1 / 40

New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 +

New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 +. Ben McCall. Dept. of Chemistry. Dept. of Astronomy. Outline. Background Importance of H 3 + Interstellar Clouds H 3 + in Diffuse Clouds Abundance: H 3 + + e - → H + H + H

heller
Download Presentation

New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 +

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H3+ Ben McCall Dept. of Chemistry Dept. of Astronomy

  2. Outline • Background • Importance of H3+ • Interstellar Clouds • H3+ in Diffuse Clouds • Abundance: H3+ + e-→ H + H + H • Ortho/Para: p-H3+ + e-→ H + H + H H3+ + H2 → H3+ + H2 • H3+ in Dense Clouds • Abundance: H3+ + O → OH+ + H2 • Puzzle: H3+ + O2 ↔ HO2+ + H2

  3. H He Ne O C N Si S Ar Mg Fe Astronomer's Periodic Table

  4. H3+ + O  H2 + OH+ OH+ + H2  H + H2O+ H2O+ + H2  H + H3O+ H3O+ + e-  H2O + H NH3 Si NH2 H2CO CH H2O C2 C OH CO Proton Affinity (eV) CH4 CO2 N2 O H2 O2 N H3+: Cornerstone of Interstellar Chemistry

  5.  Persei Interstellar Cloud Classification Dense molecular clouds: • H  H2 • C  CO • n(H2) ~ 104–106 cm-3 • T ~ 20 K Diffuse clouds: • H ↔ H2 • C  C+ • n(H2) ~ 101–103 cm-3 • [~10-15 Torr] • T ~ 50 K Snow & McCall ARAA, 44, 367 (2006) Pound ApJ 493, L113 (1998) Photo: Jose Fernandez Garcia

  6. Outline • Background • Importance of H3+ • Interstellar Clouds • H3+ in Diffuse Clouds • Abundance: H3+ + e-→ H + H + H • Ortho/Para: p-H3+ + e-→ H + H + H H3+ + H2 → H3+ + H2 • H3+ in Dense Clouds • Abundance: H3+ + O → OH+ + H2 • Puzzle: H3+ + O2 ↔ HO2+ + H2

  7. Steady State [H2]  (310-17 s-1) [H3+] = =  (2400) [e-] ke (510-7 cm3 s-1) Diffuse Cloud H3+ Chemistry Formation cosmic ray  [H2] H2 H2+ + e- H2 + H2+  H3+ + H Rate = Destruction Rate = ke [H3+] [e-] H3+ + e-  H + H2 or 3H dense cloud value = 10-7 cm-3 L ~ 3 pc ~ 1019cm N(H3+) ≡ L × [H3+] ~ 1012 cm-2  ΔI/I ~ 0.01%

  8. Lots of H3+ in Diffuse Clouds! Cygnus OB2 12 HD 183143 N(H3+) ~ 1014 cm-2?!? McCall, et al. ApJ 567, 391 (2002)

  9. (order of magnitude)  [H2] [H3+] = [e-] ke Big Problem with the Chemistry! ^ Steady State: • To increase the value of [H3+], we need: • Smaller electron fraction [e-]/[H2] • Smaller recombination rate constant ke • Higher ionization rate  ruled out by observations

  10. Enigma of H3+ Recombination • Laboratory values of ke have varied by 4 orders of magnitude! • Problem: not measuring H3+ in ground states ke (cm3 s-1) Larsson, McCall, & Orel Chem. Phys. Lett., in press

  11. H3+ 20 ns 45 ns H, H2 electron beam Ion Storage Ring Measurements CRYRING • Very simple experiment • Complete vibrational relaxation • Control H3+– e- impact energy • Rotationally cold ions from supersonic expansion source 900 keV 12.1 MeV 30 kV

  12. CRYRING Results • Considerable amount of structure (resonances) in the cross-section • ke = 2.6  10-7 cm3 s-1 • Factor of two smaller McCall et al. Nature 422, 500 (2003)

  13. Agreement with Other Work • Reasonable agreement between: • CRYRING • Supersonic expansion • TSR • 22-pole trap • Theory S.F. dos Santos, V. Kokoouline, and C. H. Greene, J. Chem. Phys. 127 (2007) 124309

  14. Astrophysics!!  [H2] [H3+] = [e-] ke Big Problem with the Chemistry! Steady State: • To increase the value of [H3+], we need: • Smaller electron fraction [e-]/[H2] • Smaller recombination rate constant ke • Higher ionization rate  =7.410-16 s-1 (25× higher thandense clouds!) N. Indriolo, T. R. Geballe, T. Oka, & B. J. McCall, ApJ 671, 1736 (2007)

  15. Low Energy Cosmic Rays? • Flux below <1 GeV essentially unconstrained • magnetic field due to solar wind • Large low E flux can reproduce observations! 1 MeV 2 MeV 10 MeV 20 MeV 50 MeV (diffuse) (dense) Photo: M.D. Stage, G. E. Allen, J. C. Houck, J. E. Davis, Nat. Phys. 2, 614 (2006) N. Indriolo, B. D. Fields & B. J. McCall, in preparation

  16. Outline • Background • Importance of H3+ • Interstellar Clouds • H3+ in Diffuse Clouds • Abundance: H3+ + e-→ H + H + H • Ortho/Para: p-H3+ + e-→ H + H + H H3+ + H2 → H3+ + H2 • H3+ in Dense Clouds • Abundance: H3+ + O → OH+ + H2 • Puzzle: H3+ + O2 ↔ HO2+ + H2

  17. R(1,1) R(1,0) + + go No -ΔE/kTex e = gp Np ΔE H3+ Ortho/Para Ratio Cygnus OB2 12 para I = 1/2 ortho I = 3/2 Tex ~ 27 K but Tkin ~ 60 K Why?

  18. experiment para H2 normal H2 para-H3+ + e- vs. ortho-H3+ + e- TSR • para-H3+ fraction • unknown (~0.55?) theory para-H3+ ortho-H3+ K Theory: S.F. dos Santos, V. Kokoouline, and C. H. Greene, J. Chem. Phys. 127, 124309 (2007) Experiment: H. Kreckel, et al. Phys. Rev. Lett. 95, 263201 (2005)

  19. Recent CRYRING Results 85% p-H3+ [100% p-H3+] ×2! 50% p-H3+ [100% o-H3+] • Big ortho-para difference • But ortho/para H3+ may be equilibrated by H3+ + H2 collisions B. Tom et al., in preparation

  20. Outline • Background • Importance of H3+ • Interstellar Clouds • H3+ in Diffuse Clouds • Abundance: H3+ + e-→ H + H + H • Ortho/Para: p-H3+ + e-→ H + H + H H3+ + H2 → H3+ + H2 • H3+ in Dense Clouds • Abundance: H3+ + O → OH+ + H2 • Puzzle: H3+ + O2 ↔ HO2+ + H2

  21. H3+ + H2→ (H5+)* → H3+ + H2 1 “identity” H5+ 3 “hop” 6 if purely statistical: α = hop/exchange = 0.5 “exchange”

  22. D2d C2v Dynamical Effects C2v ~3000 cm-1 ~1500 cm-1 ~50 cm-1 “hop” “exchange” Not obvious that “statistical” α = hop/exchange = 0.5 is valid!

  23. Energetic Effects • Angular momentum restrictions • e.g. p-H3+ + p-H2→ o-H3+ + p-H2 • At low T in pure p-H2, slow p-H3+→ o-H3+ 1/2  0 ↔ 3/2  0 ortho I = 3/2 para I = 1/2 ortho I = 1 170 K para I = 0

  24. Cordonnier et al. JCP 113, 3181 (2000) hop ~2.4 exch T ~ 400 K α = ≠ 0.5! Oka Group Experiments Pulsed Hollow Cathode Positive Column Cell p-H3+ n-H2 p-H2 o-H3+ p-H3+ o-H3+ p-H2 n-H2 How does α vary with T?

  25. Pinhole flange/ground electrode -450 V ring electrode H3+ Supersonic Expansion Ion Source Gas inlet 2 atm H2 • H3+ formed near nozzle • [p-H2] / [H2] fixed • [H2] / [H3+] >> Ncollisions • [p-H3+] / [H3+] reaches steady state in few coll. • [p-H3+] / [H3+] measured spectroscopically Solenoid valve McCall et al. PRA 70, 052716 (2004)

  26. 2.8 – 4.8 mm DFG System Nd:YAG 1064 nm 532 nm pump laser l/4 Ti:Sapph 700 – 990 nm l/2 AOM 25cm 20cm reference cavity l/2 PPLN InSb mode- matching lenses ringdown cavity Glan prism 20cm achromat dichroic

  27. Cavity Ringdown Spectra • First results from our DFG laser! • Clear enhancement of para-H3+ in para-H2 • More enhanced in argon dilution • Trot ~ 80 K • R(1,1)u vs R(2,2)l ortho-H3+ para-H3+

  28. H3+ + H2 Results o/p H3+ ratio not thermal, but steady state of H3+ + H2 Tex (Oka) ζPersei Tkin~60 K α=2.5 α=1.0 α=0.5 80 K Park & Light JCP 126, 044305 (2007)

  29. Outline • Background • Importance of H3+ • Interstellar Clouds • H3+ in Diffuse Clouds • Abundance: H3+ + e-→ H + H + H • Ortho/Para: p-H3+ + e-→ H + H + H H3+ + H2 → H3+ + H2 • H3+ in Dense Clouds • Abundance: H3+ + O → OH+ + H2 • Puzzle: H3+ + O2 ↔ HO2+ + H2

  30. CH H2O C2 C OH CO Proton Affinity (eV) CH4 CO2 N2 O H2 O2 O Ne N N H Ne He H3+ in Dense Clouds • Relatively few electrons • C → CO • H3+ destroyed by proton transfer • CO • O, O2? ? C CO

  31. Dense Cloud H3+ Chemistry Formation cosmic ray ζ [H2] H2 H2+ + e- H2 + H2+  H3+ + H Rate = (fast) 210-9 cm3 s-1 Destruction Rate = kCO [H3+] [CO] H3+ + CO  HCO+ + H2 = Rate = kO [H3+] [O] H3+ + O  OH+ + H2 ?? Steady State 0.810-9 cm3 s-1 [H2] ζ (310-17 s-1) [H3+] = =  (6700) [CO] kCO (210-9 cm3 s-1) = 10-4 cm-3 McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999) L ~ 1 pc ~ 3×1018cm → N(H3+) ~ 3×1014 cm-2

  32. H3+ + O → OH+ + H2 • At T<50, kO kCO •  ζor L ↑ by factor of ~2 Stephen Klippenstein (2008) Tcloud Ryan Bettens (1999)

  33. Outline • Background • Importance of H3+ • Interstellar Clouds • H3+ in Diffuse Clouds • Abundance: H3+ + e-→ H + H + H • Ortho/Para: p-H3+ + e-→ H + H + H H3+ + H2 → H3+ + H2 • H3+ in Dense Clouds • Abundance: H3+ + O → OH+ + H2 • Puzzle: H3+ + O2 ↔ HO2+ + H2

  34. H3+ + O2↔ HO2+ + H2 • HO2+ is last simple protonated species yet to be observed spectroscopically • O2 difficult to observe in dense clouds; HO2+ may be a useful tracer? • Nearly thermoneutral formation reaction • Our work: • Re-examine thermochemistry • Calculate spectroscopic constants S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, in preparation

  35. Thermochemical Calculations • Active Thermochemical Tables (ATcT) • PA0 K(O2) = 417.18 ± 0.11 kJ/mol • PA0 K(H2) = 417.78 ± 0.01 kJ/mol • ΔrE0 = 0.60 ± 0.11 kJ/mol = 50 ± 9 cm-1 • Ab initio calculations • ΔEe valence complete basis set (CBS) limit: +222.1 cm-1 • ΔEe core-valence contribution +28.3 • harmonic vibrational ZPE correction -199.5 • anharmonic vibrational ZPE correction +76.4 • rotational ZPE correction -63.0 • ΔE0 net +64.3 cm-1 Branko Ruscic (Argonne) Dave Woon (Illinois) S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, in preparation

  36. n ( O ) + + = n ( HO ) 2 K n ( H ) 2 T 3 n ( H ) 2 Interstellar Abundance of HO2+ DrH°298 = 1.31 ± 0.11 kJ/mol DrG°298 = -1.75 ± 0.11 kJ/mol = 2 × (10-4 cm-3) × (10-4) N(HO2+) = n(HO2+) L ~ (2×10-8 cm-3)(3×1018 cm) ~ 6×1010 cm-2 (likely undetectable) S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, in preparation

  37. HO2+ Spectroscopic Constants S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, in preparation

  38. http://bjm.scs.uiuc.edu http://astrochemistry.uiuc.edu Acknowledgments H3+ Observations: Takeshi Oka (U. Chicago) Tom Geballe (Gemini) Storage Ring Measurements: Mats Larsson (Stockholm) Richard Thomas (Stockholm) Cosmic Ray Theory: Brian Fields (Illinois) H3+ + H2: Kisam Park (U. Chicago → TTU) H3+ + O: Stephen Klippenstein (Argonne) H3+ + O2: Susanna Widicus Weaver (Illinois → Emory) Dave Woon (Illinois) Branko Ruscic (Argonne) Michael Wiczer [and many others] Andrew Mills Kyle Crabtree Nick Indriolo Brian Tom NSF Chemistry, AMO Physics Critical Research Initiative NASA Laboratory Astrophysics

  39. Spin-Modification Probability Products formed by Hop and Exchange Reactants Park & Light JCP 126, 044305 (2007)

More Related