New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 + Ben McCall Dept. of ChemistryDept. of Astronomy.

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New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 + Ben McCall Dept. of ChemistryDept. of Astronomy

Outline Background –Importance of H 3 + –Interstellar Clouds H 3 + in Diffuse Clouds –Abundance: H e - → H + H + H –Ortho/Para: p-H e - → H + H + H H H 2 → H H 2 H 3 + in Dense Clouds –Abundance: H O → OH + + H 2 –Puzzle: H O 2 ↔ HO H 2

Astronomer's Periodic Table H He CN O Ne Mg Fe SiS Ar

H 3 + : Cornerstone of Interstellar Chemistry N O2O2 H2H2 O N2N2 CO 2 CH 4 OH C C2C2 H2OH2O H 2 CO CH NH 2 Si NH 3 CO Proton Affinity (eV) H O  H 2 + OH + OH + + H 2  H + H 2 O + H 2 O + + H 2  H + H 3 O + H 3 O + + e -  H 2 O + H

Interstellar Cloud Classification Diffuse clouds: H ↔ H 2 C  C + n(H 2 ) ~ 10 1 –10 3 cm -3 –[~ Torr] T ~ 50 K  Persei Photo: Jose Fernandez Garcia Snow & McCall ARAA, 44, 367 (2006) Dense molecular clouds: H  H 2 C  CO n(H 2 ) ~ 10 4 –10 6 cm -3 T ~ 20 K Pound ApJ 493, L113 (1998)

Outline Background –Importance of H 3 + –Interstellar Clouds H 3 + in Diffuse Clouds –Abundance: H e - → H + H + H –Ortho/Para: p-H e - → H + H + H H H 2 → H H 2 H 3 + in Dense Clouds –Abundance: H O → OH + + H 2 –Puzzle: H O 2 ↔ HO H 2

Rate = k e [H 3 + ] [e - ]  [H 2 ] Diffuse Cloud H 3 + Chemistry H 2 H e - H 2 + H 2 +  H H cosmic ray H e -  H + H 2 or 3H Rate = Formation Destruction [H 3 + ]  = keke [e - ] Steady State [H 2 ] = (3  s -1 ) (5  cm 3 s -1 )  (2400) = cm -3 L ~ 3 pc ~ cm N(H 3 + ) ≡ L × [H 3 + ] ~ cm -2 dense cloud value  Δ I/I ~ 0.01%

Lots of H 3 + in Diffuse Clouds! HD McCall, et al. ApJ 567, 391 (2002) Cygnus OB2 12 N(H 3 + ) ~ cm -2 ?!?

Big Problem with the Chemistry! Steady State: [H 3 + ]  = keke [e - ] [H 2 ] To increase the value of [H 3 + ], we need: Smaller electron fraction [e - ]/[H 2 ] Smaller recombination rate constant k e Higher ionization rate  (order of magnitude) ^ ruled out by observations

Enigma of H 3 + Recombination Laboratory values of k e have varied by 4 orders of magnitude! Problem: not measuring H 3 + in ground states k e (cm 3 s -1 ) Larsson, McCall, & Orel Chem. Phys. Lett., in press

Ion Storage Ring Measurements 20 ns 45 ns electron beam H3+H3+ H, H 2 +Very simple experiment +Complete vibrational relaxation +Control H 3 + – e - impact energy +Rotationally cold ions from supersonic expansion source CRYRING 30 kV 900 keV 12.1 MeV

CRYRING Results Considerable amount of structure (resonances) in the cross-section k e = 2.6  cm 3 s -1 Factor of two smaller McCall et al. Nature 422, 500 (2003)

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)

Big Problem with the Chemistry! Steady State: [H 3 + ]  = keke [e - ] [H 2 ] To increase the value of [H 3 + ], we need: Smaller electron fraction [e - ]/[H 2 ] Smaller recombination rate constant k e Higher ionization rate  =7.4  s -1 (25× higher than dense clouds!) N. Indriolo, T. R. Geballe, T. Oka, & B. J. McCall, ApJ 671, 1736 (2007) Astrophysics!!

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

Outline Background –Importance of H 3 + –Interstellar Clouds H 3 + in Diffuse Clouds –Abundance: H e - → H + H + H –Ortho/Para: p-H e - → H + H + H H H 2 → H H 2 H 3 + in Dense Clouds –Abundance: H O → OH + + H 2 –Puzzle: H O 2 ↔ HO H 2

H 3 + Ortho/Para Ratio + ortho I = 3/2 para I = 1/2 + Cygnus OB2 12 NoNo NpNp gogo gpgp e -ΔE/kT ex = ΔEΔE R(1,0) R(1,1) T ex ~ 27 K but T kin ~ 60 K Why?

para-H e - vs. ortho-H e - Theory: S.F. dos Santos, V. Kokoouline, and C. H. Greene, J. Chem. Phys. 127, (2007) normal H 2 para H 2 experiment para-H 3 + ortho-H 3 + theory Experiment: H. Kreckel, et al. Phys. Rev. Lett. 95, (2005) TSR K para-H 3 + fraction unknown (~0.55?)

Recent CRYRING Results 85% p-H 3 + [100% p-H 3 + ] 50% p-H 3 + [100% o-H 3 + ] ×2! B. Tom et al., in preparation Big ortho-para difference But ortho/para H 3 + may be equilibrated by H H 2 collisions

Outline Background –Importance of H 3 + –Interstellar Clouds H 3 + in Diffuse Clouds –Abundance: H e - → H + H + H –Ortho/Para: p-H e - → H + H + H H H 2 → H H 2 H 3 + in Dense Clouds –Abundance: H O → OH + + H 2 –Puzzle: H O 2 ↔ HO H 2

H H 2 → (H 5 + )* → H H 2 “identity” “hop” “exchange” H5+H if purely statistical: α = hop/exchange = 0.5

Dynamical Effects C 2v D 2d C 2v “hop” “exchange” Not obvious that “statistical” α = hop/exchange = 0.5 is valid! ~3000 cm -1 ~50 cm -1 ~1500 cm -1

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

Oka Group Experiments o-H 3 + p-H 3 + Pulsed Hollow Cathode Positive Column Cell Cordonnier et al. JCP 113, 3181 (2000) p-H 2 n-H 2 o-H 3 + p-H 3 + n-H 2 p-H 2 hop exch ~2.4 T ~ 400 K α = ≠ 0.5! How does α vary with T?

Supersonic Expansion Ion Source H 3 + formed near nozzle [p-H 2 ] / [H 2 ] fixed –[H 2 ] / [H 3 + ] >> N collisions [p-H 3 + ] / [H 3 + ] reaches steady state in few coll. [p-H 3 + ] / [H 3 + ] measured spectroscopically H2H2 Gas inlet 2 atm Solenoid valve -450 V ring electrode Pinhole flange/ground electrode H3+H3+ McCall et al. PRA 70, (2004)

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

Cavity Ringdown Spectra First results from our DFG laser! Clear enhancement of para-H 3 + in para-H 2 More enhanced in argon dilution T rot ~ 80 K –R(1,1) u vs R(2,2) l ortho-H 3 + para-H 3 +

H H 2 Results α=2.5 α=1.0 α= K Park & Light JCP 126, (2007) ζ Persei T ex o/p H 3 + ratio not thermal, but steady state of H H 2 (Oka) T kin ~60 K

Outline Background –Importance of H 3 + –Interstellar Clouds H 3 + in Diffuse Clouds –Abundance: H e - → H + H + H –Ortho/Para: p-H e - → H + H + H H H 2 → H H 2 H 3 + in Dense Clouds –Abundance: H O → OH + + H 2 –Puzzle: H O 2 ↔ HO H 2

N O Ne H 3 + in Dense Clouds C CO N O2O2 H2H2 O N2N2 CO 2 CH 4 OH C C2C2 H2OH2O CH CO Proton Affinity (eV) H Ne He Relatively few electrons C → CO H 3 + destroyed by proton transfer –CO –O, O 2 ? ?

