theoretical and experimental water collisions with normal and parahydrogen brian j. drouin, jet...
TRANSCRIPT
THEORETICAL AND EXPERIMENTAL WATER COLLISIONS WITH NORMAL AND
PARAHYDROGEN
BRIAN J. DROUIN, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099;
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
Collisional excitation -> Radiative Pressure -> impedes collapse
• Abundant / perhaps in ice form though
Ortho & Para Hydrogen
• Ortho generally has larger cross sectionCollisional 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– Rates only
• Wiesenfeld & Faure PRA 2010
rates & cross-sections
Water Collisions with Hydrogen6/22/2012
6/22/2012 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)
6/22/2012 8Water Collisions with Hydrogen
New Experimental Work (II)
pH2 generator
0 50 100 150 200
10
100
PB (Å2
)
Temperature (K)
(1) Cool cell to temperature between 25 and 40 K(2) Stabilize cell temperature with thermal resistance(3) Fill cell with buffer gas (50-1500 mTorr)(4) Open flow of H2O, 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 H2O, minimize to prevent rapid accumulate ice(5) Fill with large amount of H2 (1500-4000 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 Amorphouswater 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
0 20 40 60 80 100 1200
0.5
1
1.5
2
2.5
3
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
4.1
4.2
w (MHz)
T = 24.5 K
Density (amagat) dropslinearly in static system
Linewidth (MHz)drops nonlinearly
New Theoretical Work• More Transitions
– 101-110; 111-000; 202-111• Tighter Convergence Criteria necessary
– Several months on CIMENT cluster• Pressure Shift Computation
– Imaginary portion, very sensitive to resonance
6/22/2012 10Water Collisions with Hydrogen
New Comparisons (broadening)
6/22/2012 11Water Collisions with Hydrogen
Blue Circles Exp. nH2:H2OBlue Line Theory nH2:H2ORed Diamonds Exp. pH2:H2ORed Line Theory pH2:H2ODashed Red Line add J = 2
Agreement better than30% nearly everywhere for111-202 and 101-110
Low temp. close for000-111 but divergent to 50% disagreement at 200 K
New Comparisons (shift)
6/22/2012 12Water Collisions with Hydrogen
Blue Circles Exp. nH2:H2OBlue Line Theory nH2:H2ORed Diamonds Exp. pH2:H2ORed Line Theory pH2:H2O
Temperature dependences of shifts are phenomenally well reproduced by theory
Scale factors of x1-x2
New Comparisons (Power Law)
For 202-111 and 101-110 broadening and shift cross sections agree to 30 % !For 111-000 the pH2 interactions are out 40-80 % and oH2 diverge from 0-50 %
6/22/2012 13Water Collisions with Hydrogen
J’Ka’Kc’
JKaKc
sPB0
(j=0)n sPB0
(j=1)n dPS0
(j=0)s dPS0
(j=1)s
101-110 21.6 -0.560 125.6 -0.464 1.86 -0.27 18.7 -0.83
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)
111-000 4.82 -0.269 84.5 -0.404 2.09 -0.73 -6.49 -0.71
Exp. 8.5(8) +0.13(8) 80(2) -0.24(2) 4.8(2) -0.69(5) -25.3(13) -0.79(3)
202-111 12.52 -0.202 144.3 -0.490 -6.44 -0.35 -4.71 -1.14
Exp. 14.9(5) -0.20(3) 99(2) -0.35(1) -10.9(8) -0.91(6) -7.4(70) -2.4(18)
s
PSPB
n
PBPB
TT
TT
20
20
0
0
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
6/19/2012 Water Collisions with Hydrogen 16
John Pearson
Tim Crawford
NASA - APRA