Spectroscopic Studies of the H H 2 Reaction at Astrophysically Relevant Temperatures Brian A. Tom, Brett A. McGuire, Lauren E. Moore, Thomas J. Wood, Benjamin J. McCall June 24, 20091University of Illinois at Urbana-Champaign

Astrophysical Motivation H H 2 → H 2 + H 3 + most common bimolecular reaction in the universe H 2 and H 3 + used as probes of ISM June 24, 20092University of Illinois at Urbana-Champaign ortho -H 3 + para -H 3 + Indriolo et al., ApJ., 671, 1736, (2007)

H H 2 → H 2 + H 3 + Is this really a reaction? –YES! June 24, 2009University of Illinois at Urbana-Champaign3 ½ + ½ + ½ = 3/2 ortho-H 3 + ½ + ½ - ½ = 1/2 para-H 3 + ½ + ½ = 1 ortho-H 2 ½ - ½ = 0 para-H cm cm -1 R(1,0) (ortho) cm -1 R(1,1) u (para) cm -1 Difference of ~ 0.32 cm -1

Reaction Dynamics June 24, 2009University of Illinois at Urbana-Champaign4 H H 2 → (H 5 + )* → H H 2 “identity” “hop” “exchange” H5+H if purely statistical: α = k hop /k exchange = 0.5

Nuclear Spin Considerations June 24, 2009University of Illinois at Urbana-Champaign5 o-H p-H 2 → p-H p-H 2 3/201/20≠  p2p2 p3p3 [para-H 2 ] [para-H 2 ] + [ortho-H 2 ] [para-H 3 + ] [para-H 3 + ] + [ortho-H 3 + ] 

High Temperature Model Based on work by Cordonnier and Oka Assumes all reactions are possible High energy Assume steady state for p 3 [H 2 ] >> [H 3 + ] June 24, 2009University of Illinois at Urbana-Champaign6 α +1+2 α p 2 3 α +2 p 3 ≡ [p-H 3 + ]/[H 3 + ] = p2p2 p3p3 ¼ ½ M. Cordonnier et al., J. Chem. Phys. 113, 3181 (2000)

Low Temperature Model Based on work done by Park and Light of Chicago Accounts for the energetic considerations of the reactions June 24, 2009University of Illinois at Urbana-Champaign7 Predicts a non-linear relationship between p 2 and p 3 Plug in α and T, model gives a curve which is compared to the data K. Park and J. C. Light, J. Chem. Phys., 126, (2007) p2p2 p3p3 ¼ ½

How do we get para-H 2 enrichment (p 2 )? June 24, 2009University of Illinois at Urbana-Champaign8 Method: B. A. Tom, S. Bhasker, Y. Miyamoto, T. Momose, B. J. McCall, Rev. Sci. Instr. 80, (2009) Helium Cryostat Enrichment controlled by T or mixing ~ 14 K > 99.9% p-H 2 Purity monitored by NMR and thermal conductance

How do we measure the para-H 3 + fraction (p 3 )? June 24, 2009University of Illinois at Urbana-Champaign9 Takayoshi Amano H2H2 pump Hollow cathode plasma cell ~310 K ~130 K ~180 K

Data Analysis June 24, 2009University of Illinois at Urbana-Champaign10 ανLανL ανLανL ItIt IoIo =e -ανL-ανL ανLανL |μ2||μ2|  nL p 3 =  n para  n para +  n ortho

Results – High Temperature Model June 24, 2009University of Illinois at Urbana-Champaign11

310 K June 24, 2009University of Illinois at Urbana-Champaign12

180 K June 24, 2009University of Illinois at Urbana-Champaign13

130 K June 24, 2009University of Illinois at Urbana-Champaign14

Experimental Conclusions α is dependant on T –k hop and k exchange not constant –Lower T → Lower α → exchange dominates Neither models work well –High temperature Linear relationship Ignores energetics –Low temperature Must use high temperatures to replicate data p 3 does not converge to 0.5 June 24, 2009University of Illinois at Urbana-Champaign15

Astronomical Observations June 24, 2009University of Illinois at Urbana-Champaign16

Future Work More low temperature measurements Refine our models for α More astronomical observations June 24, 2009University of Illinois at Urbana-Champaign17

June 24, 2009University of Illinois at Urbana-Champaign18 Acknowledgments Lauren Moore Brian Tom Tom Wood Team Hydrogen McCall Group

June 24, 2009University of Illinois at Urbana-Champaign19 Acknowledgments Lauren Moore Brian Tom Tom Wood Team Hydrogen McCall Group

Temperature June 24, 2009University of Illinois at Urbana-Champaign20