Implications of the H H 2  H 2 + H 3 + reaction for the ortho- to para-H 3 + ratio in interstellar clouds Kyle N. Crabtree, Lt. Col. Brian A. Tom, USAF, Carrie A. Kauffman, Brett A. McGuire, and Benjamin J. McCall University of Illinois 22 March

Overview  H 3 + in interstellar clouds  Symmetry, Nuclear Spin, and H 3 +  H H 2  H 2 + H 3 +  Experimental Details  Results

Periodic Table

Astronomer’s Periodic Table

H 3 + : Why is it important?  Simplest polyatomic species– theoretical benchmark  Dominant ionic species in hydrogenic plasma  Low proton affinity  Cornerstone of gas- phase ion-molecule 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 3 + Chemistry  Formation: 1. H 2 + cosmic ray  H e - (slow) 2. H H 2  H H (fast)  Destruction: H e -  H 2 + H or 3H (diffuse clouds) H CO  HCO + + H 2 (dense clouds)

Astronomical Spectroscopy of H 3 + R(1,0) Å R(1,1) u Å B. J. McCall Ph.D. Thesis, University of Chicago (2001).

B. J. McCall, T. R. Geballe, K. H. Hinkle, and T. Oka ApJ (1999), 522, H 3 + Spectroscopy N. Indriolo Private Communication

H 3 + Temperature  Observed R(1,0) and R(1,1) u lines  T ex  T ex = 30 K in both diffuse and dense clouds  T 01 (H 2 J=0,1 states) = 60 K in diffuse clouds  Dense cloud temperatures: K

Overview  H 3 + in interstellar clouds  Symmetry, Nuclear Spin, and H 3 +  H H 2  H 2 + H 3 +  Experimental Details  Results

H 3 + Symmetry S3*S3* E(12)(123)E*E* (12) * (123) * A1+A A2+A E+E A1-A A2-A E-E E+E+ A1+A1+ para ortho

Nuclear Spin Constraints on Rotational States

 Ortho and para-H 3 + are distinct species  T ex ≠ temperature  n (1,0) /n (1,1) related to ortho/para ratio  “Low” T ex  overabundance of para-H 3 + H H 2  H 2 + H 3 + reaction allows H 3 + population to transfer between ortho and para spin configurations

Overview  H 3 + in interstellar clouds  Symmetry, Nuclear Spin, and H 3 +  H H 2  H 2 + H 3 + › Reaction Outcomes › High Temperature › Low Temperature  Experimental Details  Results

H H 2  H 2 + H 3 + “identity” “hop” “exchange” H5+H not well understood: branching ratio α = hop/exchange quantum effects at low T simplest bimolecular reaction involving a polyatomic most common bimolecular reaction in the universe: ~10 52 s -1

Nuclear Spin Statistical Weights 1/2  0 = 1/2 3/2  0 = 3/2 + → para ortho H3+H3+ H2H2 Typeo-H 3 + p-H 3 + paraorthohop2/31/3 paraorthoexch.1/32/3 para hop01 para exch.1/32/3 p 3 ≡ [p-H 3 + ]/[total H 3 + ] p 2 ≡ [p-H 2 ]/[total H 2 ]  ≡ k hop /k exchange M. Cordonnier et al., J. Chem Phys (2000), 113, T. Oka, J. Mol. Spec. (2004), 228, 635.

High Temperature Model

Key Features Linear p 3 = 0.5 w/n-H 2 M. Cordonnier et al., J. Chem Phys (2000), 113, 3181.

Need for Another Model? p-H 2 ; J = 0 o-H 2 ; J = 1 ΔE = 170 K K. Park and J. Light, J. Chem. Phys. (2007), 126, Dynamics of Floppy Molecules Session 123N Moscone Center Thursday 1:30 pm Pub #705 Experimental measurements of the H H 2 → (H 5 + )* → H 2 + H 3 + reaction Kyle N Crabtree, Brian A Tom, Carrie A Kauffman, Brett A McGuire, Benjamin J McCall

Low Temperature Model

Key Features Curvature p 3 not necessarily 0.5 with n-H 2

Model Limitations  HT model only considers conservation of angular momentum; LT model adds energetic considerations  Neither model takes into account the H 5 + potential energy surface  LT model only uses rate coefficients from (1,0) and (1,1) states, not all ortho and para states

Outlook

Overview  H 3 + in interstellar clouds  Symmetry, Nuclear Spin, and H 3 +  H H 2  H 2 + H 3 +  Experimental Details › Difference Frequency Generation Laser › Para-H 2 Production › Supersonic Ion Source/cw-CRDS › Hollow Cathode/Direct Absorption  Results

Difference Frequency Generation Laser (DFG) Spectral Range: μm Output Power: μW

Para-H 2 Production 15 K >99.9% purity B. A. Tom, S. Bhasker, Y. Miyamoto, T. Momose, and B. J. McCall Rev. Sci. Inst (2009), 80, Ferric Oxide catalyst

Hollow Cathode Cell T = 130 – 300 K

Piezo Pulsed Supersonic Expansion Ion Source Piezo disc Plunger T < 130 K

Overview  H 3 + in interstellar clouds  Symmetry, Nuclear Spin, and H 3 +  H H 2  H 2 + H 3 +  Experimental Details  Results

Hollow Cathode: T=310 K

Hollow Cathode: T=180 K

Hollow Cathode: T=130 K

Supersonic Expansion: T=110 K

Low Temperature Model

Diffuse Cloud Observations  Survey of diffuse cloud sightlines with known H 2 (1)/(0) measurements  H 3 + measured in: › ζ-Per UKIRT (CGS4) › X-Per UKIRT (CGS4) › HD Gemini South (Phoenix)  More data from VLT (CRIRES) and Keck (NIRSPEC) UKIRT Gemini South

Diffuse Cloud Observations

Conclusions  Observed (1,1):(1,0) ratio  ortho:para- H 3 + ratio, not temperature  Likely represents steady state of H H 2 reaction, not thermalization  Decrease of  with temperature  H 3 + ortho:para ratio possibly allows determination of H 2 ortho:para ratio in dense clouds where H 2 not observable

Acknowledgements  McCall Research Group  Kisam Park  Funding: