1. The H3+ + H2 Reaction; A Possible Mechanism for para- H3+ Enrichment in the Diffuse Interstellar Medium
Lieutenant Colonel Brian A. Tom, USAF
University of Illinois at Urbana-Champaign
Inaugural MWAM, 2008

2. H3+ - Abundant and Reactive
(Science, 279, 1910, 1998)
(Nature, 384, 334, 1996)
(ApJ, 183, L17-20, 1973)
(ApJ, 185, 505, 1973)
(Phys. Rev., 26, 44, 1925)

3. H3+ - An Astrophysical Probe
Ortho
Para
(3:1)
H3+
Ortho
Para
(2:1)
T. Oka, PNAS, 103, 12235, (2006)

4. Para-H3+ is Enriched in Diffuse Clouds
Texcitation measured
from H3+ is consistently
~ 40 K lower than H2 –
derived Trot ~ 70 K
This indicates para-H3+
is enriched to ~ 65%
ortho-H3+
para-H3+
Indriolo et al., Ap. J., 671, 1736, (2007)
The cause?
Reaction-induced spin modification
H3+ + H H2 + H3+

5. Theory of spin modification in H3+ + H2 →H2 + H3+

6. H3+ + H2 H2 + H3+ + + + Identity Hop Exchange
M. Cordonnier et al., J. Chem. Phys., 113, 3181, (2000)

7. H3+ + H2 H2 + H3+ ortho-H3+ + ortho-H2 ortho-H2 + ortho-H3+
para-H ortho-H → ortho-H para-H3+
Hop 2/ /3
Exchange / /3
para-H para-H → ortho-H para-H3+
Hop
Exchange / /3
ortho-H3+ + ortho-H2
ortho-H3+ + para-H2
para-H3+ + ortho-H2
para-H3+ + para-H2
ortho-H2 + ortho-H3+
ortho-H2 + para-H3+
para-H2 + ortho-H3+
para-H2 + para-H3+
T. Oka, J. Mol. Spec., 228, 635, (2004)

8. H3+ + H2 H2 + H3+ Problem Previous model does not account for energy
ortho-H2
para-H2
120 cm-1
Previous model does
not account for energy
differences between
ortho and para ground
states
Problem
Park and Light have
addressed this
K. Park and J. Light, J. Chem. Phys., 126, , (2007)

9. Experiment

10. Spectroscopy Temperature Para-H3+ fraction J 1 2 3 K
(ortho) (para) (para) (ortho) J
500 1 2 3
E(cm-1) K
R(1,0)
R(1,1)u
64 cm-1
169 cm-1
315 cm-1
237 cm-1
87 cm-1
Temperature
Para-H3+ fraction

11. Spectroscopy Pulsed Source Para-H2 generator Telescope InSb LiNbO3
Achromat
Glan Prism
Nd:YAG
λ/4
λ/2
AOM
Wavemeter
Ti:Sapphire
λ/2

12. Results

13. Temperature and Para Fraction

14. An Explanation for Enrichment?
0.79 ± 0.11
Using  = 0.51 ± 0.11, and  = 0.79 ± 0.11;
The average para-H3+ fraction measured from 9 diffuse cloud sightlines is 0.65 ± 0.15
Y. Sheffer et al., accepted for Astrophys. J. (2008)

15. Conclusions H3+ plays an important role in the interstellar medium
We have inferred  = kH/kE = 0.79 ± 0.11 for K
Could explain para-H3+ enrichment in diffuse ISM
More experiments are necessary to understand the underlying process
Texcitation by itself is not a reliable measure of rotational temperature in the ISM

16. Acknowledgments Andrew Mills, Kyle Crabtree, Michael Wiczer
McCall Group

17. Backup slides

18. H Motivation H2+ + H2 → H3+ + H H+ and H2+ He Cosmic Rays H2+ H3+ H+
O, C, N, Ne, Mg, Si, Fe…
T. R. Hogness and E. G. Lunn, Phys. Rev., 26, 44, (1925)
T. Oka, PNAS, 103, 12235, (2006)

19. DR - Spectroscopy