Anion slow electron velocity-map imaging (SEVI): applications to spectroscopy and dynamics Columbus June, 2009

Motivation: spectroscopy and dynamics of transient species Reactive free radicals play key role in combustion, planetary atmospheres, interstellar chemistry Reactive free radicals play key role in combustion, planetary atmospheres, interstellar chemistry Map out electronic and vibrational structure, with special focus on vibronic coupling between close-lying electronic statesMap out electronic and vibrational structure, with special focus on vibronic coupling between close-lying electronic states Optical spectroscopy (LIF, infrared, microwave) are well- established probesOptical spectroscopy (LIF, infrared, microwave) are well- established probes Our (complementary) approach: anion photoelectron spectroscopy (PES)and its variants Our (complementary) approach: anion photoelectron spectroscopy (PES)and its variants Slow electron velocity-map imaging (SEVI), a high resolution version of PES

Specific systems: Open-shell radicals and reactive species (C 3 O, C 3 S, C n H, C 2 H 3 O, i- C 3 H 5 O, HCO 2 ) Open-shell radicals and reactive species (C 3 O, C 3 S, C n H, C 2 H 3 O, i- C 3 H 5 O, HCO 2 ) Use SEVI to resolve low-frequency vibrational modes, fine-structure, low- lying electronic statesUse SEVI to resolve low-frequency vibrational modes, fine-structure, low- lying electronic states High resolution of SEVI particularly useful for probing vibronic coupling, Duschinsky mixing, internal rotations, …High resolution of SEVI particularly useful for probing vibronic coupling, Duschinsky mixing, internal rotations, …

Anion photoelectron spectroscopy: h

Why negative ions? Easy to mass-select Easy to mass-select Hard to make in large concentrations, but can usually photodetach at h <4 eV Hard to make in large concentrations, but can usually photodetach at h <4 eV Can access many interesting neutral species by anion photodetachment Can access many interesting neutral species by anion photodetachment Radicals, clusters, transition states…Radicals, clusters, transition states…

Selection rules in PES Electronic: all “one-electron” transitions are allowed BN¯: X 2  + (…2p  2p  4 ) Detachment from 2p  MO  X 3 , b 1  states of BN Detachment from 2p  MO  a 1  +, A 3  + states of BN So can directly measure a-X singlet-triplet splitting (0.031 eV in BN) Asmis, 1998

Vibrational selection rules: Mode i totally symmetric(sym stretch in CO 2 ) : any  i allowed. “Active” modes: large changes in normal coordinate upon photodetachment Mode i non-totally symmetric (CO 2 bend, antisym stretch) : only even  i, but  i =0 typically dominates Simplest possible expression-product of 1-dim Franck-Condon factors No non-adiabatic effects, i.e. for anion, neutral

Odd  i transitions in non-totally symmetric modes are signature of vibronic coupling: Jahn-Teller coupling (degenerate neutral state, i.e. CH 3 O X 2 E state) or Herzberg-Teller coupling between nearby electronic states (X 2  u +, A 2  g + states of BNB ) Vibronic coupling in PES PES of BNB - (Asmis 1999) odd  3 transitions (i.e ) occur only because of vibronic coupling between X and A states of BNB

Photoelectron angular distributions e¯ E  limiting cases:  =0: s wave  =2: p wave  =-1: s+d wave overlapping electronic bands, vibronic coupling 2A12A1 2B22B2 X 1, A 5 are transitions: intensity borrowing Xu, 1998 BNB -

How to improve resolution? Photoelectron spectroscopy Photoelectron spectroscopy Very general, limited to 5-10 meV ZEKE (zero electron kinetic energy) spectroscopy ZEKE (zero electron kinetic energy) spectroscopy High resolution ( meV) Experimentally challenging SEVI SEVI Resolution comparable to ZEKE without expt’l complications Fixed h Tunable h ZEKESEVI

SEVI apparatus Adaptation of ideas by Chandler, Houston, Parker Adaptation of ideas by Chandler, Houston, Parker Electrons with meV fill detector Electrons with meV fill detector Very high resolution for the slow electrons Very high resolution for the slow electrons Energy and angular distributions Energy and angular distributions

Grid Discharge Source Expansion goes in a  2.5mm x 30mm canal and pass through 2 fine stainless steel grids separated by 1mm with the 2 nd grid held at ~-500VDC TEFLON Aluminum

SEVI of Cl -  Cl( 2 P 3/2 ), Cl*( 2 P 1/2 ) Quadrant symmetrized SEVI image Inverse Abel transformed image 2 P 1/2 2 P 3/2 Cl Cl*

