab

49 On-Resonance Interpretation NO 2 rocking (475 cm -1 ) NO 2 Wag (603 cm -1 ) NO 2 Scissor (657 cm -1 ) CN stretch (918 cm -1 ) 2 quanta NO 2 rocking (475X2 cm -1 ) 1 quanta NO 2 rocking (475 cm -1 ) and 1 quanta NO 2 Wag (603 cm -1 ) CH 3 rocking (1096 cm -1 ) 1 quanta NO 2 rocking (475 cm -1 ) and 1 quanta NO 2 Scissor (657 cm -1 )

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


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