1. The impact of long range interactions on low temperature pressure broadening: the case of OCS-He Daniel R. Willey & Kelly N. Salb Department of Physics Allegheny College Supported by the National Science Foundation with special thanks to R. L. Le Roy

2. The Mystery: Why do calculations using well-founded, state-of-the-art potential surfaces consistently disagree with sub - 10K pressure broadening experiments?

3. The Mystery: Why do calculations using well-founded, state-of-the-art potential surfaces consistently disagree with sub - 10K pressure broadening experiments? The Victims: - CO - He - H 2 S - He - HCN - He - OCS - He

4. Pressure broadening of OCS by He: 4.2 K to 23 K Experiment: Ross & Willey, JCP 122, 2005 Theory: Howson & Hutson, JCP 115, 2001 (CO-He, H 2 S-He and HCN-He all very similar) The Evidence:

5. - Large (~ 1000 Å 2 ) resonances at low energy (Feshbach, shape…) - Both inelastic & elastic collisions contribute - Thermal average smoothes resonances, but causes cross sections to rise at low temperature. Pressure broadening cross section vs collision energy (theoretical)

6. The Suspects:

7. Experimentalists: - Wrong temperature (cell vs gas)? - Wrong pressure (transpiration)?

8. The Suspects: Experimentalists: - Wrong temperature (cell vs gas)? - Wrong pressure (transpiration)? Theorists: - Breakdown of collision theory at low energy (resonances)?

9. The Suspects: Experimentalists: - Wrong temperature (cell vs gas)? - Wrong pressure (transpiration)? Theorists: - Breakdown of collision theory at low energy (resonances)? - Inaccurate potential at long range?

10. Will modifying the long range, asymptotic region of the surface reduce resonances and bring experiment and theory into closer agreement?

11. Ansatz: Reduce low energy resonances by forcing asymptotic region to approach zero more rapidly.

12. Will modifying the long range, asymptotic region of the surface reduce resonances and bring experiment and theory into closer agreement? Ansatz: Reduce low energy resonances by forcing asymptotic region to approach zero more rapidly. Two Modifications: Isotropic Modification (IM): reduction starts at same distance relative to CM for all angles. Anisotropic Modification (AM): reduction starts at same distance relative to local minima

13. Transformation from Jacobi (R,  ) coords to elliptical (   ) coords  - ‘distance-like’ coord  - ‘angle-like’ coord

14. V(R) for three angles:  = 0˚: helium at oxygen end  = 70˚: global minimum (IM and AM identical here)  = 180˚: helium at sulfur end

15. Original, Howson & Hutson Surface: largest, intermediate anisotropy IM surface: smallest, least anisotropic AM surface: intermediate size, most anisotropic

16. Calculated pressure broadening cross section vs collision energy - Resonances reduced for both IM and AM surfaces, more for IM. - Cross sections all converge near 20 cm -1

17. Pressure broadening cross sections vs temperature: Experiment and Theory - Both IM and AM surfaces show improved agreement - IM surface is best; drops at lowest temperatures - Cross sections merge near 25 K

18. Mystery Solved? Some loose ends: - Is it size or is it roughness? Or both? - Both AM and IM surfaces are smaller than H&H surface - Least anisotropic IM shows greatest decrease - Most anisotropic AM surface still shows decrease - Does either surface agree with observations of bound complex? - CO-He, H 2 S-He, HCN-He??