FE06 : Collisional Cross- Sections at Low Temperatures David L. Graff T. J. Ronningen F. C. De Lucia The Ohio State University

Low Energy Molecular Interactions

Typical Cross-Section as a Function of Temperature

Doppler Width –Std Dev < 1.5% Temperature –Std Dev < 3% Molecular Thermometry Low Temperature Molecules

Analysis: Extract from each Lineshape Line Shape Parameter Pressure Dependence Temperature Dependence Line Origin Independent Doppler Width IndependentSquare root Line Center Linear shiftNot analytical Lorentzian Width Linear increaseNot analytical

HC 14 N - He : Broadening / Shifting Experimental errors were reduced to 3% Data are compared to three PES Ronningen et al. JCP (2005)

Translation Stage P2P2 P1P1

Adjust Mirror Until Absorption “Fits” 0 Steps-2 Steps2 Steps

Fit Each Lineshape for Dispersion Functional Form is Voigt Absorption plus Lorentzian Dispersion term –Each term has the same line-center and Lorentzian linewidth

Include Dispersion in the Fit

Comparison of Methods The two analyses do not yield the same answer

Compare Cross-Sections Temp (K)Broadening (A 2 )Shifting (A 2 )Broadening (A 2 )Shifting (A 2 ) (0.5)-0.8(1.0)17.0(0.4)0.5(1.9) (0.6)-1.1(1.3)19.4(0.4)0.7(2.0) (0.6)-0.5(1.5)21.6(0.4)0.3(2.2) (0.7)-0.8(1.7)22.9(0.5)0.0(2.6) HC 15 NHC 14 N

Conclusions Low temperature pressure broadening measurements continue to disagree with theoretical predictions Careful consideration of our experimental variables have sharpened this difference We have improved our method of measuring the pressure shift parameter to gain greater precision

Acknowledgements Co-Authors –T.J. Ronningen –Frank De Lucia De Lucia Group –Brenda, Manfred –Ivan, Atsuko –Sarko, Kerra and Corey Funding –ARO and NSF

CaH Buffer Gas Trapping CaH -He(3) PES was calcuated by Balakrishnan et. Al. Diffusion rate was measured to be 2-8 times less than predicted values

Doppler Width as Thermometer Standard deviation of retrieved Doppler width from thermometer measurement is < 1.5% This corresponds to < 3% standard deviation in Temperature measurement

Linewidths retrieved from data simulated with incorrect transpiration corrections were not linear with pressure

CO - He : Broadening / Shifting Measurements extended from K Thachuk et al. compared data with four PES Agreement was very good down to 20K Thachuk et al. JCP

CO - He : Broadening / Shifting Measurements extended from K Thachuk et al. compared data with four PES Agreement was very good down to 20K Thachuk et al. JCP

Measured both Pressure Broadening / Shifting and Inelastic Depopulation Rates Substantial Disagreement Below 10K Ball, et al. JCP H 2 S - He : Broadening / Shifting

HCN - He Pressure Broadening