Velocity Map Imaging Study of a Mass-Selected Ion Beam: the Photoinitiated Charge-Transfer Dissociation of and Ag + (C 6 H 6 ) Jonathon A. Maner, Daniel T. Mauney and Michael A. Duncan ISMS 2015 RE04

Important in organometallics, biochemistry, and catalysis – protein structure (Dougherty) – transition metal catalysts Electrostatic cation-pi interaction Condensed phase – UV-Vis, FTIR, NMR spectroscopy and X-ray diffraction Gas phase – UV photodissociation (Duncan) – CID TM + (bz), BE’s kcal/mol (Armentrout) – IR spectroscopy (Duncan, Lisy) – ZEKE, MATI (Yang) Charge transfer – UV photodissociation of M + (C 6 H 6 ), M = Ag, Bi, Cu, Fe, Mg (Duncan) Metal Ion-Benzene Complexes Gallivan, J. P. and Dougherty, D. A. Proc. Natl. Acad. Sci. USA, 2014, 96, Duncan, M. A. Int. J. Mass Spectrom. 2008, 272, Meyer, F. et al. J. Am. Chem. Soc. 1995, 117, Van Heijnsbergen, D. et al. J. Am. Chem. Soc. 2002, 124, Dibenzene chromium

Original Apparatus for Photodissociation Experiments Metal ion-C 6 H 6 complexes produced by laser vaporization in a supersonic expansion Ions are pulse extracted into a time-of-flight mass spectrometer Mass-selection with a pulsed deflection plates Laser excitation at turning region of the reflectron Parent and fragment ions are reaccelerated and detected with an EMT

Full Mass Spectrum Photodissociation Mass Spectrum (355 nm) Cu + (C 6 H 6 ) and Ag + (C 6 H 6 ) dissociate by charge transfer exclusively! Duncan, M. A. J. Phys. Chem. 1992, 96, Photodissociation of Cu + (C 6 H 6 ) and Ag + (C 6 H 6 ) Cu + (C 6 H 6 )C6H6+C6H6+ C6H6+C6H6+ Ag + (C 6 H 6 )

Photoexcitation Spectrum of Ag + (C 6 H 6 ) Duncan, M. A. J. Phys. Chem. 1992, 96, C 6 H 6 + yield 418 nm 418 nm is not enough to excite transitions of Ag + (126 nm) or benzene (260 nm) Charge transfer state at difference in IP D 0 ” ≤ hv − ∆IP For 418 nm and ∆IP = 1.67 eV, D 0 ” ≤ 1.30 eV or 30 kcal/mol Need to measure ∆E excess

Discrepancies in the literature Armentrout and coworkers determined the binding energy of Ag + (C 6 H 6 ) to be 37.4 kcal/mol – Threshold collision-induced dissociation in a guided ion beam mass spectrometer Bauschlicher reported a value of 36.5 kcal/mol using MCPF/DZP Our BE (30 kcal/mol) is lower – Ions may be internally excited – Multiphoton process Potential problems with CID – Relies extensively on kinetic modeling – BE’s will appear lower for internally excited ions – BE’s may appear higher due to the finite time scale for dissociation and incomplete energy transfer with the collision gas Armentrout, P. B. Chem. Phys. Lett. 1993, 210, Bauschlicher, C. W. et al. J. Phys. Chem. 1992, 96,

The VMI Experiment Measure angular distribution and displacement from center of mass Infer nature of transition and energetics involved Dick, B. Phys. Chem. Chem. Phys. 2014, 16, Whitaker, B., ed., Imaging In Molecular Dynamics: Technology and Applications, Cambridge University Press, UK, 2003.

Apparatus for Photofragment Imaging of Mass-Selected Ions Modified reflectron TOF MS for photodissociation experiments Incorporating VMI and slice imaging techniques NuACQ and FiniteSlice software (Suits Lab)

Calibration vs Ar + from Ar pixels For Ar 2 + D 0 ″ = 1.33 eV (Moseley) Dissociation at 355 nm (3.49 eV) Available excess energy 3.49 eV ‒ 1.33 eV = 2.16 eV Conservation of energy and momentum leads to 1.08 eV per fragment Velocity is 2280 m/s → cm → 363 pixels Resolution must be 40 pixels or better 40 pixels Moseley, J. T. et al. J. Chem. Phys. 1977, 67, 1659−1668.

Imaging C 6 H 6 + from Ag + (C 6 H 6 ) Ag + (C 6 H 6 ) nm → Ag + C 6 H 6 + Velocity of bz + is m/s KER(bz + ) = eV TKER = eV D 0 ″ ≤ eV – eV – eV = eV D 0 ″ ≤ 29.2 ± 1.2 kcal/mol Edge of signal using resolution from Ar 2 + Some vibrational and translational excitation Zero KE?

Imaging C 6 H 6 + from Ag + (C 6 H 6 ) Ag + (C 6 H 6 ) nm → Ag + C 6 H 6 + Maximum velocity of bz + is 1066 m/s KER(bz + ) = eV TKER = eV D 0 ″ ≤ 50.6 ± 3.1 kcal/mol → not much help Vibrational excitation of all bz + Intense central spot from bz + with no translational energy Some loss of anisotropy Edge of signal using resolution from Ar 2 + Some vibrational and translational excitation Translationally cold/ internally excited

Conclusion A new instrument for velocity map imaging of photofragments of a mass-selected ion beam has been constructed. This apparatus provides new information previously inaccessible by photodissociation experiments. We have used this apparatus to estimate the upper limit on the binding energy of Ag + (C 6 H 6 ) (29.2 ± 1.2 kcal/mol). This new apparatus will allow us to study the photodissociation dynamics of many systems that can be generated by laser vaporization or electrical discharge.

Acknowledgments The Duncan Lab Arthur Suits Hannah Reisler DOE