Photoelectron Spectroscopy of Multiply-Charged Anions

Washington State University

Pacific Northwest National Laboratory

Photoelectron Spectroscopyof Multiply-Charged Anions

Lai-Sheng WangDepartment of Physics, Washington State University &

Chemical & Materials Sciences Division, Pacific Northwest National Laboratory

Anion 20076/29 – 7/3, 2007 Park City, Utah

Ken: Congrat to your (60±1)th B-Day!



Multiply-charged anions were rarely observed in the gas phase

e-M- M2-

e2

R

R (e- -- M-)

Formation of multiply charged anions by sequential electron attachments:

improbable

+

E

M2-



J. Chem. Phys. 50, 1896 (1969)

The earliest observed doubly charged anion



Phys. Rev. Lett. 67, 1242 (1991)

Doubly-charged carbon clusters and fullerenes

Cn2– (n = 7-28) by Cs+ sputtering of graphite

Phys. Rev. Lett. 65, 625 (1990)

J. Am. Chem. Soc. 113 , 6795 (1991)



Experimental:

Mass spectrometry observations (70’)

Compton et al.: first observation of Cn2- (n = 7-28) (1990)

C602-: Coe et al. and Compton et al. (91’)

Kebarle, Lau, etc: electrospray (mid-90’s)

Theoretical: (1990’s)

Boldyrev/Gutsev (J. Phys. Chem. 94, 2256, 1990)

high EA species; MkXk+1- or (MX)kX-, stable dianions MkXk+2

2-

Cederbaum (Scheller, Compton & Cederbaum, Science 270, 1160, 1995)

Simons/Boldyrev/Gutowski (Acc. Chem. Res. 29, 497, 1996)

Ortiz

Landman

…

Prior research on multiply-charged anions



Multiply-charged anionsare common in the condensed phase

• Simple oxo-anionsSO4

2-, CO32-, PO4

3-, C2O42-, S2O8

2-,

Si2O52-, CrO4

2-, Cr2O72-, VO4

3-, RuO43-…

• Inorganic and organometallic multiply- charged anionsMX4

2-, MX62-, (X = Metal, X = halides)…

• Organic multiply-charged anions

• Biomolecules



Electrospray IonizationInterface between solution and gas phase

John B. Fenn et al. J. Phys. Chem. 88, 4451 & 4671 (1984): Nobel Prize 2002

-HV

Electrospray Capillary

Heated Desolvation

Capillary Skimmer

Mn- Mn- beam

Evaporation Breakup

Charged droplets

Kebarle, Lau, etc: ESI MS of MCA (mid-90’s)



Photodetachment Spectroscopy Facility for Multiply Charged Anions

2.5m(TOF-MS)

Einzel-1

Syringe

Desolvation Capillary

4m

(M

TO

F-P

ES

)

ES

I S

ou

rce

Einzel-2

Ion Guide

Ion Trap

Ion Extraction

Mass Gate &

Decelerator

Ion Detector

Electron Detector

eee

Wang, Ding, Wang, & Barlow Rev. Sci. Instrum. 70, 1957-1966 (1999)



Solution-phase chemistry in the gas phaseUnique properties of multiply-charged anions

Electronic structure of solution anions and complexes in the gas phase

Solvation and solvent stabilization of complex and multiply-charged anions

Probing the electronic structure of Fe-S clusters, complexes and Fe-S proteins

Re2Cl82-, S2O8

2-, Cr2O72-, H2P2O7

2-, Ru6(CO)182-, ZrF6

2-, ML62- (M = Re, Os, Ir, Pt), …

J. Chem. Phys. 111, 4497 (1999); 112, 6959 (2000); J. Am. Chem. Soc. 112, 2096 (2000); 112, 2339 (2000);J. Phys. Chem. A 104, 4429 (2000); 105, 10468 (2001); J. Am. Chem. Soc. 122, 8305 (2000)…

SO42-(H2O)n, C2O4

2-(H2O)n, NO3-(H2O)n, F-(H2O)n, -O2C-(CH2)x-CO2

-(H2O)n, … Science 294, 1322 (2001); J. Chem. Phys. 113, 10837 (2001); 115, 2889 (2001); 116, 561 (2002);J. Phys. Chem. A 106, 7607 (2002); J. Am. Chem. Soc. 124, 10182 (2002); JACS 126, xxx (2004)…

Phys. Rev. Lett. 89, 163401 (2002); J. Phys. Chem. A 107, 1703 (2003); 107, 2821 (2003); 107, 2898 (2003); J. Phys. Chem. A 107, 4612 (2003); J. Am. Chem. Soc. 125, 14072 (2003)…

Nature 400, 245 (1999); Phys. Rev. Lett. 81, 2667 (1998); 81, 3351 (1998); J. Chem. Phys. 110, 3635 (1999); Phys. Rev. Lett. 83, 3402 (1999); J. Phys. Chem. A 104, 1978 (2000); Chem. Phys. Lett. 307, 391 (1999);…

AB2-

AB-EB

hKE

AB2-AB-

- EB

h KE

AB2- --> AB- + e-

RAB-- e-

E

FeS

S

S

S

L

L

L

L

FeS

S

S

S

L

L

L

LFe

S

S

Fe S

S Fe

S Fe

Fe S

SLS L

SL

S L



Current experimental effort on multiply charged anions

Compton: Coulomb barrier, electron attachment (ESI)

Kappes:PES, laser detachment, lifetime in a Penning trap ESI/Laser ablation

Denmark group (Nielsen, Andersen, Hvelplund): electron scattering, charge transfer in storage ring (ESI)

Mainz group (Herlert, Schweikhard):Multiply charged anion formation in a Penning trap

Neumark/Woste/Meijer: IR of SO42-(H2O)n-

Wiliams: MS and IR of SO42-(H2O)n-

Dessent ……

ESI is gaining popularity as a powerful ion source not just for analytical mass spectrometry, but also for physical chemistry and spectroscopy



Photoelectron spectroscopy of singly and multiply-charged anions

Ek Eb

= Eb + Ek h

A

B

C D E F

Ele

ctro

n A

ffin

ity

Eb

h

Eb

Ek

X M

X M- Mn-

M(n-1)-

M- h

M e- + 1

1

+ Mn- M(n-1)- e- 2

2

h



E

RAB-e-

Singly Charged Anions:

AB- AB + e-h

Attractive (Polarization/Dipole)

Multiply Charged Anions:

AB2- AB- + e-h

(Repulsive)

RAB-- e-

E

Difference between photodetachment of singly and multiply-charged anions



AB2-AB-Meta-Stable Dianion

- EB

h KE

h h

2

AB2-

AB-

EB

Stable Dianion

1

hKE

Repulsive coulomb barrier (RCB) and negative electron binding energies



The first PES of a doubly charged anion: Direct observation of the RCB

Wang, Ding & Wang, Phys. Rev. Lett. 81, 3351 (1998)

Citrate

355 nm

266 nm



C

C

C

C

C

OO

OO

H H H H

1.54

1.36

109

H H

C

C

C

C

OO

C

OO

H H H H

H H

C

H H

C

C

C

C

OO C

OO

H H H H

H H

C

CH H

H H

C

C

C

C

OO

C

OO

H H H H

H H

C

H H

CH H

C

H H

C

C

C

C

OO C

OO

H H H H

H H

C

CH H

H H

C

CH H

H H

C

C

C

C

OO

C

OO

H H H H

H H

C

H H

CH H

C

H H

CH H

C

H H

C

C

C

C

OO C

OO

H H H H

H H

C

CH H

H H

C

CH H

H H

C

C

C

C

OO

C

OO

H H H H

H H

C

H H

CH H

C

H H

CH H

C

H H

C

CH H

H H

CH H

C

H H

-OOC(CH2)nCOO-

n

3

4

5

6

7

8

9

10

-OOC(CH2)nCOO-

n=3

n=5

n=6

n=4

n=7

n=9

n=10

n=8

0 1 2 3Binding Energy (eV)

355nm

0 1 2 3 4Binding Energy (eV)

n=3

n=5

n=6

n=4

n=7

n=9

n=8

n=10

-OOC(CH2)nCOO-

266nm

Re

lati

ve

Ele

ctr

on

In

ten

sit

y

(a) (b)X A X

Wang, Ding, Wang & Nicholas Phys. Rev. Lett. 81, 2667-2670 (1998)