PES of C 3 O¯ Poorly-resolved progression (550 cm -1 ) Poorly-resolved progression (550 cm -1 ) Difficult to determine EA Difficult to determine EA EA=1.34 EA=1.34±0.15 eV Calculated EA 0.93±0.1 eV 1) J.M Oakes and G.B. Ellison, Tetraedron, 42, 6263 (1986) 2) J.C. Rienstra-Kiracofe, G.B. Ellison, B.C. Hoffman, and H.F. Schaefer III, J. Chem. Phys. A, 104, 2273 (2000) 1+1+ 2 A’

SEVI of C 3 O ¯ and C 3 S ¯ closely spaced doublets photoelectron angular distributions are different Garand, submitted

SEVI of C 3 O¯ EA = ± eV v 5 (CCC bend) = 109 cm -1 v 4 (CCO bend) = 581 cm -1 v 3 (sym. str.) = 935 cm -1 EA

SEVI of C 3 S¯ EA = ± eV v 5 (CCC bend) = 151 cm -1 v 4 (CCS bend) = 478 cm -1 v 3 (sym. str.) = 721 cm -1 EA

C 3 O Simulations B3LYP/AVTZ C 3 O¯ Anion 148  172  Optimized

C 3 S Simulations B3LYP/AVTZ C 3 S Anion 160  175  Optimized

C 3 O, C 3 S summary C 3 O¯ SEVI spectrum represents dramatic improvement over previous PES C 3 O¯ SEVI spectrum represents dramatic improvement over previous PES Accurate EA’s for C 3 O, C 3 S, several vibrational frequencies determined for first time Accurate EA’s for C 3 O, C 3 S, several vibrational frequencies determined for first time Analysis still needs work Analysis still needs work large-amplitude bending motion in anions, R-T effectslarge-amplitude bending motion in anions, R-T effects

SEVI of C n H¯ anions anions and neutrals seen in interstellar medium anions and neutrals seen in interstellar medium even n: closely spaced 2  +, 2  states in neutral even n: closely spaced 2  +, 2  states in neutral odd n: evidence for linear and cyclic isomers in anion, neutral odd n: evidence for linear and cyclic isomers in anion, neutral Taylor, 1998

C 4 H - ( 1  + )  C 4 H ( 2  + and 2  ) 2+2+ 22 2   splitting is only 213 cm -1 Progressions in bending modes  vibronic coupling Zhou, 2007 B, C have different PAD’s

C n H, odd n C 3 H: cyclic isomers slightly lower energy than linear isomers in C 3 H¯ and C 3 H C 3 H: cyclic isomers slightly lower energy than linear isomers in C 3 H¯ and C 3 H C 5 H: numerous low- lying structures calculated for anion, neutral C 5 H: numerous low- lying structures calculated for anion, neutral Structures I, II have been observed by microwave spectroscopy Structures I, II have been observed by microwave spectroscopy

PES/SEVI of C 5 H¯ X 0 : “linear”  linear A 0, B 0 : cyclic  cyclic (Sheehan, 2008) PES SEVI

C 5 H simulations B3LYP/AVTZ Nearly everything can be fit with linear-linear simulation

SEVI of C 7 H¯ and C 9 H¯ Spin-orbit splittings of 28 cm -1 seen for both species: transitions to linear 2  neutral states

Vinoxy radical: C 2 H 3 O Combustion intermediate Combustion intermediate Studied extensively by Terry Miller (BX transition using LIF) Studied extensively by Terry Miller (BX transition using LIF) X, B states well- characterized, less known about A state, anion X, B states well- characterized, less known about A state, anion Anion Neutral

Vinoxy: SEVI versus PES L. S. Alconcel, H. J. Deyerl, V. Zengin, and R. E. Continetti, J. Phys. Chem. A 103, 9190 (1999). XXXX AXAX

Main Vibrations ν 9 : CCO bend 524;498;423 ν 4 : CO stretch exp:---;1528;1533 ν 7 : CC stretch ---;1137;1533 a' a" ν 10 : CH wag exp: 813;---;--- thy: 939;942;n/a ν 11 : all CH wag 358;---; ;734;n/a ν 12 : CH 2 twist 643;---; ;429;n/a anion radical anion radical ; ;

Franck-Condon Simulations Franck-Condon Factor Franck-Condon Factor Duschinsky Rotation Duschinsky Rotation Q’ and Q : normal coordinatesQ’ and Q : normal coordinates J : Duschinsky Rotation Matrix : mixing of normal modesJ : Duschinsky Rotation Matrix : mixing of normal modes K : mass-weighted geometry change between normal coordinatesK : mass-weighted geometry change between normal coordinates

Vinoxy Simulations Vinoxy Simulations Parallel mode approximation: J = 1 Manually match modes Full Duschinsky rotation SEVI overview spectrum

Assignments: a ' modes ν 4 : CO stretch ν 7 : CC stretch ν 9 : CCO bend 0-0 ν 4 : CC stretch ν 5 : CH 2 scissors ν 6 : OCH bend ν 8 : CH 2 rock ν 9 : CCO bend

Assignments : a " modes Combination bands with * * * * * v 10 : CH wag ν 11 : all H wag ν 12 : CC torsion ν 11 : out-of-plane mode

1-Methylvinoxy: SEVI vs PES L. S. Alconcel, H. J. Deyerl, and R. E. Continetti, J. Am. Chem. Soc. 123, (2001).

