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

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)

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

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

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).

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

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

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 340-270 nm, β values extracted I− • CH3CN I−

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

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

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

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=350-270 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−

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

Conclusions Photoelectron angular distributions were extracted over a 0-1.3 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).

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