70 th International Symposium on Molecular Spectroscopy University of Illinois at Champaign-Urbana Jun 22-26, 2015 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

Introduction Wang, L. S.; Ding, C. F.; Wang. X. B.; Nicholas, J. B. Phys. Rev. Lett. 1998, 81, 2667 Photoelectron spectroscopy EA = 3.40 ± 0.30 eV At 355 nm (3.49 eV) EA= 3.47 ± 0.01 eV Lu, Z.; Continetti, R. E. J. Phys. Chem. A 2004, 108, 9962 Binding Energy (eV) X. B. Wang, H. K. Woo, L. S. Wang, B. Minofar, and P. Jungwirth, J. Phys. Chem. A, 110, 5047 (2006) EA = ± eV Large geometry change from anion to radical Yu, D.; Rauk, A.; Armstrong, D. A. J. Chem. Soc., Perkin Trans. 2, 1994, 2207 O-C-O angle, ~20° smaller C-C length, Å shorter Very weak FC factor for 0-0 transition, difficult to determine the EA. hνhν - e Franck-Condon Principle

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 M. Yamashita, J.B. Fenn, J. Phys. Chem., 1984, 88(20), CH 3 COONa MeOH/H 2 O=9:1 X.B. Wang, L.S.Wang, Rev. Sci. Instrum. 79, (2008) I. Leon, Z. Yang, H.T. Liu, L.S. Wang, Rev. Sci. Instrum. 85, (2014)

High-resolution non-resonant photoelectron spectra of CH 3 COO - At the same range, more peaks were resolved. EA = ± eV ± eV D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, (2015) X. B. Wang, H. K. Woo, L. S. Wang, B. Minofar, and P. Jungwirth, J. Phys. Chem. A, 110, 5047 (2006)

Theoretical vibrational frequencies and assignment Harmonic frequencies of CH3COO were calculated using the B3LYP/6-31+G(d,p) method and Cs symmetry. Theoretical calculation Peak assignment D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, (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 anion ground state neutral ground state Binding energy neutral state dipole-bound state anion dipole-bound ground state EA Binding energy of dipole-bound state Autodetachment propensity rule: Δν = -1 + resonant intensity Final intensity = regular non- resonant intensity hνhν hνhν J. Simons, J. Am. Chem. Soc. 1981, 103, 3971 Autodetachment 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. K. R. Lykke, R. D. Mead, and W. C. Lineberger, Phys. Rev. Lett. 52, 2221 (1984) phenoxide anion, radical μ = 4.0 D H. T. Liu, C. G. Ning, D. L. Huang, P. D. Dau, and L. S. Wang, Angew. Chem., Int. Ed. 52, 8976 (2013) Autodetachment can greatly enhance the intensity. non-resonant A. H. Zimmerman, J. I. Brauman, J. Chem. Phys. 66, 5823 (1977).

Photodetachment spectrum of CH 3 COO - Scanned near the threshold of peak a and b, we got the photodetachment spectrum of CH 3 COO - and observed two vibrational levels of dipole-bound states. Used PGOPHER program to simulate the rotational profile of peak 2 and got the T rot =20-35 K Photoelectron spectrum at nm Photodetachment spectrum, nm CH3COO μ ~ 3.5 D D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, (2015)

High-resolution resonant photoelectron spectra Resonant spectra Non-resonant spectrum 0-0 transition peak intensity was greatly enhanced, confirming the electron affinity. Energy level diagram nm nm nm hνhν hνhν D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, (2015)

Summary  Used the high-resolution photoelectron imaging to determine the electron affinity of CH 3 COO to be ± eV;  The binding energy of the dipole-bound ground state was measured as 53 (8) cm -1.  Observed two vibrational levels of dipole-bound states for CH 3 COO - and confirmed the electron affinity of CH 3 COO;

Acknowledgement Prof. Lai-Sheng Wang Daoling Huang Dr. Gary Lopez Thank you!

14 Resonant two-photon detachment for dipole-bound ground state Energy level diagramScan below the detachment threshold 0.02 nm step 0.03 nm step threshold dipole-bound ground state D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, (2015)

Principles for photoelectron spectroscopy Franck-Condon principle 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. Geometry change from anion to radical Yu, D.; Rauk, A.; Armstrong, D. A. J. Chem. Soc., Perkin Trans , 2207 O-C-O bond angle change: ~20° smaller C-C bond length change: Å shorter Very weak FC factor, weak 0-0 transition peak, difficult to resolve

Cold ion trap The Paul trap is attached to the second stage of the cooling device. Collision with He/H 2 The first stage can achieve K The second stage can achieve 4.2 K Two-stage closed cycle helium refrigerator Gifford-McMahon cycle 3D view of cold ion trap X.B. Wang, L.S.Wang, Rev. Sci. Instrum. 79, (2008) Cold heads Displex TM models operating manual. Sumitomo (SHI) Cryogenics of America, INC. June 2007 Cooling the temperature to 4.4 K, eliminating the vibrational hot band.

Velocity-map imaging Assume the interaction zone as a point source R = v t Kinetic energy KE = ½ mv 2 = ½ m(R 2 /t 2 ) or KE = a R 2 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 I. Leon, Z. Yang, H.T. Liu, L.S. Wang, Rev. Sci. Instrum. 85, (2014) I. León, Z. Yang, and L. S. Wang, J. Chem. Phys. 138, (6) (2013) Photoelectron imaging of Au 2 -, at nm Raw imaging inverse-Abel transformation

High-resolution non-resonant photoelectron spectra of CH 3 COO - At the same range, more peaks are resolved. EA = ± eV ± eV D. L. Huang, G. Z. Zhu, L. S. Wang, J. Chem. Phys. 142, (2015) l=0, s-wave =1, p-wave =2, d-wave Wigner’s threshold law For one photon photodetachment process, the cross section of the transition is related with the angular momentum of the outgoing electrons. X. B. Wang, H. K. Woo, L. S. Wang, B. Minofar, and P. Jungwirth, J. Phys. Chem. A, 110, 5047 (2006) K. J. Reed, A. H. Zimmerman, H. C. Andersen, and J. I. Brauman, J. Chem. Phys. 64 (4), 1368 (1976) E. P. Wigner, Phys. Rev. 73 (9), 1002 (1948)

How to find the dipole-bound state anion ground state neutral ground state Wavelength /nm anion dipole-bound ground state resonant electron signal regular non-resonant electron signal Total electron signal Scan the laser wavelength near the threshold EA Binding energy of dipole-bound state resonant two- photon detachment one-photon detachment Photodetachment spectrum And the jump of the baseline shows the threshold of 0-0 transition. 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. neutral state dipole-bound state hνhν