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 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.
• 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