Anion slow electron velocity-map imaging (SEVI): applications to

spectroscopy and dynamics

ColumbusJune, 2009

Motivation: spectroscopy and dynamics of transient species

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 states

• Optical spectroscopy (LIF, infrared, microwave) are well-established probes

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 (C3O, C3S, CnH, C2H3O, i-C3H5O, HCO2)• Use 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, …

Anion photoelectron spectroscopy:

N orm al C oord ina te

ele

ctro

n K

inet

ic E

ne

rgy

()

eKE

C ross S ectionM -

M

M

0

Po

ten

tial E

ne

rgy

ele

ctro

n B

indi

ng

En

erg

y (

)eB

Ehn

eBE h eKE

Why negative ions?

Easy to mass-select Hard to make in large

concentrations, but can usually photodetach at h <4 eV

Can access many interesting neutral species by anion photodetachment• Radicals, clusters, transition states…

Selection rules in PESElectronic: all “one-electron” transitions are allowed

BN¯: X 2+ (…2p2p4)

Detachment from 2p MO X 3, b1 states of BNDetachment 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:

1( , 1... ) ( ,..., )hi nABC i n ABC e

Mode i totally symmetric(sym stretch in CO2) : any i allowed. “Active” modes: large changes in normal coordinate upon photodetachment

Mode i non-totally symmetric (CO2 bend, antisym stretch) : only even i, but i =0 typically dominates

2

1

'n

i ii

I

Simplest possible expression-product of 1-dim Franck-Condon factorsNo non-adiabatic effects, i.e.

,v e v 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. CH3O X 2E state)or Herzberg-Teller coupling between nearby electronic states (X 2u

+, A 2g+ states of BNB )

1 1 1 2 2 2,v e c v e c v e

Vibronic coupling in PES

PES of BNB- (Asmis 1999)odd 3 transitions (i.e. 30

1) occur only because of vibronic coupling between X and A states of BNB

Photoelectron angular distributions

2(1 (cos ))4

dP

d

e¯E

limiting cases:=0: s wave=2: p wave=-1: s+d wave

overlapping electronic bands, vibronic coupling

2A1

2B2

X1, A5 are 301 transitions:

intensity borrowing

Xu,1998

BNB-

How to improve resolution?

Photoelectron spectroscopy• Very general, limited to 5-10

meV ZEKE (zero electron kinetic

energy) spectroscopy• High resolution (0.1-0.2 meV)• Experimentally challenging

SEVI• Resolution comparable to

ZEKE without expt’l complications

An ion

Neu tral

eKE

Fixed hn

Tunable hn

ZEKE SEVI

SEVI apparatus

Adaptation of ideas by Chandler, Houston, Parker Electrons with 50-100 meV fill detector Very high resolution for the slow electrons 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 2nd grid held at ~-500VDC

TEFLON

Aluminum

SEVI of Cl- Cl(2P3/2), Cl*(2P1/2)

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

r = 2.3 pixels

E = 19 cm-1

eKE = 906 cm-1

Radius (pixels)

r = 2.1 pixels

E = 2.8 cm-1

eKE = 23.3 cm-1

Quadrant symmetrized SEVI image

Inverse Abel transformed image

2P1/2

2P3/2

Cl

Cl*

PES of C3O¯

Poorly-resolved progression (550 cm-1) Difficult to determine EA

EA=1.34EA=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+

2A’

SEVI of C3O¯ and C3S¯

closely spaced doubletsphotoelectron angular distributions are different

Garand, submitted

SEVI of C3O¯

n04

n04

9000 10000 11000 12000 13000 14000 15000 16000 17000

Ele

ctro

n S

igna

l (A

rb. u

nits

)

eBE (cm-1)

B2

D1E1

D2D3

D4

E2E3

E4

x5 A1

A4

A5

A6

A7

A8

A9

A10

A11

B3

B4

B5

B6

B7

B8

B9

B10

B11

A0A3

A2

D5

D6 D7

D8

D9

E5

E6 E7

E8

E9

B1

D0

E0

B0

C3O

C1

C2

An : 4n

0

Bn :4n

o51

0

Dn: 31

04n

0

En: 31

04n

051

0

EA = 1.238 ± 0.003 eV

v5 (CCC bend) = 109 cm-1

v4 (CCO bend) = 581 cm-1

v3 (sym. str.) = 935 cm-1

EA

SEVI of C3S¯

11500 12000 12500 13000 13500 14000 14500 15000 15500 16000

C3S

Ele

ctro

n Sig

nal (

arb.

