Observation of dipolebound state and highresolution photoelectron imaging

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 70 th 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 e. V 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 e. V) EA= 3. 47 ± 0. 01 e. V Wang, L. S. ; Ding, C. F. ; Wang. X. B. ; Nicholas, J. B. Phys. Rev. Lett. 1998, 81, 2667 EA = 3. 250 ± 0. 010 e. V Binding Energy (e. V) 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? Ø High-resolution anion photoelectron spectroscopy cold ion trap velocity-map imaging detector Ø Autodetachment from dipole-bound state from non-resonant to resonant transition

High-resolution anion photoelectron spectroscopy apparatus CH 3 COONa Me. OH/H 2 O=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 CH 3 COO- 3. 250 ± 0. 010 e. V 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 e. V 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 CH 3 COO • were calculated using the B 3 LYP/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? Ø High-resolution anion photoelectron spectroscopy cold ion trap velocity-map imaging detector Ø 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) dipole-bound state neutral state non-resonant t en hm ac et d to Au neutral ground state hν EA Binding energy of dipole-bound state hν anion dipole-bound ground state Binding energy non-resonant Final intensity = regular non+ 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 CH 3 COO • 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 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 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 Ø Used the high-resolution photoelectron imaging to determine the electron affinity of CH 3 COO • to be 3. 2528 ± 0. 0010 e. V; Ø Observed two vibrational levels of dipole-bound states for CH 3 COOand confirmed the electron affinity of CH 3 COO • ; Ø 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 3 D view of cold ion trap Two-stage closed cycle helium refrigerator Gifford-Mc. Mahon 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/H 2 Cold heads Displex. TM 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 = v t I. Leon, Z. Yang, H. T. Liu, L. S. Wang, Rev. Sci. Instrum. 85, 083106(2014) Photoelectron imaging of Au 2 -, at 590. 41 nm Raw imaging 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 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 CH 3 COOWigner’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 e. V 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 e. V 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 dipole-bound state Wavelength /nm neutral ground state EA Binding energy hν of dipole-bound state 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.