1. Superoxide Photoelectron Angular Distributions: Vibrational Dependence as a Consequence of Born-Oppenheimer Behavior
Richard Mabbs, Matthew Van Duzor, Foster Mbaiwa, Jie Wei
Department of Chemistry, Washington University in St. Louis
Stephen T. Gibson, Steven J. Cavanagh, Brenton R. Lewis
Research School of Physical Sciences and Engineering, ANU, Canberra

2. Outline Photoelectron Imaging
Photoelectron Angular Distribution – Signature of Parent Orbital
H− and I- Photodetachment
Vibrational Effects on the Angular Distribution
O2− Photodetachment 2

3. Anion Imaging TOF Mass Spectrometer
Detection Region
Ion source Chamber
CCD Camera
Pulsed Nozzle
Imaging
detector
e- gun e-
Ion optics
Ion beam
detector
MCP ion
Pulsed repeller
Ion Time of Flight MS
ns tunable dye Laser 3

4. Imaging a Quantum Object
Accumulated Image is ≡ Electron Event Density:
≡ Probability Density of Electron Location
Frame by Frame
Image Accumulation
E.g. Eppink & Parker; Rev. Sci. Instrum (1997) 4

5. Navigating the Image: E.g. I− + hν → I + e−
2P3/2 Angular Distribution r 
εp Z
I−, 280 nm
Inverse Abel Transform
E.W. Hansen, P.L. Law; J. Opt. Soc. Am. A 2 (1985) 510
Photoelectron Spectrum
(280 nm) 1 2
eKE / eV
I(2P3/2)←I−(1S0)
I(2P1/2)←I−(1S0)
I()  1 + β·P2(cos )
β - Anisotropy Parameter
+ve: I() Max at 0 & 180°
−ve: I() Max at 90°
eKE = hν − eBE 5

6. PAD Interpretation “Simple” Example – H− Detachment
Image ≡│yfree│2
yfree related to initial ybound by
selection rules: D1, Dm=0
Angular Components: Y00 Y10 hn
l = 787 nm H- H + e-
Angular Part
Probability Density
2D - Projection z y x
Y10  cos q z y x
│Y10*Y10│ cos2 q εp
Max
-Max
Mabbs, R.; Grumbling, E. R.; Pichugin, K.;Sanov, A. Chem. Soc. Rev. 2009, 38, 2169

7. PAD Interpretation I− Detachment
 = 1; m = 1,0  ′ = 2,0; m′ = 1,0
s & d wave interference
Cooper & Zare J.Chem.Phys.48 (1968) 942; J.Chem.Phys 49 (1968) 4252
Rℓ+1/Rℓ−1 = A·eKE
f = δℓ+1− δℓ−1 relative phase
D. B. Hanstorp et.al. Phys. Rev. A 40 (1989) 670
eKE independent
initial decrease, then rise to asymptotic limit
Limiting values −1 ≤ β ≤ +2
β(0) = +2
β(0) = 0
β(0) = +0.2
Parent Orbital
threshold s p d

8. Molecular Anion Photodetachment
O2- pg – HOMO,
d-like orbital:  = 2
O2 X2Σg− ← O2− X2Πg
β values by vibronic transition
396 nm
488 nm
Atomic model fit
Change Wavelength
Expect points
to conform to curve
780 nm
F. A. Akin, L. K. Schirra, and A. Sanov, J. Phys. Chem. A 110, 8031 (2006).
R. J. Celotta et al. Phys. Rev. A 6, 631 (1972).

9. PADs for O2 X2Σg− ← O2− X2Πg O2− Detachment O2− : 454 nm ANU
WU St. Louis
M. Van Duzor et al. Phys. Rev. A. Rapid Communications (in Press)

10. Photodetachment between 900 and 440 nm
PADs for O2 X2Σg− (v') ← O2− X2Πg (v = 0)
Photodetachment between 900 and 440 nm
Experimental
Comparison
v' = 2
v' = 1
v' = 3
v' = 4
O2− anion β(eKE) trends depend
on level of vibrational excitation of neutral O2

11. Vibrational dependence, why?
, (4)
Differential Cross Section
One Electron Approximation
ϕk – Plane Wave
ϕdo – United Atom d - Orbital
eBE
R. M. Stehman and S. B. Woo, Phys. Rev. A 23, 2866 (1981).

12. Vibrational Dependence: Insight
Angular Distribution
Determining Factors
Ehν
eBE
eKE
Ehν
K. M. Ervin, I. Anusiewicz, P. Skurski, J. Simons, and W. C. Lineberger, J. Phys. Chem. A 107, 8521 (2003)
P. H. Krupenie, J. Phys. Chem. Ref. Data 1, 423 (1972)

13. Average over all Ωm – extract β
Vibrational Dependence: Insight
Average over all Ωm – extract β 5 4 2 3 1
v' =0
R. M. Stehman and S. B. Woo, Phys. Rev. A 23, 2866 (1981).

14. Molecular PAD Indicate
Summary
Average over all Ωm – extract β
eBE is ROO dependent
Each Vibronic Transition Selects a subset of ROO
Trends in β(eKE) reflect parametric dependence of ϕi on ROO
Molecular PAD Indicate
Parent Orbital & Coupling of Electronic and Vibrational Degrees of Freedom
R. M. Stehman and S. B. Woo, Phys. Rev. A 23, 2866 (1981).

15. Acknowledgements Post Doc Dr. Jie Wei Matt. Van Duzor Foster Mbaiwa
Funding
NSF Grant # CHE
Joshua Lasinski
Nicholas Holtgrewe
Diep Dao
Stephen T. Gibson
Stephen J. Cavanagh
Brenton R. Lewis