ζ [H 2 ] Dense Cloud H 3 + Chemistry H 2 H e - H 2 + H 2 +  H H cosmic ray H CO  HCO + + H 2 Rate = Formation Destruction Rate = k CO [H 3 + ] [CO] ζ [H 3 + ] = k CO [CO] Steady State = (3  s -1 ) (2  cm 3 s -1 ) [H 2 ]  (6700) = cm -3 (fast) McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999) L ~ 1 pc ~ 3×10 18 cm → N(H 3 + ) ~ 3×10 14 cm -2 H O  OH + + H 2 Rate = k O [H 3 + ] [O] 2  cm 3 s  cm 3 s -1 = ??

H O → OH + + H 2 Stephen Klippenstein (2008) Ryan Bettens (1999) At T<50, k O  k CO  ζ or L ↑ by factor of ~2 T cloud

Outline Background –Importance of H 3 + –Interstellar Clouds H 3 + in Diffuse Clouds –Abundance: H e - → H + H + H –Ortho/Para: p-H e - → H + H + H H H 2 → H H 2 H 3 + in Dense Clouds –Abundance: H O → OH + + H 2 –Puzzle: H O 2 ↔ HO H 2

H O 2 ↔ HO H 2 HO 2 + is last simple protonated species yet to be observed spectroscopically O 2 difficult to observe in dense clouds; HO 2 + 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

Thermochemical Calculations Active Thermochemical Tables (ATcT) –PA 0 K (O 2 ) = ± 0.11 kJ/mol –PA 0 K (H 2 ) = ± 0.01 kJ/mol –Δ r E 0 = 0.60 ± 0.11 kJ/mol = 50 ± 9 cm -1 Ab initio calculations –ΔE e valence complete basis set (CBS) limit: cm -1 –ΔE e core-valence contribution –harmonic vibrational ZPE correction –anharmonic vibrational ZPE correction –rotational ZPE correction –ΔE 0 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

Interstellar Abundance of HO 2 +  r H° 298 = 1.31 ± 0.11 kJ/mol  r G° 298 = ± 0.11 kJ/mol )H( )O( )H( n n nK T   )HO( 2 n  = 2 × (10 -4 cm -3 ) × (10 -4 ) N(HO 2 + ) = n(HO 2 + ) L ~ (2×10 -8 cm -3 )(3×10 18 cm) ~ 6×10 10 cm -2 (likely undetectable) S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, in preparation

HO 2 + Spectroscopic Constants S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, in preparation

Acknowledgments NASA Laboratory Astrophysics NSF Chemistry, AMO Physics H 3 + Observations: Takeshi Oka (U. Chicago) Tom Geballe (Gemini) Storage Ring Measurements: Mats Larsson (Stockholm) Richard Thomas (Stockholm) Cosmic Ray Theory: Brian Fields (Illinois) H H 2 : Kisam Park (U. Chicago → TTU) H O: Stephen Klippenstein (Argonne) H O 2 : Susanna Widicus Weaver (Illinois → Emory) Dave Woon (Illinois) Branko Ruscic (Argonne) Brian Tom Nick Indriolo Kyle Crabtree Michael Wiczer Andrew Mills [and many others] Critical Research Initiative

Spin-Modification Probability Total Io-H o-H 2 o-H p-H 2 p-H o-H 2 p-H p-H 2 o-H o-H 2 5/ /24/91/3605/125/9 00 1/21/9 008/92/902/3 o-H p-H 2 3/205/1201/411/300 p-H o-H 2 3/25/9 11/34/910/900 1/28/92/9001/917/1815/6 p-H p-H 2 1/202/30015/611/2 Reactants Products formed by Hop and Exchange Park & Light JCP 126, (2007)