Slow Electron Velocity-map Imaging of Negative Ions: Applications to Spectroscopy

and Dynamics

ColumbusJune 2012

Spectroscopy and dynamics of free radicals, transition states, clusters

• Reactive free radicals play key role in combustion, planetary atmospheres, interstellar chemistry– Map out electronic and vibrational structure, with special focus on vibronic coupling

• Spectroscopy of potential energy surfaces for chemical reactions– Pre-reactive van der waals complexes– Transition state spectroscopy

• Clusters: evolution of properties of matter with size– Semiconductor clusters, metal oxides, water clusters, He droplets

• How do we do this? Anion photoelectron spectroscopy (PES) and its variants– Anion slow electron velocity-map imaging (SEVI), a high resolution version of PES– Combine with ion trapping and cooling to maximize resolution

How to improve energy resolution of photoelectron spectroscopy?

• Photoelectron spectroscopy– Very general, limited to 5-10 meV

• ZEKE (zero electron kinetic energy) spectroscopy– High resolution (0.1-0.2 meV)– Experimentally challenging– restricted to s-wave detachment

• SEVI– Resolution comparable to ZEKE

without expt’l complications– Versatile structural probe

An ion

Neu tral

eKE

Fixed hn

Tunable hn

ZEKE SEVI

The ZEKE Queen

SEVI apparatus

• Adaptation of ideas by Chandler, Houston, Parker• Energy and angular distributions• Electrons with 300-500 meV fill detector• Very high resolution for the slow electrons

Slow electron velocity-map imaging

• Low VMI voltages, long flight tube– Photoelectrons with 4500 cm-1 (0.5 eV) or 2500 cm-1 (0.3 eV) fill the detector

• Optimized VMI conditions– Collinear geometry, pulsed detector– -metal shielding, large VMI electrodes, DC voltages only– Small interaction region, finely adjustable extraction voltage

• Best resolution for the slower electrons (E R2)– Tune photon energy closer to a given transition threshold

Flight tube: 50 cm

-350V -255V GND

μ-metal shielding (2 layers)

Mass-selected anion beam

-200V -146V GND

1024x1024

Pulsed MCP detector

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*

SEVI of NeSˉ

NeS : Dˉ 0=79 cm-1

NeS: D0=34 cm-1

X2-I1 splitting (A-B)=9 cm-1

Sˉ

(m-1)

SEVI of ArS , KrSˉ ˉ

ArS : Dˉ 0=409 cm-1

ArS: D0=120 cm-1

A, B, E are X2, I1, II0 origins

KrS : Dˉ 0=630 cm-1

KrS: D0=163 cm-1

A, B, G are X2, I1, II0 origins

SEVI of S-(D2)

17000 17200 17400 17600 17800

S-(D2)

Ele

ctro

n S

igna

l (ar

b. u

nits

)

eBE (cm-1)

j= 0 1 2 30 1 2

S

D

D

Progressions in hindered rotor, S-D2 stretch

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

PE spectra

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

2S+

2P

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

Progressions in bending modesvibronic coupling

Zhou, 2007B, C have different PAD’s

Zhou, 2007

SEVI of CnH , odd nˉ

• Direct measurement of S-O splitting in X state of C5H (25 cm-1) and T0 for a state (1.309 eV)

• FC simulations show anion has linear X3g ˉground state

Garand, Chem. Sci. 2010

Longer chains

Next generation of SEVI experiments:

• Peak widths in SEVI spectra of polyatomic molecular anions are typically 20-30 cm-1 wide (i.e. spin-orbit splitting in CnH ground state)

• Why is resolution worse than for atomic species?

• Ion temperature limits resolution– Unresolved rotational contours, incomplete

vibrational cooling• Implement anion trapping and cooling

Lai-Sheng Wang

Modified SEVI apparatus

Another view

Buffer gas: H2 (35 K) or He (5K)Trapping time: 49 ms (20 Hz rep rate)Gas density: 3*1013 cm-3

Determination of Ion Temperature

22900 22950 23000 23050 23100

elec

tron

sig

nal (

a.u.

)

eBE (cm-1)

SEVI spectrum of C5¯

Population of anion spin-orbit states (splitting 26.5 cm-1) serves as temperature probe. Distribution corresponds to 30K. Taken with He at 5K.

