1. Experimental Investigation of H2 Singlet-ungerade States by Electron Impact
Xianming Liu, Paul V. Johnson, Charles P. Malone
Jason A. Young, Geoffrey K. James, and Isik Kanik
Donald E. Shemansky
Jet Propulsion Laboratory
California Institute of Technology

2. Outline Comparison of experiment and theory
Experimental setup
Localized ro-vibronic coupling
Ro-vibrational coupling via centrifugal potential
Continuum emission
Excitation and emission cross sections
Lyman and Werner bands
EF-X band
Planetary Applications
Jupiter aurora H2 emission modeling
Comet H2 spectral assignment
Saturn stellar occultation modeling

3. 3 m spectrometer and electron impact apparatus
Instrumental range and resolution (with a 1200 G/mm grating)
1st order – 64 mÅ, Å
2nd order – 32 mÅ, Å
3rd order – 21 mÅ, Å
(FWHM with e+H2 point source and 10m slits)

4. Transition labeled as J”(v’,v”)J.
Old model, used to analyze Voyager e+H2 data, failed to consider coupling between v=14 of the B state and v=3 of the C state.
New model uses semi-ab initio transition probabilities calculated by Abgrall et al.

5. The model uses band transition probabilities partitioned by Hönl-London factors.
Due to the low population at J”>1 levels, intensity difference understates the strong rotational dependence of transition probabilities.
The J’=0-7 of v’=6 & 7 levels of the B state do not couple to any levels in any significant way (100% to 99.7% pure).

6. Top figure
semi ab initio P- & R-branch A values
A values obtained from partitioning P(1) A-value with Hönl-London factors.
Bottom figure
strong rotational dependence of Frank-Condon factor (left axis)
Weak rotational dependence of the total A values (right axis, unit: 109 s-1)

7. New model uses semi ab initio A values calculated by Abgrall et al.

8. Lyman continuum profiles of P-branch transitions of v=9 of the B state

9. Top: continuum part of model based the calculated profiles of (v’, J’=0) levels partitioned by Hönl-London factors.
Bottom: continuum part based on the calculation for individual (v’,J’) levels

10. Mechanisms of H2 excitation by electron
Direct excitation
Cascade excitation
X to singlet gerade (eg. X to EF) followed by singlet gerade to ungerade state (eg EF to B)
contribute to all ungerade levels, but disproportionally to the low v levels of the B state.
Resonance excitation
formation and autoionization of H2 anion
primarily populate the low v levels of the B state

11. v’=8 of the B state has very small cascade and resonance contributions
v’=8 of the B state has very small cascade and resonance contributions. The measured shape function represents that of the direct excitation.

12. The v’= 1 level of the C state also has negligible cascade excitation
Note that the measurement was carried out at a single rovibronic threshold.

13. T = 300 K
Direct excitation cross section to the discrete level of the B and C state.
Excitation into the continuum levels of B and C states is ~1% of the values shown

14. v’ = 0 has significant contribution from resonance and cascade excitations
cascade excitation takes place via dipole-forbidden excitation of EF,GK,I,J and other singlet gerade states
Dipole allowed component includes the direct excitation and indirect singlet-ungerade → singlet-gerade → (B,v=0,J=2) excitation

17. J”=22
J”=22
J”=14
J”=20

18. Mechanism of cometary H2 VUV emission
Solar photon (esp. H Lyman-) dissociation of H2O
H2O + hν → O (1D) + H (branching ratio 1216A)
H2O + hν → H(2S) + OH(X 2 or A 2Σ) (64% and
H2O + hν → 2H(2S) + O(3P) 1216A)
Highly excited H2 (X) has very long spontaneous emission lifetime (5-200 days)
Solar photoexcitation of excited H2 (X) primarily by Lyman- to singlet ungerade states to produce observed emission lines
Very hot OH(X 2 or A 2Σ) has been observed in lab with 1216A radiation
It has been known that H2O + hν → O (1D) + H2 should also produce rotationally hot H2 (X)
The FUSE observation yields the first “experimental” evidence at ro-vibrational level that H2 (X) is rotationally hot

19. Rotational temperature is LTE
Vibrational temperature is Non-LTE
Relative vibrational population determined by a fit of 15 sets of vibrational column vectors
Width very sensitive to rotational temperature

20. Acknowledgement Dr. H. Abgrall Dr. J. M. Ajello
Dr. S. M. Ahmed Dr. M. Glass-Maujean
Dr. E. Roueff
NASA/ORAU Senior NPP Program
NASA/NRC Associateship Program
NASA Planetary Atmosphere Program
NASA Cassini UVIS Contract
NSF Atmosphere Aeronomy Program
NSF Astronomy and Astrophysics Research Grant