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

Columbus June 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

Neutral Fixed h n How to improve energy resolution of photoelectron spectroscopy?

ZEKE SEVI Tunable h n • • • 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 Anion

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

-350V -255V GND -200V -146V GND Mass-selected anion beam Pulsed MCP detector 1024x1024 • • • Flight tube: 50 cm μ-metal shielding (2 layers) 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  R 2 ) – Tune photon energy closer to a given transition threshold

Quadrant symmetrized SEVI image Inverse Abel transformed image

SEVI of Cl



Cl(

2

P

3/2

), Cl*(

2

P

1/2

)

Cl* Cl 2 P 3/2 1.0

0.8

0.6

0.4

0.2

0.0

0 50 2 P 1/2  r = 2.1 pixels  E = 2.8 cm -1 eKE = 23.3 cm -1  r = 2.3 pixels  E = 19 cm -1 eKE = 906 cm -1 100 150 200 Radius (pixels) 250 300

(m -1 )

SEVI of NeSˉ

Sˉ NeSˉ: D 0 =79 cm -1 NeS: D 0 =34 cm -1 X2-I1 splitting (A-B)=9 cm -1

SEVI of ArSˉ, KrSˉ

ArSˉ: D 0 =409 cm -1 ArS: D 0 =120 cm -1 A, B, E are X2, I1, II0 origins KrSˉ: D 0 =630 cm -1 KrS: D 0 =163 cm -1 A, B, G are X2, I1, II0 origins

SEVI of S

-

(D

2

)

j= 0 1 2 3 n 0 1 2 S (D 2 ) S D D Progressions in hindered rotor, S-D 2 stretch 17000 17200 17400 eBE (cm -1 ) 17600 17800

SEVI of C

n

Hˉ anions

• • • anions and neutrals seen in interstellar medium 2 even n: closely spaced  + , 2  states in neutral odd n: evidence for linear and cyclic isomers in anion, neutral PE spectra Taylor, 1998

C

4

H

-

(

1  +

)

 2  2  +

C

4

H (

2  +

and

2 

)

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

SEVI of C

n

Hˉ, odd n

• • Direct measurement of S-O splitting in X state of C 5 H (25 cm -1 ) and T 0 for a state (1.309 eV) FC simulations show anion has linear X 3  g ˉ ground 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 C n H 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: H 2 (35 K) or He (5K) Trapping time: 49 ms (20 Hz rep rate) Gas density:  3*10 13 cm -3

Determination of Ion Temperature

SEVI spectrum of C 5 ˉ

C

5  (

X

2  1/ 2,3/ 2 ) 

C X

5 ( 1 

g e

 3/2  =1/2 22900 22950 23000 eBE (cm -1 ) 23050 23100 Population of anion spin-orbit states (splitting 26.5 cm -1 ) serves as temperature probe. Distribution corresponds to 30K. Taken with He at 5K.

Impact of ion cooling on SEVI spectrum of S 3 ˉ (bent anion and neutral) trap at 35K EL Valve buffer gas H 2 buffer gas He 18000 19000 eBE [cm -1 ] 20000 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. 17000 18000 19000 20000 eBE [cm -1 ] 21000 For S 3 ˉ, the choice of buffer gas plays a crucial role. Both spectra were recorded at trap temperatures of 35K with very similar H 2 and He densities inside the ion trap.

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

• • • B3LYP/ aug-cc-pVTZ Harmonic frequencies C 2v geometry E rel (eV) 2.7

1.7

Radical: 2 B 1 Radical: 2 A 2 hv 0.0

Anion: 1 A 1

• • • • Cooled to 35 K with H 2 buffer gas in ion trap FC simulation, 130 cm -1 FWHM EA = 1.802(1) eV T 0 ≈ 0.86 eV

Overview

220 cm -1 FWHM

Closer look

20 cm -1 FWHM 11 cm -1 FWHM

Compare to simulation

• • • Non-FC allowed transitions Mix of s- and p-wave Vibronic coupling to 2 B 1 state?

Spectroscopy of reactive potential energy surfaces?

• • F CH 4 has a C 3v structure short F —HCH 3 bond – Near transition state of F + CH 4 reaction

F + CH

4

reaction

E assympt

Czako et al, JCP 2009.

Cheng et al. JCP 2011.

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

Comparison to Recent Published Results

F( 2 P 3/2 )CH 4 F( 2 P 1/2 )CH 4

SEVI overview Cheng et al.

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

FCH 4

SEVI of Fˉ CH

4

Bound van der

E asympt 29400 29500 29600 29700 eBE (cm -1 ) 29800 29900 30000 • • • Structure below E asympt 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

Cold, near threshold Fˉ CD

4 FCD 4 29400 29500 29600 29700 eBE (cm -1 ) 29800 29900 Distance between vertical lines 115 cm -1 • • • • See structure above E asympt associated with TS region Considerably less signal from vdW region Progression(s) at 115 cm -1 Assignment in progress (new data!)

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

Andreas Osterwalder Matt Nee Jia Zhou $$$ AFOSR

Many thanks:

Etienne Garand Tara Yacovitch Jongjin Kim Christian Hock

… and the rest of the group!

Why is SEVI spectrum of H

2

Fˉ so sensitive to photon energy?

• • • Detachment occurs by p-wave (l=1) Wigner threshold law comes into play Features at low eKE are less intense h n 1 h n 2   (

h

n 

E th

)  1 2

Bound van der Waals states

F + CH

4

ground state

Intermolecular stretch Hindered methyl rotation or intermolecular bend narrow: resonances?

• • • Tentative assignments : no TS simulations yet Large geometry differences Isotope effects