1. The low-temperature nuclear spin equilibrium of H 3 + in collisions with H 2 Kyle N. Crabtree, * Benjamin J. McCall University of Illinois, Urbana, IL Florian Grussie, Max H. Berg, Andreas Wolf, Holger Kreckel Max-Planck Institut für Kernphysik, Heidelberg, Germany Sabrina Gärtner, Stephan Schlemmer I. Physikalisches Institut, Universität zu Köln, Köln, Germany * Present address: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA

2. The setting: diffuse molecular clouds T: 50—70 K; n: 10 2 cm -3 ; Ionization: 10 -4 n(C + ) >> n(C) & n(CO); f(H 2 )  0.9 Chemistry dominated by cosmic ray ionization, ion-molecule reactions, and electron dissociative recombination  Per California Nebula

3. The players: H 3 + and H 2 para-H 2 ; I = 0 ortho-H 2 ; I = 1 ortho-H 3 + ; I = 3/2 para-H 3 + ; I = 1/2  E = 170 K  E = 32 K UV Absorption (Spitzer; FUSE) IR Absorption (Keck;UKIRT;VLT) Only chemical reactions can interconvert o/p spin modifications e.g. o-H 2 + H +  p-H 2 + H + ; o-H 3 + + p-H 2  p-H 3 + + o-H 2

4. The problem: “spin” temperature T(H 2 ) ≈ 50-75 K T(H 3 + ) ≈ 20-35 K H 3 + colder than H 2 ! Crabtree et al. (2011) ApJ 729, 15 ζ-Per X-Per λ-Cep HD 73882 HD 110432 HD 154368

5. Possible explanations Kinetic limit: H 2  H 2 + -----------------------------------Cosmic ray ionization H 2 + + H 2  H 3 + + H ----------------------Fast H 3 + formation H 3 + + H 2  H 2 + H 3 + ----------------------Incomplete thermalization H 3 + + e -  H 2 + H (or 3H) --------------Fast recombination Thermodynamic limit “identity” “hop” “exchange” H5+H5+  = k hop /k exch Nonthermal outcome at low T?

6. Previous H 3 + + H 2 studies Cordonnier et al. (2000) JCP 113, 3181Crabtree et al. (2011) JCP 134, 194310 & 194311  (450 K) = 2.4  (350 K) = 1.6  (135 K) = 0.5

7. Experimental strategy 5 cm He H2H2 1.Prepare H 2 with known o/p ratio (T 01 ) 2.Set trap temperature to T 01 3.Introduce and cool H 3 + 4.Add prepared H 2, allow to react to steady state 5.Measure H 3 + o/p ratio

8. Sample preparation/verification Fe(III) oxide catalyst Cryogenic container (10K) Raman spectroscopyPara hydrogen converter

9. LIR spectrometer Laser H2H2 H3+H3+ H3+H3+ He (buffer gas) Ar (probe gas) p-H 2 (variable p 2 ) H3+H3+ ArH + T trap = T H 2 (p 2 ) 500ms storage time laser on for the last 50ms ~ 500 H 3 + ions

10. Results p2p2 T 01 Each panel is >1 week of experiment time! Steady state verified: doubling storage time does not affect results.

11. Experimental complications H3+H3+ (H 3 + )* ArH + h Ar H2H2 ArH + + H 2  H 3 + + Ar H 3 + regenerated during laser interaction Net effect: H 3 + o/p ratio shifted towards ArH + + H 2 value

12. Summary of results

13. Consistent with prior experiments

14. Possible explanations Kinetic limit: H 2  H 2 + -----------------------------------Cosmic ray ionization H 2 + + H 2  H 3 + + H ----------------------Fast H 3 + formation H 3 + + H 2  H 2 + H 3 + ----------------------Incomplete thermalization H 3 + + e -  H 2 + H (or 3H) --------------Fast recombination Thermodynamic limit “identity” “hop” “exchange” H5+H5+  = k hop /k exch Nonthermal outcome at low T?

15. Kinetic limit in diffuse clouds… Still no satisfactory explanation