266 nm 355 nm

Intramolecular coulomb repulsion and RCB

Wang, Ding, Wang & Nicholas, Phys. Rev. Lett. 81, 2667 (1998)



0.00 0.05 0.10 0.15(Å-1)1/rn

0.0

0.5

1.0

1.5

2.0

2.5

3.0

EB

X &

RC

B (

eV)

89 7 345610n

EBX (eV) = 3.21(4) - 16.7(3) rn (Å)

RCB(eV) = 0.00(5) + 16.8(3)

rn (Å)

Wang, Ding, Wang & NicholasPhys. Rev. Lett. 81, 2667 (1998) EB + RCB = constant = 3.2 eV !

= electron binding energy of R-CO2–

Intramolecular coulomb repulsion and RCB

C—–(CH2)n—–CO

O

O

OΘ Θ

rn

r

RCB = e2/r = 14.4/r (eV.Å)



-2 -1 0 1 2 3 4 5Binding Energy (eV)

-2 -1 0 1 2 3 4 5Binding Energy (eV)

X

AB

X

X

AB

X

266 nm

193 nm

266 nm

193 nm

[CuPc(SO3)4H]3-[CuPc(SO3)4]4-

Observation of negative electron binding energies:photoelectron spectra of [CuPc(SO3)4]4- and [CuPc(SO3)4H]3-

X. B. Wang & L. S. Wang, Nature 400, 245 (1999)

1

2

3

4

R12 = 12.47 Å

5

R13 = 14.68 Å R14 = 8.53 Å R23 = 14.37 Å R24 = 17.10 Å R34 = 14.37 Å R15 = 6.50 Å R25 = R35 = R45 = 8.66 Å

Cu S ON C H

Kappes et al., isomer-dependent life times: J. Phys. Chen. A 107, 794 (2003)



• Isolated SO42- and PO4

3- Anions Do Not Exist

– Boldyrev & Simons, J. Phys. Chem. 98, 2298 (1994)

– SO42- ----> SO4

- + e-, -1.6 eV

• Blades & Kebarle, J. Am. Chem. Soc. 116, 10761 (1994)

– ESI of Na2SO4 solution

– Observed SO42-(H2O)n, n = 4-16

• Questions:

– Minimum number of H2O needed to stabilize SO42-

– How is SO42- solvated?

– Inside or outside? – Solvation shell?

Solvation and solvent stabilization of multiply-charged anions



50 60 70 80 90

HSO4- SO4-

n = 3 4 5 6 7 8 9 10 12 14 16

x3

(a)

Time of Flight (s) 115 120 125 130

100 105 110 115

20 22 24 26 28 30

36 38 40 34 32 30

(b)

(c)

ESI Mass Spectra of SO42-(H2O)n

Blades & KebarleJ. Am. Chem. Soc. 116, 10761 (1994)



10

9

8

7

6

5

4

Na+(SO42-)

6 0 1 2 3 4 5 Binding energy (eV)

HSO4-(H2O)n-1OH- X J. Chem. Phys. 113, 10837 (2000)

X A

X

A

PES of SO42–(H2O)n and solvent stabilization of SO4

2–

1 3 5 7 9 11 13Number of H2O molecules

-1

0

1

2

3

4

Bin

din

g E

ner

gy

(eV

)

Wang, Nicholas & Wang, J. Chem. Phys. 113, 10837 (2000)

VDE

ADE



1.88

1.

