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

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

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

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. 68 3477 (1997) 4

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

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

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

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).

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)

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

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).

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)

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).

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).

Acknowledgements Post Doc Dr. Jie Wei Matt. Van Duzor Foster Mbaiwa Funding NSF Grant # CHE 0748738 Joshua Lasinski Nicholas Holtgrewe Diep Dao Stephen T. Gibson Stephen J. Cavanagh Brenton R. Lewis