Vinylidene! Stephen Gibson,1 Benjamin Laws,1 Ravin Fernando,2 A. G. Suits,2 and Robert W. Field3 1Australian National University, 2Wayne State University, 3MIT

It all starts with Lineberger’s PES spectrum of the C2H2- ion How long does it live? 400 fs or >3.5 μs? Coulomb Explosion Imaging 193 nm photolysis of Vinyl Cyanide: 3-Center makes HCN + H2C2 Chirped Pulse mm-Wave Spectrum: Where’s Vinylidene? Isomerization path: vinylidene in-plane wag acetylene local bend Rotation-Induced IVR on the Acetylene side of the barrier: Perry and Herman Heff: Essential to create non-rotating vinylidene Infrared Multiphoton Dissociation of Vinyl Chloride: Suits‘ translation-, vibration-, rotation-distribution in HCl Source of low-J rotational levels of vinylidene! Negative Ion Photoelectron Spectroscopy with Velocity Mapped Imaging: Gibson Vinylidene Structure AND Vinylidene-Acetylene Isomerization Mechanism

Vinylidene is the “elusive” high-energy isomer of acetylene Negative Ion Photo-Electron Spectrum

A beam of C2H2- is accelerated, crossed by a photodetachment laser beam, 3.5μs later neutral C2H2 impacts a thin foil on which all electrons are removed, and 6 fully stripped ions are detected as a single event on a multiparticle coincidence detector.

Is the Coulomb Explosion Interpretation Wrong? Eigenstates prepared by detachment of electron from C2H2− are 50:50 divided between acetylene and vinylidene geometric structures Eigenstates live forever The isomerization coordinate Vinylidene in-plane wag Acetylene local-bend mm-wave spectrum of 193 nm VCN photofragments Prozument CPmmW experiment Expect vinylidene and acetylene local-benders

The Law According to Quantum Mechanics Eigenstates of a time-independent Hamiltonian are stationary: no dynamics! Patterns of eigenstate energies and transition intensities encode dynamics The code is written in the eigenvectors So what is all of this nonsense about short lifetimes for vinylidene?

193 nm Photolysis of Vinyl Cyanide Detect HCN and HNC by CPmmW Spectroscopy, but no trace of vinylidene or local-bender acetylene HCN (v1,v2,v3) HNC [v1,v2,v3] J=0-1 Prozument et al., Faraday Discussion 163, 33 (2013)

How to prepare vinylidene Eigenstates? Challenge: rotation promotes IVR in vibrationally hot acetylene J=2 J=30 J=100 Vinylidene must be born rotationally cold Red curves are survival probability of local-bender character

UV Photolysis vs. IRMPD of VCl Arthur Suits’ group experiments Vinyl Chloride fragments unimolecularly into HCl and C2H2 (is it vinylidene?) 3 vs. 4-center mechanisms 193 nm vs. Infrared Multi-Photon Dissociation 193 nm photolysis is far above threshold IRMPD is near threshold

Vinyl Chloride Photodissociation Extensively studied at 193nm

3-Center vs. 4-Center HCl Elimination 193 nm photolysis Rotationally very hot

Suits group uses IRMPD to dissociate at 3-C threshold Probe using state-resolved imaging of HCl HCl Rotation Expt Theory 270K 510K spectrum rotational population distribution

Translational Distributions for HCl (v,J) Levels

IRMPD of Vinyl Chloride makes cold Vinylidene via 3-Center elimination of HCl Ideal for direct CPmmW detection of Vinylidene CPmmW results promised soon!

Steve Gibson at Australian National University Negative ion photoelectron spectrometer Up to 100x higher resolution than Lineberger’s for near threshold photodetachment Velocity Map Imaging of the ejected electron Higher sensitivity Angular distribution of ejected electron s, p, d, f character of ejected electron Magic happens!

Velocity Map Image of Photelectrons from C2H2− background removed raw image Each ring is a vibrational level of vinylidene Note angular dependence

C2H2 Anisotropy Parameter - From photoelectron measurements at 494nm, 579nm, 619nm, 639nm, and 656nm 0.2 00 PES 0.0 31 1 2 0 62,40 1 1 -0.2 2 03 0 2 3 0 ? β 2500 4460 8130 -1 -0.4 eBE (cm ) -0.6 ANU 00 Ervin et al. ---- p-like orbital -0.8 p-like 2 2 β(ε)=2Aε[Aε-2c]/[1+2A ε ] A=0.335, c=0.89 3 1 2 Electron binding energy (eV)

Above barrier (low eKE): acetylene? 250 o __ right horizontal angular slice 80°-100 2131 top vertical angular slice -20° - +20 o ? 200 Above barrier (low eKE): acetylene? x5 intensity 150 100 50 200 400 image radius (pixels) 600

What does Quantum Mechanics Allow? How do we design an experiment to see what we are allowed to see? Photodetachment of an electron “Franck-Condon” weighted coherent superposition on nonrotating vinylidene vibrational states UV photodissociation Superposition of high-J rotating vinylidene vibrational states Infrared Multi-Photon Dissociation Threshold weighted incoherent ensemble of low-J vinylidene vibrational states

Conclusions Lineberger was right in 1989 PES samples low-J Vinylidene Photodetachment of electron makes low-J Vinylidene PES samples low-J Vinylidene Lowest vibrational levels are of nearly pure vinylidene character Something happens at low eKE = high excitation energy Beta parameter changes Sharp structure in PES disappears IRMPD also makes low-J Vinylidene We are all waiting for the CPmmW spectrum!