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
How to improve energy resolution of
photoelectron spectroscopy?
eK E
• Photoelectron spectroscopy
– Very general, limited to 5-10 meV
ZEKE
SEVI
• ZEKE (zero electron kinetic
energy) spectroscopy
– High resolution (0.1-0.2 meV)
– Experimentally challenging
– restricted to s-wave detachment
N e u tra l
• SEVI
Fixed
hn
A n io n
Tunable
hn
– Resolution comparable to ZEKE
Thewithout
ZEKE Queen
expt’l complications
– Versatile structural probe
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
Pulsed MCP detector
Mass-selected
anion beam
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 R2)
– Tune photon energy closer to a given transition threshold
SEVI of Cl-  Cl(2P3/2), Cl*(2P1/2)
Quadrant
symmetrized
SEVI image
2P
3/2
1.0
Cl*
Cl
0.8
r = 2.3 pixels
0.6
E = 19 cm
2P
1/2
0.4
eKE = 906 cm
r = 2.1 pixels
Inverse Abel
transformed
image
E = 2.8 cm
0.2
-1
eKE = 23.3 cm
-1
0.0
0
50
100
-1
150
200
Radius (pixels)
250
300
-1
SEVI of NeSˉ
Sˉ
(m-1)
NeSˉ: D0=79 cm-1
NeS: D0=34 cm-1
X2-I1 splitting (A-B)=9 cm-1
SEVI of ArSˉ, KrSˉ
ArSˉ: D0=409 cm-1
ArS: D0=120 cm-1
A, B, E are X2, I1, II0 origins
KrSˉ: D0=630 cm-1
KrS: D0=163 cm-1
A, B, G are X2, I1, II0 origins
SEVI of S-(D2)
j= 0 1
n 0
12
2
3
Electron Signal (arb. units)
-
S (D2)
D
S
D
Progressions
in hindered
rotor, S-D2
stretch
17000
17200
17400
-1
eBE (cm )
17600
17800
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
PE spectra
Taylor, 1998
C4H-(1+)  C4H (2+ and 2)
2
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 CnHˉ, odd n
• 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 X3gˉ
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 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
SEVI spectrum of C5¯


C 5 ( X  1/ 2 ,3 / 2 )  C 5 ( X  g )  e
2
1

electron signal (a.u.)
3/2
=1/2
22900
22950
23000
23050
23100
-1
eBE (cm )
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
S3¯ (bent anion and neutral)
buffer gas H2
trap at 35K
EL Valve
electron signal [a.u.]
electron signal [a.u.]
buffer gas He
18000
19000
20000
-1
eBE [cm ]
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
21000
-1
eBE [cm ]
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.
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
Erel (eV)
• B3LYP/ aug-cc-pVTZ
• Harmonic frequencies
• C2v geometry
2.7
Radical: 2B1
1.7
Radical: 2A2
hv
0.0
Anion: 1A1
Overview
220 cm-1 FWHM
• 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
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 2B1
state?
Spectroscopy of reactive potential energy surfaces?
F + CH4 reaction
• F-CH4 has a C3v
structure
• short F-—HCH3
bond
– Near transition
state of F + CH4
reaction
Eassympt
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(2P3/2)CH4
F(2P1/2)CH4
Cheng et al.
SEVI overview
Cheng, M.; Feng, Y.; Du, Y. K.; Zhu, Q. H.; Zheng, W. J.; Czako, G.; Bowman, J. M. J. Chem. Phys. 2011,
134.
SEVI of F¯ CH4
Bound van der
Waals states
-
electron signal (a.u.)
FCH4
Easympt
29400
29500
29600
29700
29800
-1
eBE (cm )
29900
30000
• Structure below
Easympt is from
bound states
• Structure at higher
eBE is from
transition state
region
• Partially-resolved
features;
combination of
internal rotor and CF stretch expected
Cold, near threshold F¯ CD4
-
electron signal (a.u.)
FCD4
29400
29500
29600
29700
29800
29900
-1
eBE (cm )
Distance between vertical lines 115 cm-1
• 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!)
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
Jongjin
Kim
Andreas Osterwalder
Tara Yacovitch
Matt Nee
Jia Zhou
$$$
AFOSR
Christian
Hock
… and the rest of the group!
Why is SEVI spectrum of H2Fˉ 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
hn1
hn2
  ( hn  E th )
 12
Bound van der
Waals states
F + CH4 ground state
Intermolecular stretch
Hindered methyl rotation
or intermolecular bend
narrow: resonances?
• Tentative assignments : no
TS simulations yet
• Large geometry differences
• Isotope effects