Vibrational Assignments ν 9 : CC stretch ν 10 : CH 2 a' rock ν 14 : CCC bend ν 20 : CH 2 wag 0-0 ?

Hindered Rotor Simulation 1D Potential Hamiltonian & free rotor basis

Simulation Results: H vs D i-C 3 H 5 Oi-C 3 D 5 O EAEA? EA

Vinoxy, methyl-vinoxy: Precise EA & T 0 Precise EA & T 0 Vinoxy: new frequencies for A state, Duschinsky rotation needed to simulate X state Vinoxy: new frequencies for A state, Duschinsky rotation needed to simulate X state Methyl-vinoxy: resolve hindered rotor progressions in X band as well as vibrational structure Methyl-vinoxy: resolve hindered rotor progressions in X band as well as vibrational structure Electron AfinityFirst Term Energy Vinoxy ± eV0.996 ± eV 1-Methylvinoxy ± eV1.039±0.003 eV

HCO 2 and DCO 2 Intermediate in H+CO 2 reaction Intermediate in H+CO 2 reaction Anion is closed-shell, C 2v species Anion is closed-shell, C 2v species Neutral has nearly degenerate 2 A 1, 2 B 2 states ( 2 A 2 state somewhat higher) Neutral has nearly degenerate 2 A 1, 2 B 2 states ( 2 A 2 state somewhat higher) vibronic coupling via 5, 6 modes (b 2 ) vibronic coupling via 5, 6 modes (b 2 ) artifactual symmetry breaking problem for electronic structure calculations artifactual symmetry breaking problem for electronic structure calculations

PES vs. SEVI J. Chem. Phys. 103, 7801 (1995)

Results from SEVI: Many more peaks resolved, including hot- bands Many more peaks resolved, including hot- bands Band A is an apparent origin Band A is an apparent origin Peaks exhibit two distinct PAD’s: Peaks exhibit two distinct PAD’s: For HCO 2, A, D, F, H are p-likeFor HCO 2, A, D, F, H are p-like B, C, E, G are s-likeB, C, E, G are s-like Implies transitions to two electronic states Implies transitions to two electronic states Sort out with simulations including vibronic coupling, Duschinsky rotation (with John Stanton) Sort out with simulations including vibronic coupling, Duschinsky rotation (with John Stanton)

HCO 2 Simulations Vibronic symmetry: Red : A 1 Blue : B 2 EA = ± eV Ground State: 2 A 1 T 0 ( 2 B 2 ) : 318 ± 8 cm -1 Peaks D, F have contributions from ( 2 A 1 ) (in blue), higher peaks strongly mixed 2 3 6

DCO 2 Simulations EA = ± eV Ground State: 2 A 1 T 0 ( 2 B 2 ) : 87 ± 8 cm -1 Vibronic symmetry: Red : A 1 Blue : B again see vibronic coupling via 6 mode

Summary SEVI offers “next generation” of anion photodetachment experments SEVI offers “next generation” of anion photodetachment experments First technique that systematically improves resolution of anion PES without sacrificing (much) generalityFirst technique that systematically improves resolution of anion PES without sacrificing (much) generality Where are we headed? Where are we headed? Bare and complexed metal/semiconductor clustersBare and complexed metal/semiconductor clusters Pre-reactive complexes and transition states (in progress)Pre-reactive complexes and transition states (in progress) Cold ions via trapping/coolingCold ions via trapping/cooling Development of improved methods to simulate spectra beyond simple harmonic analysisDevelopment of improved methods to simulate spectra beyond simple harmonic analysis

SEVI Group: Jia Zhou Etienne Garand Tara Yacovitch $$$ from AFOSR Matt Nee Andreas Osterwalder John Stanton

X( 1 A g ) (SEVI) A( 3 B 3u ) (ZEKE) ZEKE, SEVI of Si 4 - ZEKE, SEVI spectra show much more vibrational structure ZEKE, SEVI spectra show much more vibrational structure Why is SEVI better? Why is SEVI better? ZEKE is experimentally challenging! Band X is missing in ZEKE spectrum- Wigner threshold law s-wave only Arnold, 1993 Garand, unpub

Example: Si 4 - Si 4 - : 2 B 2g […(a g ) 2 (b 1u ) 2 (b 2g ) 1 ] 2 mode (  300 cm -1 ) most active PES Kitsopoulos, 1991