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

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 :

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) 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) ; Schneider et al., JPCA 114 (2010) 4017 Low-energy modes crucial for IVR in the molecule, needed for full characterization of ion. ^ Motivation

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

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

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

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

CH stretching region of CH 3 NO 2 - J. M. Weber et al., JCP 115 (2001) 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 Ar

CH 3 NO h  CH 3 NO 2 + e - J. M. Weber et al., JCP 115 (2001) 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 -

Low energy vibrations of CH 3 NO Ar predissociation CH 3 NO 2 - ·Ar + h  CH 3 NO 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)

CH 3 NO 2 - ·Ar 2 + h  CH 3 NO 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. Low energy vibrations of CH 3 NO Ar predissociation Thompson et al., JCP 142 (2015)

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 Ar predissociation Thompson et al., JCP 142 (2015)

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 Ar predissociation Thompson et al., JCP 142 (2015)

Low energy vibrations of CH 3 NO 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)

Low energy vibrations of CH 3 NO 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)

Low energy vibrations of CH 3 NO 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)

Low energy vibrations of CH 3 NO 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 cm -1 / calc cm -1 Antisymmetric NO 2 stretch: 12 Exp cm -1 / calc cm -1 Thompson et al., JCP 142 (2015)

Low energy vibrations of CH 3 NO 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, & 4 15 ) CN stretch / NO 2 symmetric stretch (opposite phase) : 5 Exp cm -1 / calc cm -1 (interaction with ) Thompson et al., JCP 142 (2015)

Low energy vibrations of CH 3 NO 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, & 4 15 ) CN stretch / NO 2 symmetric stretch (opposite phase) : 5 Exp cm -1 / calc cm -1 (interaction with ) NEWLY IDENTIFIED FUNDAMENTAL TRANSITIONS Thompson et al., JCP 142 (2015)

modecalculated a experimentalcharacterization harmonicanharmonic (77.2)2925symmetric CH 2 stretch (64.0)2776symmetric CH 3 stretch (2.94)1420HCH bend (0.295)1354CN stretch / CH 3 umbrella (36.0) 1198 CN stretch / NO 2 symmetric stretch opposite phase (6.62)1080out of plane HCN bend / C-N-O 2 bend (2.70)845 CN stretch / NO 2 symmetric stretch in phase (14.9)N/ANO 2 bend (17.4)380 ± 56NO 2 wag (50.5)2969antisymmetric CH 2 stretch (0.727) 1446 (1.61) (1420)HCH asymmetric bend (179)1241antisymmetric NO 2 stretch (75.4)1035HCN bend (2.71)N/AONC bend (0.434)N/Ahindered CH 3 rotor Low energy vibrations of CH 3 NO 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) Weber et al., JCP 115 (2001) 10718; Schneider et al.,JPCA 112 (2008) 7498; Adams et al., JCP 130 (2009) Schneider et al., JPCA 114 (2010) 4017;

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

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

Low energy vibrations of CH 3 NO Electron Detachment Expectation: AEA = 1387 cm -1, detachment should only occur above AEA. Observation: Detachment starts at ca cm -1.  Signals should come from vib. excited molecules. Most Franck-Condon active mode: 412 cm ± 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 cm -1, in agreement with observation. Thompson et al., JCP 142 (2015)

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

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)

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

THE END

Photoelectron Spectroscopy AEA E kin = h - E B

What happens upon vibrational excitation?