1. The Photoelectron Angular Distribution as a Probe of Energetically Indistinguishable Channels in Photodetachment
Matthew Van Duzor, Richard Mabbs, Foster Mbaiwa and Jie Wei
OSU International Symposium on Molecular Spectroscopy
6/24/10

2. Outline Background: Experiment: Results & Interpretation:
I− • CH3CN Photodetachment and dipole bound state (DBS)
Experiment:
Photoelectron Imaging of I− • CH3X (X=CN, I, Br, Cl).
Results & Interpretation:
Evidence of autodetachment in photoelectron angular
distributions (PADs)

3. I−CH3CN Cluster Anion
2P3/2
SO splitting 0.94 eV
eBE = 3.54 eV
(0.48 eV > I−)
Transitions referred to as “2P3/2”, “2P1/2” throughout
Both I−, I−CH3CN display prominent spectral feature
I−, 273 nm
2P1/2
I− • CH3CN,
273 nm

4. I−CH3CN Photoelectron Images
Evolution on the 2P3/2 angular distribution is focus of present work
2P3/2
2P1/2 ε θ
Transitions are fit to
I(θ) ∝ C{1+βP2(cosθ)}
where θ is defined with respect to the linearly polarized electric (ε) field of the detachment laser
I− • CH3CN, 273 nm

5. CH3CN− Dipole Bound Anion
The I− • CH3CN cluster anion is a well known precursor of the CH3CN− dipole-bound state (DBS)
CH3CN μ = 3.92 D (μcritical ≥ 2.0 D)
photoneutral action spectrum
I−(1S0) • CH3CN
I(2P3/2) + CH3CN−
355 nm
VDE
Intensity peaks ~18 meV below 2P3/2 threshold
C.G. Bailey, C.E.H. Dessent, M.A. Johnson, K. H. Bowen, J. Chem. Phys. 104, 6976 (1996).
C.E.H. Dessent, C.G. Bailey, and M.A. Johnson, J. Chem. Phys. 103, 2006 (1995).

6. Total Cross Section: Near 2P1/2 Threshold
Cross section displays enhancement below the 2P1/2 channel opening
Measured position of peak ~17 meV below 2P1/2
Suggests DBS excitation similar to that below 2P3/2 channel
2P1/2 channel opening

7. Energetically equivalent!
Experimental Scheme
I(2P1/2) • CH3CN
I(2P1/2) • CH3CN−
355 nm Excitation near “2P3/2”:
I(2P3/2) • CH3CN−
279 nm Excitation near “2P1/2”:
I(2P1/2) • CH3CN−
I(2P3/2) • CH3CN + e− (~0.92 eV)
Energetically equivalent!
I(2P3/2) • CH3CN + e− (~0.92 eV)
Relaxation,
autodetachment
I(2P3/2) • CH3CN
I(2P3/2) • CH3CN−
I−(1S0) • CH3CN

8. I−CH3CN, I−: 2P3/2 Channel
2P1/2 channel opening
“Peak” β value occurs ~25 meV prior to 2P1/2 channel opening
I− and I− • CH3CN images were collected at photodetachment wavelengths from nm, β values extracted
I− • CH3CN I−

9. Detail: Near 2P1/2 Threshold
279 nm
278 nm
Evolution of PAD from
λ = 288 to 275 nm (~200 meV)
277 nm
280 nm
283 nm
281 nm
276 nm
275 nm
288 nm
285 nm
2P1/2 channel opening

10. Interpretation
Calculation of DBS illustrates spatial overlap of 2P1/2 excited I and DB orbital
e− localized long enough to allow SO relaxation
I(2P1/2)−•CH3X → I(2P3/2)•CH3CN + e−
aug-cc-pvdz/CRENBL

11. Sub-Critical μ: I−CH3X (X=I, Br, Cl)
2P3/2
I−, 280 nm
eBE=3.06
2P1/2
Molecular dipoles are in all cases <2 D.
I− • CH3X (X=1, Br, Cl) spectra display 2 prominent features
Splitting = 0.94 eV, consistent with I−
I− • CH3Cl,
280 nm
eBE=3.41,
μ = 1.89 D
I− • CH3Br,
280 nm
eBE=3.42,
μ = 1.82 D
I− • CH3I,
280 nm
eBE=3.43,
μ = 1.62 D

12. I−CH3X (X=I, Br, Cl): 2P3/2 Channel
I−CH3CN β “peak”
Change in position (eKE) and relative magnitude compared to I−CH3CN
I− and I−CH3X (X=I, Br, Cl) images were collected at λ= nm, β values extracted
2P1/2 channel opening
For all clusters, “peaks” appear near 2P1/2 channel opening
I− • CH3I
I− • CH3Br
I− • CH3Cl I−

13. Detail: Near I−CH3X (X=I, Br, Cl) 2P1/2 Threshold
Peak positions coincident with 2P1/2 channel opening
Suggests that threshold e− are “delayed” long enough to allow competition by autodetahcment
2P1/2 channel opening

14. Conclusions
Photoelectron angular distributions were extracted over a eV (eKE) range for I− • CH3CN, I− • CH3I, I− • CH3Br, I− • CH3Cl and I−.
Rapid variation in the I− • CH3X (X=CN, I, Br, Cl) β parameter at eKEs consistent with 2P3/2 photodetachment was observed at energies near the 2P1/2 channel opening
These features were attributed to autodetachment associated with the 2P1/2 excited state of the I moiety
General feature of I− • X clusters studied (X= polar molecule).

15. Foster Mbaiwa, Jie Wei and Richard Mabbs
Acknowledgements
Foster Mbaiwa, Jie Wei and Richard Mabbs
Funding: