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

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 m spectrometer and electron impact apparatus Instrumental range and resolution (with a 1200 G/mm grating) 1st order – 64 mÅ, 300-3700Å 2nd order – 32 mÅ, 300-1850Å 3rd order – 21 mÅ, 300-1230 Å (FWHM with e+H2 point source and 10m slits)

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.

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).

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)

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

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

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

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

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.

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.

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

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

J”=22 J”=22 J”=14 J”=20

Mechanism of cometary H2 VUV emission Solar photon (esp. H Lyman-) dissociation of H2O H2O + hν → O (1D) + H2 (branching ratio 11% @ 1216A) H2O + hν → H(2S) + OH(X 2 or A 2Σ) (64% and 11% @1216A) H2O + hν → 2H(2S) + O(3P) (11% @ 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

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

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