Observation of dipole-bound state and high-resolution
photoelectron imaging of cold acetate anions
Guo-Zhu Zhu, Dao-Ling Huang, Lai-Sheng Wang,
Department of Chemistry, Brown University, Providence, RI
70th International Symposium on Molecular Spectroscopy
University of Illinois at Champaign-Urbana
Jun 22-26, 2015
Introduction
Large geometry change from anion to radical
Photoelectron spectroscopy
EA = 3.40 ± 0.30 eV
hν
-e
O-C-O angle, ~20° smaller
C-C length, 0.071 Å shorter
Franck-Condon
Principle
Very weak FC factor for
0-0 transition, difficult
to determine the EA.
Yu, D.; Rauk, A.; Armstrong, D. A. J. Chem. Soc., Perkin Trans. 2, 1994, 2207
At 355 nm (3.49 eV)
EA= 3.47 ± 0.01 eV
Wang, L. S.; Ding, C. F.; Wang. X. B.; Nicholas, J.
B. Phys. Rev. Lett. 1998, 81, 2667
EA = 3.250 ± 0.010 eV
Binding Energy (eV)
Lu, Z.; Continetti, R. E. J. Phys. Chem. A 2004, 108, 9962
X. B. Wang, H. K. Woo, L. S. Wang, B. Minofar, and P.
Jungwirth, J. Phys. Chem. A, 110, 5047 (2006)
How to do it better?
 High-resolution anion photoelectron spectroscopy
cold ion trap
velocity-map imaging detector
 Autodetachment from dipole-bound state
from non-resonant to resonant transition
High-resolution anion photoelectron spectroscopy apparatus
CH3COONa
MeOH/H2O=9:1
X.B. Wang, L.S.Wang, Rev. Sci. Instrum.
79, 073108 (2008)
M. Yamashita, J.B. Fenn, J. Phys. Chem., 1984, 88(20), 4451-4459
I. Leon, Z. Yang, H.T. Liu, L.S. Wang, Rev. Sci. Instrum.
85, 083106(2014)
High-resolution non-resonant photoelectron spectra of CH3COO-
3.250 ± 0.010 eV
X. B. Wang, H. K. Woo, L. S. Wang, B. Minofar, and
P. Jungwirth, J. Phys. Chem. A, 110, 5047 (2006)
At the same range, more
peaks were resolved.
EA = 3.2528 ± 0.0010 eV
D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, 091103 (2015)
Theoretical vibrational frequencies and assignment
Theoretical calculation
Peak assignment
Harmonic frequencies of CH3COO• were
calculated using the B3LYP/6-31+G(d,p)
method and Cs symmetry.
D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, 091103 (2015)
How to do it better?
 High-resolution anion photoelectron spectroscopy
cold ion trap
velocity-map imaging detector
 Autodetachment from dipole-bound state
from non-resonant to resonant transition
Dipole-bound state
Neutral molecules, with a dipole moment greater than about 2.0 D,
can bind an electron to form weakly bound anions by dipole field.
The dipole-bound anion can support excited dipole-bound states
near the detachment threshold.
phenoxide anion,
radical μ = 4.0 D
A. H. Zimmerman, J. I. Brauman, J. Chem. Phys. 66, 5823 (1977).
H. T. Liu, C. G. Ning, D. L. Huang, P. D. Dau, and L.
S. Wang, Angew. Chem., Int. Ed. 52, 8976 (2013)
K. R. Lykke, R. D. Mead, and W. C. Lineberger, Phys. Rev. Lett. 52, 2221 (1984)
neutral state
dipole-bound state
non-resonant
neutral ground state
hν
Binding energy
of dipole-bound
state
hν
anion dipole-bound
ground state
EA
Binding energy
Binding energy
non-resonant
Final intensity =
regular non+ resonant intensity
resonant intensity
anion ground state
Autodetachment propensity rule: Δν = -1
Autodetachment can
greatly enhance the intensity.
J. Simons, J. Am. Chem. Soc. 1981, 103, 3971
Photodetachment spectrum of CH3COOCH3COO• μ ~ 3.5 D
Photodetachment spectrum, 373.65-375.25 nm
Photoelectron spectrum at 372.68 nm
Scanned near the threshold of peak a and
b, we got the photodetachment spectrum
of CH3COO- and observed two vibrational
levels of dipole-bound states.
