Experimental Investigation of H2 Singletungerade States by Electron Impact Xianming Liu, Paul V. Johnson, Charles P. Malone Jason A. Young, Geoffrey K. James, and Isik Kanik Jet Propulsion Laboratory California Institute of Technology Donald E. Shemansky 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, 3003700Å – 2nd order – 32 mÅ, 3001850Å – 3rd order – 21 mÅ, 3001230 Å (FWHM with e+H2 point source and 10m 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. • 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) 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. 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 dipoleforbidden excitation of EF,GK,I,J and other singlet gerade states • Dipole allowed component includes the direct excitation and indirect singletungerade → singletgerade → (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 – H2O + hν → H(2S) + OH(X 2 or A 2Σ) – H2O + hν → 2H(2S) + O(3P) (branching ratio 11% @ 1216A) (64% and 11% @1216A) (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 rovibrational 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. S. M. Ahmed Dr. E. Roueff Dr. J. M. Ajello Dr. M. Glass-Maujean 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