92

1.98

1.93

1.96

1.99

1.99

1.97

2.08

1.92

1.94

1.94

1.98

2.06 2.11

2.10

2.06

1.80

1.99

2.09

Wang, Nicholas & Wang, J. Chem. Phys. 113, 10837 (2000)

Calculated structures of SO42-(H2O)n, n = 1-6



PES of SO42-(H2O)n, n = 4-40

SO42-(H2O)n

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

0 1 2 3 4 5 6 7Binding Energy (eV)

SO42-(H2O)n

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

0 1 2 3 4 5 6 7Binding Energy (eV)

Wang, Yang, Nicholas & Wang Science 294, 1322 (2001)

Large hydrated clusters:a single sulfate dianion in

the center of a water droplet



Recent IR studies on SO42-(H2O)n

J. Chem. Phys. 125, 111102 (2006)Zhou, Santambrogio, Brummer, Moore,

Woste, Meijer, Neumark & Asmis

IR for n = 3-24 using FELIX at 17 K

J. Am. Chem. Soc. 129, 2220 (2007) Bush, Saykally & Williams

IR for n = 6 at OH stretching at 130 K



Isomers of SO42–(H2O)n

J. Am. Chem. Soc. 129, 2220 (2007) Bush, Saykally & Williams

Gao & Liu, J. Chem. Phys. 123, 224302 (2005)



Dissociation of SO42–(H2O)n for n = 3-17 (BIRD)

Wong & Williams J. Phys. Chem. A 107, 10976 (2003)

Two dissociation Channels:

SO42–(H2O)n → SO4

2–(H2O)n-k + kH2O (1)

→ HSO4– (H2O)k + (H2O)mOH– (2)

For n = 3, 4: (2) exclusively

n = 5, 6: both (1) and (2), but for n = 5, (2) dominates, and for n = 6, (1) dominates

n ≥ 7: (1) exclusively



Size-dependent charge separation reactions of SO4

2–(H2O)n for n = 3-7

Gao & Liu, J. Chem. Phys. 123, 224302 (2005)



Second generation ESI-PES apparatus at low-temperatures

5 m



Radiation shield 1st stage 50K

Adapter

ARS: DE-204 2nd stage 2.0 W@10K 9.0 W@20K Dia. 1.17 O.D.

AA

Sapphire

1/8

sst

tu

be

1.17

Indium foilRotable

Sealed by Indium wire

The low-temperature ion trap

Ion entrance Ion exit

Cold Head10 K

Rotatable



Wang, Woo & Wang, J. Chem. Phys. 123, 051106 (2005)

Hotbands

Vibrationally-cold photoelectron spectrum of C60–



Vibrationally resolved PES of cold C70-

2.0 3.0 4.0 5.0 6.0

Binding Energy (eV)

(a) 355 nm

(b) 266 nm

(c)193 nm

1.55 eV

Chem. Phys. Lett. 233, 52 (1995)

EA(C70) =

2.765 ± 0.010 eV

EA(C60) =

2.683 ±

0.008 eV

EA(C60) = 2.666 ± 0.001 eVEA(C70) = 2.676 ± 0.001 eV

Too low!!!



Photoelectron spectroscopy of cold C702-

0.0 1.0 2.0 3.0 4.0

Binding Energy (eV)

(b) 355 nm

(c) 266 nm

(a) 532 nm

1.55 eV

2.0 3.0 4.0 5.0 6.0

Binding Energy (eV)

1.55 eV

0.0 1.0 2.0 3.0 4.0

C702-

C702-

C70-

EA2(C70) = ADE(C702-) = 0.02 eV

Wang, Woo, Huang, Kappes & Wang, Phys. Rev. Lett. 96, 143002 (2006)



Electron-electron repulsion in C702-

HOMO e1”

LUMO

e1”

a1”

7.94

7.14

Å

Å

EA1(C70) – EA2(C70) =

2.765 – 0. 02 = 2.745 eV

e2/r =

1.8 to 2.0 eV

Wang, Woo, Huang, Kappes & Wang, Phys. Rev. Lett. 96, 143002 (2006)



AcknowledgmentsAcknowledgments

Dr. Xue-Bin Wang

Dr. Jie Yang

Dr. Chuan-Fan Ding (Fudan U.)

Dr. Xin Yang (Fudan U.)

Dr. You-Jun Fu (U. Kentucky)

Dr. Hin-Koon Woo (Scripps)

Dr. Tom Waters (U. Melbourne)

Supported by: DOE, NSF & Guggenheim Foundation

Collaboration:

J. B. Nicholas (theo)

M. M. Kappes (fullerenes)

Photoelectron Spectroscopy of Multiply-Charged Anions

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