2 15 1/2,3/2 5( ) ( )gC X C X e S

=1/2

3/2

17000 18000 19000 20000 21000

elec

tron

sig

nal [

a.u.

]

eBE [cm-1]

buffer gas H2

buffer gas He

18000 19000 20000

elec

tron

sig

nal [

a.u.

]

eBE [cm-1]

trap at 35K EL Valve

Comparison of SEVI spectra recorded with ions that come straight from the Even-Lavie Valve and ions that have been thermalized in the rf trap at 35K.

For S3¯, the choice of buffer gas plays a crucial role. Both spectra were recorded at trap temperatures of 35K with very similar H2 and He densities inside the ion trap.

Impact of ion cooling on SEVI spectrum of S3¯ (bent anion and neutral)

Indenyl Radical

• Combustion intermediate– acetylene-oxygen-argon

flames

• Intermediate in the formation of PAHs

Marinov, N. M.; Castaldi, M. J.; Melius, C. F.; Tsang, W. Combust. Sci. Technol. 2007, 128, 295.

Calculations

Anion: 1A10.0

Erel (eV)

Radical: 2A2

Radical: 2B1

1.7

2.7

hv

• B3LYP/ aug-cc-pVTZ• Harmonic frequencies• C2v geometry

Overview

• Cooled to 35 K with H2 buffer gas in ion trap

• FC simulation, 130 cm-1 FWHM

• EA = 1.802(1) eV• T0 ≈ 0.86 eV

220 cm-1 FWHM

Closer look

20 cm-1 FWHM 11 cm-1 FWHM

p-wave

Compare to simulation

• Non-FC allowed transitions

• Mix of s- and p-wave• Vibronic coupling to 2B1

state?

Spectroscopy of reactive potential energy surfaces?

Czako et al, JCP 2009.

F + CH4 reaction

• F-CH4 has a C3v structure

• short F-—HCH3 bond– Near transition

state of F + CH4

reaction

Cheng et al. JCP 2011.

K. Liu et al: evidence for reactive resonances in correlated product distributions (PRL, 2004)

Eassympt

Comparison to Recent Published Results

Cheng, M.; Feng, Y.; Du, Y. K.; Zhu, Q. H.; Zheng, W. J.; Czako, G.; Bowman, J. M. J. Chem. Phys. 2011, 134.

SEVI overview

F(2P3/2)CH4

F(2P1/2)CH4

Cheng et al.

SEVI of F¯ CH4

• Structure below Easympt is from bound states

• Structure at higher eBE is from transition state region

• Partially-resolved features; combination of internal rotor and C-F stretch expected

29400 29500 29600 29700 29800 29900 30000

elec

tron

sig

nal (

a.u.

)

eBE (cm-1)

FCH-

4

Bound van der Waals states

Easympt

Cold, near threshold F¯ CD4

• See structure above Easympt associated with TS region

• Considerably less signal from vdW region

• Progression(s) at 115 cm-1

• Assignment in progress (new data!)

29400 29500 29600 29700 29800 29900

elec

tron

sig

nal (

a.u.

)

eBE (cm-1)

FCD-

4

Distance between vertical lines 115 cm-1

Summary

• SEVI offers “next generation” of anion photodetachment experiments– First technique that systematically improves resolution

of anion PES without sacrificing (much) generality• Where are we headed?

– Cold ions via trapping/cooling– Bare and complexed metal/semiconductor clusters– Pre-reactive complexes and transition states (in

progress)– Theory needed to simulate TS spectra, vibronic coupling

Many thanks:Etienne Garand

Tara Yacovitch

JongjinKim

ChristianHock

$$$AFOSR

Andreas Osterwalder

Matt NeeJia Zhou

… and the rest of the group!

Why is SEVI spectrum of H2F so sensitive to ˉphoton energy?

• Detachment occurs by p-wave (l=1)

• Wigner threshold law comes into play

• Features at low eKE are less intense

12( )thh E

h1 h2

F + CH4 ground state

• Tentative assignments : no TS simulations yet

• Large geometry differences• Isotope effects

Bound van der Waals states

Hindered methyl rotationor intermolecular bend

narrow: resonances?

Intermolecular stretch