1. Heavy Atom Vibrational Modes and Low-Energy Vibrational Autodetachment in Nitromethane Anions Michael C. Thompson, Joshua H. Baraban, Devin A. Matthews, John F. Stanton and J. Mathias Weber 70th International Symposium on Molecular Spectroscopy June 25, 2015

2. Motivation What if the electron binding energy E B of an anion is lower than some of its vibrational transition energies? That molecule’s answer to a photon with ħ  = E vib > E B :

3. Properties of nitromethane anion: Low electron binding energy: AEA = (172±6) meV = 1387 cm -1.  vibrational autodetachment possible by excitation of vibrational modes with ħ  ≥ AEA. Adams et al., J. Chem. Phys. 130 (2009) 074307 So far: vibrational spectrum of CH 3 NO 2 - well characterized in CH stretching region, but not in region around AEA. Weber et al., JCP 115 (2001) 10718; Schneider et al.,JPCA 112 (2008) 7498; Adams et al., JCP 130 (2009) 074307; Schneider et al., JPCA 114 (2010) 4017 Low-energy modes crucial for IVR in the molecule, needed for full characterization of ion. ^ Motivation

4. Experimental Method I: IR Photodissociation cluster + h hot cluster fragments

5. Vibrational Spectroscopy of Mass Selected Anions 600 – 4500 cm -1 0.1-10 mJ / 5 ns IR-OPO/OPA Nd:YAG CH 3 NO 2 - CH 3 NO 2 - Ar n

6. Experimental Method II: Vibrational Autodetachment bare anion + h hot molecule electron loss, formation of neutral molecule

7. Vibrational Spectroscopy of Mass Selected Anions IR-OPO/OPA Nd:YAG CH 3 NO 2 - CH 3 NO 2 - Ar n 600 – 4500 cm -1 0.1-10 mJ / 5 ns

8. CH stretching region of CH 3 NO 2 - J. M. Weber et al., JCP 115 (2001) 10718 H. Schneider et al. JPCA 112 (2008) 7498       1 : CH 2 antisymm. stretch 2 : CH 2 symmetric stretch 3 : CH stretch 4 : HCH bend 5 : HCH bend 6 : CH 3 umbrella      CH 3 NO 2 - ·Ar 4 + h  CH 3 NO 2 - + 4Ar

9. CH 3 NO 2 - + h  CH 3 NO 2 + e - J. M. Weber et al., JCP 115 (2001) 10718 H. Schneider et al. JPCA 112 (2008) 7498 electron loss spectrum: vibrational resonances on broad background due to direct detachment same vibrational features 3 slightly shifted by Ar solvation photofragment yield [arb. units] photoneutral yield [arb. units] CH stretching region of CH 3 NO 2 -

10. Low energy vibrations of CH 3 NO 2 - - Ar predissociation CH 3 NO 2 - ·Ar + h  CH 3 NO 2 - + Ar Parent ion CH 3 NO 2 - ·Ar Features at energies down to ca. 850 cm -1. Intensities become weaker towards higher wavenumbers. Thompson et al., JCP 142 (2015) 234304

11. CH 3 NO 2 - ·Ar 2 + h  CH 3 NO 2 - + 2Ar Parent ion CH 3 NO 2 - ·Ar Features at energies down to ca. 850 cm -1. Intensities become weaker towards higher wavenumbers. Parent ion CH 3 NO 2 - ·Ar 2 Loss of 2 Ar atoms: Features at low energies suppressed. Intensities become stronger towards higher wavenumbers. Rel. intensities of dominant features change. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Thompson et al., JCP 142 (2015) 234304

12. CH 3 NO 2 - ·Ar 2 + h  CH 3 NO 2 - ·Ar + Ar Parent ion CH 3 NO 2 - ·Ar Features at energies down to ca. 850 cm -1. Intensities become weaker towards higher wavenumbers. Parent ion CH 3 NO 2 - ·Ar 2 Loss of 2 Ar atoms: Features at low energies suppressed. Intensities become stronger towards higher wavenumbers. Rel. intensities of dominant features change. Loss of 1 Ar atom: Features at low energies return. Features at higher energies suppressed. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Thompson et al., JCP 142 (2015) 234304

13. Parent ion CH 3 NO 2 - ·Ar Features at energies down to ca. 850 cm -1. Intensities become weaker towards higher wavenumbers. Parent ion CH 3 NO 2 - ·Ar 2 Loss of 2 Ar atoms: Features at low energies suppressed. Intensities become stronger towards higher wavenumbers. Rel. intensities of dominant features change. Loss of 1 Ar atom: Features at low energies return. Features at higher energies suppressed. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Thompson et al., JCP 142 (2015) 234304

14. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Product ion stabilities: Parent ion CH 3 NO 2 - ·Ar Product ion loses electron at high wavenumbers: energy content after losing Ar atom > AEA Thompson et al., JCP 142 (2015) 234304

15. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Product ion stabilities: Parent ion CH 3 NO 2 - ·Ar Product ion loses electron at high wavenumbers: energy content after losing Ar atom > AEA Parent ion CH 3 NO 2 - ·Ar 2 Low wavenumbers: Energy only sufficient for evaporation of one Ar atom. Intermediate wavenumbers: Energy sufficient to evaporate two Ar atoms for the “warmer” part of the ensemble. High wavenumbers: Energy too high for survival of CH 3 NO 2 - ·Ar  only loss of two Ar atoms observed Thompson et al., JCP 142 (2015) 234304

16. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Parent and product ion stabilities: Estimate of Ar binding energy: “Rollover” of product ion from loss of one Ar to loss of two Ar at ca. (1160 ± 50) cm -1.  E B (Ar) = (580±25) cm -1. From photoelectron spectra: E B (Ar) = (63±7) meV = (508±60) cm -1. Thompson et al., JCP 142 (2015) 234304

17. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Assignments through anharmonic calculations (Stanton group): Geometry & force constants: CCSD(T)/ANO1 Anharmonic frequencies & intensities: VPT2 Resonances treated separately Dominant peaks: CN stretch / NO 2 symmetric stretch (opposite phase): 5 Exp. 1198 cm -1 / calc. 1215 cm -1 Antisymmetric NO 2 stretch: 12 Exp. 1241 cm -1 / calc. 1252 cm -1 Thompson et al., JCP 142 (2015) 234304

18. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Some strong anharmonic interactions: CN stretch / NO 2 symmetric stretch (in phase) : 7 Exp. 845 cm -1 / calc. 846 cm -1 (interaction with 2 9, 9 +2 15 & 4 15 ) CN stretch / NO 2 symmetric stretch (opposite phase) : 5 Exp. 1198 cm -1 / calc. 1215 cm -1 (interaction with 13 + 15 ) Thompson et al., JCP 142 (2015) 234304

19. Low energy vibrations of CH 3 NO 2 - - Ar predissociation Some strong anharmonic interactions: CN stretch / NO 2 symmetric stretch (in phase) : 7 Exp. 845 cm -1 / calc. 846 cm -1 (interaction with 2 9, 9 +2 15 & 4 15 ) CN stretch / NO 2 symmetric stretch (opposite phase) : 5 Exp. 1198 cm -1 / calc. 1215 cm -1 (interaction with 13 + 15 ) NEWLY IDENTIFIED FUNDAMENTAL TRANSITIONS Thompson et al., JCP 142 (2015) 234304

20. modecalculated a experimentalcharacterization harmonicanharmonic 1 30592910 (77.2)2925symmetric CH 2 stretch 2 29622801 (64.0)2776symmetric CH 3 stretch 3 14681420 (2.94)1420HCH bend 4 13801343 (0.295)1354CN stretch / CH 3 umbrella 5 12551215 (36.0) 1198 CN stretch / NO 2 symmetric stretch opposite phase 6 11021069 (6.62)1080out of plane HCN bend / C-N-O 2 bend 7 871846 (2.70)845 CN stretch / NO 2 symmetric stretch in phase 8 583569 (14.9)N/ANO 2 bend 9 436412 (17.4)380 ± 56NO 2 wag 10 31052948 (50.5)2969antisymmetric CH 2 stretch 11 1480 1428 (0.727) 1446 (1.61) (1420)HCH asymmetric bend 12 12891252 (179)1241antisymmetric NO 2 stretch 13 10501023 (75.4)1035HCN bend 14 440431 (2.71)N/AONC bend 15 213186 (0.434)N/Ahindered CH 3 rotor Low energy vibrations of CH 3 NO 2 - - Ar predissociation 11 out of 15 vibrational modes of CH 3 NO 2 - are now experimentally characterized! High level calculations can be used for the remaining modes. present work earlier IR earlier PES Thompson et al., JCP 142 (2015) 234304 Weber et al., JCP 115 (2001) 10718; Schneider et al.,JPCA 112 (2008) 7498; Adams et al., JCP 130 (2009) 074307 Schneider et al., JPCA 114 (2010) 4017;

21. Low energy vibrations of CH 3 NO 2 - - Electron Detachment Thompson et al., JCP 142 (2015) 234304 Expectation: AEA = 1387 cm -1, detachment should only occur above AEA. Observation: Detachment starts at ca. 1000 cm -1.  Signals should come from vib. excited molecules.

22. Low energy vibrations of CH 3 NO 2 - - Electron Detachment Expectation: AEA = 1387 cm -1, detachment should only occur above AEA. Observation: Detachment starts at ca. 1000 cm -1.  Signals should come from vib. excited molecules. Most Franck-Condon active mode: 9 @ 412 cm -1 (calc.) @ 380 ± 56 cm -1 (exp.) Note: Detachment from excited state will have higher cross section than from ground state Thompson et al., JCP 142 (2015) 234304

23. Low energy vibrations of CH 3 NO 2 - - Electron Detachment Expectation: AEA = 1387 cm -1, detachment should only occur above AEA. Observation: Detachment starts at ca. 1000 cm -1.  Signals should come from vib. excited molecules. Most Franck-Condon active mode: 9 @ 412 cm -1 (calc.) @ 380 ± 56 cm -1 (exp.) Note: Detachment from excited state will have higher cross section than from ground state For v=1 in 9 : onset expected at ca. 1000 cm -1, in agreement with observation. Thompson et al., JCP 142 (2015) 234304

24. Low energy vibrations of CH 3 NO 2 - - Electron Detachment Smooth “Background”:  Direct detachment. Superimposed features correlate with vibrational states:  Vibrational autodetachment. Thompson et al., JCP 142 (2015) 234304

25. Summary Vibrational Ar predissociation and vibrational autodetachment spectroscopy used to characterize vibrational spectrum of nitromethane anion. “Titration” of fragment ion survival with number of Ar atoms. Electron detachment involving vibrationally excited states (v = 1 in NO 2 wag). Nearly complete description of vibrational fundamentals and many resonances by experiment and high level ab initio calculations. Characterization over the years required several experimental techniques (IR, PES) and strong collaboration with theory. Article appeared last week: M.C. Thompson, J.H. Baraban, D.A. Matthews, J.F. Stanton, J.M. Weber, JCP 142 (2015) 234304

26. Dramatis Personae NSF AMO PFC Michael Thompson John Stanton Joshua Baraban Devin Matthews

27. THE END

29. Photoelectron Spectroscopy AEA E kin = h - E B

30. What happens upon vibrational excitation?