Used PGOPHER program to simulate
the rotational profile of peak 2 and got
the Trot=20-35 K
D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, 091103 (2015)
High-resolution resonant photoelectron spectra
Energy level diagram
Non-resonant spectrum
372.68 nm
374.27 nm
376.36 nm
Resonant spectra
hν
hν
D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, 091103 (2015)
0-0 transition peak intensity was greatly
enhanced, confirming the electron affinity.
Summary
 Used the high-resolution photoelectron imaging to determine the
electron affinity of CH3COO• to be 3.2528 ± 0.0010 eV;
 Observed two vibrational levels of dipole-bound states for CH3COOand confirmed the electron affinity of CH3COO•;
 The binding energy of the dipole-bound ground state was measured
as 53 (8) cm-1.
Acknowledgement
Prof. Lai-Sheng Wang
Daoling Huang
Dr. Gary Lopez
Thank you!
Resonant two-photon detachment for dipole-bound ground state
Energy level diagram
Scan below the detachment threshold
0.02 nm step
dipole-bound ground state
0.03 nm step
threshold
D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, 091103 (2015)
14
Principles for photoelectron spectroscopy
Franck-Condon principle
Geometry change from anion to radical
O-C-O bond angle
change:
~20° smaller
C-C bond length
change:
0.071 Å shorter
The peak intensity is proportional to the overlapping of the
initial and final wavefunctions (Franck-Condon factor).
Large geometry change results in a very weak FC factor, and
then weak peak.
Very weak FC
factor, weak 0-0
transition peak,
difficult to resolve
Yu, D.; Rauk, A.; Armstrong, D. A. J. Chem.
Soc., Perkin Trans. 2 1994, 2207
Cold ion trap
3D view of cold ion trap
Two-stage closed cycle
helium refrigerator
Gifford-McMahon cycle
The first stage can
achieve 25-45 K
The second stage can
achieve 4.2 K
Cooling the temperature
to 4.4 K, eliminating the
vibrational hot band.
The Paul trap is attached
to the second stage of the
cooling device.
Collision with He/H2
DisplexTM
Cold heads
models operating manual. Sumitomo
(SHI) Cryogenics of America, INC. June 2007
X.B. Wang, L.S.Wang, Rev. Sci. Instrum. 79, 073108 (2008)
Velocity-map imaging
Assume the interaction zone as a
point source
R=vt
I. Leon, Z. Yang, H.T. Liu, L.S. Wang, Rev. Sci. Instrum. 85, 083106(2014)
Photoelectron imaging of Au2-, at 590.41 nm
Raw imaging
Kinetic energy
KE = ½ mv2 = ½ m(R2/t2)
or
KE = a R2
a is a constant depending on the geometry
of the VMI spectrometer and the repeller
voltage. Using Au- and I- to calibrate.
BE = hv - KE
inverse-Abel
transformation
I. León, Z. Yang, and L. S. Wang, J. Chem. Phys. 138, 184304(6) (2013)
High-resolution non-resonant photoelectron spectra of CH3COOWigner’s threshold law
For one photon photodetachment
process, the cross section of the
transition is related with the angular
momentum of the outgoing electrons.
3.250 ± 0.010 eV
X. B. Wang, H. K. Woo, L. S. Wang, B. Minofar, and
P. Jungwirth, J. Phys. Chem. A, 110, 5047 (2006)
l=0, s-wave
=1, p-wave
=2, d-wave
At the same range, more
peaks are resolved.
EA = 3.2528 ± 0.0010 eV
E. P. Wigner, Phys. Rev. 73 (9), 1002 (1948)
K. J. Reed, A. H. Zimmerman, H. C. Andersen, and J. I. Brauman, J. Chem. Phys. 64 (4), 1368 (1976)
D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, 091103 (2015)
How to find the dipole-bound state
Scan the laser wavelength near the threshold
neutral state
resonant
regular non-resonant
electron signal electron signal
dipole-bound state
Wavelength /nm
neutral ground state
Binding energy hν
of dipole-bound
state
EA
anion dipole-bound
ground state
one-photon
detachment
resonant twophoton detachment
Total electron signal
Photodetachment spectrum
anion ground state
Peaks represent the dipole-bound states. At the wavelength corresponding to the
dipole bound state, the total electron signal is the sum of regular non-resonant and
resonant (from autodetachment) signal.
And the jump of the baseline shows the threshold of 0-0 transition.