1. 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

2. 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

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

4. 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.

5. 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

6. 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?

7. 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)

8. 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

9. 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

10. Vinyl Chloride Photodissociation
Extensively studied at 193nm

11. 3-Center vs. 4-Center HCl Elimination
193 nm photolysis
Rotationally very hot

12. 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

13. Translational Distributions
for HCl (v,J) Levels

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

15. 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!

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

17. 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
Electron binding energy (eV)

18. 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

19. 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

20. 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!