THEORETICAL AND EXPERIMENTAL WATER COLLISIONS WITH NORMAL AND PARAHYDROGEN BRIAN J. DROUIN, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA ; LAURENT WIESENFELD, UJF-Grenoble 1/CNRS, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France.

Water in the Interstellar Medium Primary coolant Primary coolant Collisional excitation -> Radiative Pressure -> impedes collapse Abundant / perhaps in ice form though Abundant / perhaps in ice form though Ortho & Para Hydrogen Ortho generally has larger cross section Ortho generally has larger cross section Collisional excitation -> Radiative Pressure -> impedes collapse Water Collisions with Hydrogen6/22/2012

Radiative Pressure? Intermolecular collisions promote conversion from translational (i.e. gravitational) energy to heat or FIR light that escapes the cloud Water Collisions with Hydrogen6/22/2012

Two Methodologies Calculate Collisional Cross Sections from PES – Non-equilibrium OK – Results are known to be good for simple systems – Low symmetry creates computational burden Measure Collision Efficiencies via Pressure Broadening – Equilibrium only – Symmetry irrelevant – Absolute bound for theory Water Collisions with Hydrogen6/22/2012

Previous Experimental Work Dick et al PRA 2010 Dramatic Decrease in Collision Efficiency at low temperature Difficulties (for me) understanding theory Water Collisions with Hydrogen6/22/2012

Previous Theoretical Work Phillips et al 1996 Phillips et al 1996 – Rates only Wiesenfeld & Wiesenfeld & Faure PRA 2010 Faure PRA 2010 rates & cross-sections Water Collisions with Hydrogen6/22/2012

7Water Collisions with Hydrogen New Experimental Work (I) Cell designed for collisional cooling, pathlength is 15 cm, windows are mylar Temperature range is 18 – 200 K Modified Injector T(cell) = T(gas) (measured via Doppler width)

6/22/20128Water Collisions with Hydrogen New Experimental Work (II) pH2 generator (1) Cool cell to temperature between 25 and 40 K (2) Stabilize cell temperature with thermal resistance (3) Fill cell with buffer gas ( mTorr) (4) Open flow of H 2 O, minimize to prevent rapid accumulate ice (5) Perform spectral scan (6) Change pressure through addition or subtraction of buffer gas (7) Repeat spectral scan (8) Extract Lorentz width from linear linewidth plot (1) Cool cell to temperature between 18 and 25 K (2) Stabilize cell temperature with thermal resistance (3) Fill cell with buffer gas (10mTorr) (4) Open flow of H 2 O, minimize to prevent rapid accumulate ice (5) Fill with large amount of H 2 ( mT) (6) Wait for pressure to stabilize (5-10% drop from adsorption) (7) reduce pressure through subtraction of buffer gas (8) Perform spectral scan (return to step 7) (9) Extract Lorentz width from linear linewidth plot Modified Procedures

Why o/p Conversion?  High Density Amorphous water ice exists at < 68 K and  Contains ‘cracks’ that enable adsorption of gases  The longer the residence time, The more likely an interaction with a dipole T = 24.5 K Density (amagat) drops linearly in static system Linewidth (MHz) drops nonlinearly

New Theoretical Work More Transitions – ; ; Tighter Convergence Criteria necessary – Several months on CIMENT cluster Pressure Shift Computation – Imaginary portion, very sensitive to resonance 6/22/201210Water Collisions with Hydrogen

New Comparisons (broadening) 6/22/201211Water Collisions with Hydrogen Blue Circles Exp. nH 2 :H 2 O Blue Line Theory nH 2 :H 2 O Red Diamonds Exp. pH 2 :H 2 O Red Line Theory pH 2 :H 2 O Dashed Red Lineadd J = 2 Agreement better than 30% nearly everywhere for and Low temp. close for but divergent to 50% disagreement at 200 K

New Comparisons (shift) 6/22/201212Water Collisions with Hydrogen Blue Circles Exp. nH 2 :H 2 O Blue Line Theory nH 2 :H 2 O Red Diamonds Exp. pH 2 :H 2 O Red Line Theory pH 2 :H 2 O Temperature dependences of shifts are phenomenally well reproduced by theory Scale factors of x1-x2

New Comparisons (Power Law) For and broadening and shift cross sections agree to 30 % ! For the pH2 interactions are out % and oH2 diverge from 0-50 % 6/22/201213Water Collisions with Hydrogen J’ Ka’Kc’ J KaKc  PB0 (j=0) n  PB0 (j=1) n  PS0 (j=0) s  PS0 (j=1) s Exp.16.9(2)-0.30(3)110.1(13)-0.36(1)-0.09(5)+0.3(4)11.0(3)-0.61(3) Exp.8.5(8)+0.13(8)80(2)-0.24(2)4.8(2)-0.69(5)-25.3(13)-0.79(3) Exp.14.9(5)-0.20(3)99(2)-0.35(1)-10.9(8)-0.91(6)-7.4(70)-2.4(18)

Results Theoretical rates for low-symmetry systems can be reliable, but only with tight convergence criteria and thus expensive computational time. Experimental results must be carefully checked for systematic errors and compared thoroughly with theory.

Outlook New rates should be used for ISM Broadening/shift can be used for planetary atmospheres Ice studies for OPC Water Collisions with Hydrogen

Acknowledgements 6/19/2012Water Collisions with Hydrogen16 John Pearson Tim Crawford NASA - APRA