uni

ts)

eBE (cm-1)

A0

A1

A2

A3

A4 A5

A6

B0

B1

B2

B3

B4

B5

B6

C0

C1

C2

C3

C4

C5D0

E0

D1

D2F1

F2

G

D3

a

An: 4n

0

Bn: 4n

051

0

Dn: 31

04n

0

En: 31

04n

051

0

EA = 1.5957±0.0010 eV

v5 (CCC bend) = 151 cm-1

v4 (CCS bend) = 478 cm-1

v3 (sym. str.) = 721 cm-1

EA

C3O Simulations

B3LYP/AVTZ

C3O¯ Anion

148

172Optimized

C3S Simulations

B3LYP/AVTZ

C3S Anion

160

175Optimized

C3O, C3S summary

C3O¯ SEVI spectrum represents dramatic improvement over previous PES

Accurate EA’s for C3O, C3S, several vibrational frequencies determined for first time

Analysis still needs work• large-amplitude bending motion in

anions, R-T effects

SEVI of CnH¯ anions

anions and neutrals seen in interstellar medium

even n: closely spaced 2+, 2 states in neutral

odd n: evidence for linear and cyclic isomers in anion, neutral

Taylor, 1998

C4H-(1+) C4H (2S+ and 2P)

2S+

2P

2S+ - 2 splitting is only 213 cm-1

Progressions in bending modesvibronic coupling

Zhou, 2007

B, C have different PAD’s

CnH, odd n

C3H: cyclic isomers slightly lower energy than linear isomers in C3H¯ and C3H

C5H: numerous low-lying structures calculated for anion, neutral

Structures I, II have been observed by microwave spectroscopy

PES/SEVI of C5H¯

19000 19500 20000 20500 21000 21500 22000 22500 23000 23500

Ele

ctro

n S

ign

al (

arb

. u

nits

)

eBE (cm-1)

C5HA

B C D

E

F

G

29750 30000 30250 30500 30750 31000 31250 31500 31750 32000

Ele

ctro

n S

ign

al (

arb

. u

nits

)

eBE (cm-1)

C5HH

I J

X0: “linear”linearA0, B0: cycliccyclic

(Sheehan, 2008)

PES

SEVI SEVI

C5H simulations

19000 19500 20000 20500 21000 21500 22000 22500 23000 23500

Ele

ctro

n S

ignal (

arb

. units

)

eBE (cm-1)

C5H (X2)A

B C D

E

F

G

51

0 62

0 41

0

31

0

21

0

11

0

v1 = C-H str.

v2-v5 = C-C backbone str.

v6 = CCH bend

29750 30000 30250 30500 30750 31000 31250 31500 31750 32000

Ele

ctro

n S

ignal (

arb

. units

)

eBE (cm-1)

C5H (a4-

)

51

0

B3LYP/AVTZ

Nearly everything can be fit with linear-linear simulation

SEVI of C7H¯ and C9H¯

22500 23000 23500 24000 24500 25000 25500 26000 26500 27000

Ele

ctro

n S

igna

l (ar

b. u

nits

)

eBE (cm-1)

C7H

32500 33000 33500 34000 34500 35000 35500

Ele

cron

Sig

nal (

arb.

uni

ts)

eBE (cm-1)

C7H

25000 25500 26000 26500 27000 27500

Ele

ctro

n S

igna

l (ar

b.un

its)

eBE (cm-1)

C9H

34500 34750 35000 35250 35500 35750 36000

Ele

ctro

n S

igna

l (ar

b. u

nits

)

eBE (cm-1)

C9H

Spin-orbit splittings of 28 cm-1 seen for both species: transitions to linear 2 neutral states

Vinoxy radical: C2H3O

Combustion intermediate

Studied extensively by Terry Miller (BX transition using LIF)

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).

XXAX

Main Vibrations

ν9: CCO bend524;498;423

ν4: CO stretchexp:---;1528;1533

ν7: CC stretch---;1137;1533

a'

a"ν10: CH wagexp: 813;---;---

thy: 939;942;n/a

ν11: all CH wag358;---;---

469;734;n/a

ν12: CH2 twist643;---;---

670;429;n/a

anion radical

anion radical ; ;

Franck-Condon Simulations

Franck-Condon Factor

Duschinsky Rotation

• Q’ and Q : normal coordinates• J : Duschinsky Rotation Matrix : mixing of normal modes• K : mass-weighted geometry change between normal

coordinates

2' viv

fv dIntensity

' 'Q Q K J

Vinoxy SimulationsParallel mode approximation: J = 1

Manually match modes

Full Duschinsky rotation

SEVI overview spectrum

Assignments: a' modes

ν4 : CO stretchν7 : CC stretchν9 : CCO bend

10

1079

10

30 4,9

107

209

109

10

2079

0-0

019

0-0

10

10 54

109 1

08

106

105

104

ν4 : CC stretchν5 : CH2 scissorsν6 : OCH bendν8 : CH2 rockν9 : CCO bend

Assignments : a" modes

1111

1110

1112

1111 1

110 114

1111

Combination bands with

*

*

** *

v10: CH wagν11 : all H wagν12 : CC torsion

ν11 : out-of-plane mode

1-Methylvinoxy: SEVI vs PES

L. S. Alconcel, H. J. Deyerl, and R. E. Continetti, J. Am. Chem. Soc. 123, 12675 (2001).

Vibrational Assignments

10141

020

10

20 10,14

109

ν9 : CC stretchν10 : CH2 a' rockν14 : CCC bendν20 : CH2 wag

0-0

10

102014 1

014

2014 1

010149

109

0-0

?

Hindered Rotor Simulation

imm e)(

1D Potential

)()(2

2

3

VBCH

H

Hamiltonian & free rotor basis

...)3cos1(2

)( 3

VV

Simulation Results: H vs D

i-C3H5O i-C3D5O

EA EA?EAEA

Vinoxy, methyl-vinoxy:

Precise EA & T0

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

Electron Afinity First Term Energy

Vinoxy 1.8250 ± 0.0014 eV 0.996 ± 0.003 eV

1-Methylvinoxy 1.7471±0.0018 eV 1.039±0.003 eV

HCO2 and DCO2

Intermediate in H+CO2 reaction

Anion is closed-shell, C2v species

Neutral has nearly degenerate 2A1, 2B2 states (2A2 state somewhat higher)

vibronic coupling via 5, 6 modes (b2)

artifactual symmetry breaking problem for electronic structure calculations

1

6

32

5 4

PES vs. SEVI

J. Chem. Phys. 103, 7801 (1995)

Results from SEVI:

Many more peaks resolved, including hot-bands

Band A is an apparent origin Peaks exhibit two distinct PAD’s:

• For HCO2, A, D, F, H are p-like• B, C, E, G are s-like

Implies transitions to two electronic states Sort out with simulations including vibronic

coupling, Duschinsky rotation (with John Stanton)

HCO2 Simulations

Vibronic symmetry:

Red : A1

Blue : B2

EA = 3.4961 ± 0.0010 eV

Ground State: 2A1

T0(2B2) : 318 ± 8 cm-1 113

Peaks D, F have contributions from 601 (2A1)

(in blue), higher peaks strongly mixed

2 3

6

DCO2 Simulations

EA = 3.5164 ± 0.0010 eV

Ground State: 2A1

T0(2B2) : 87 ± 8 cm-1

Vibronic symmetry:

Red : A1

Blue : B2

2 3

6

again see vibronic coupling via 6 mode

Summary

SEVI offers “next generation” of anion photodetachment experments• First technique that systematically improves

resolution of anion PES without sacrificing (much) generality

Where are we headed?• Bare and complexed metal/semiconductor

clusters• Pre-reactive complexes and transition states (in

progress)• Cold ions via trapping/cooling• Development of improved methods to simulate

spectra beyond simple harmonic analysis

SEVI Group:

Jia ZhouEtienne Garand

Tara Yacovitch

$$$ from AFOSR

Matt Nee

AndreasOsterwalder

John Stanton

18000 17800 17600 17400 17200 17000 16800 16600 16400 16200 16000

eBE (cm-1)

X(1Ag)(SEVI)

A(3B3u)(ZEKE)

ZEKE, SEVI of Si4-

ZEKE, SEVI spectra show much more vibrational structure

Why is SEVI better?• ZEKE is experimentally

challenging!• Band X is missing in ZEKE

spectrum- Wigner threshold law12( )lthE E

s-wave only

Arnold, 1993Garand, unpub

Example: Si4-

Si4-: 2B2g […(ag)2(b1u)2(b2g)1]

2 mode (300 cm-1) most active

PES

Kitsopoulos, 1991