High Resolution Laboratory UV
Spectroscopy for Cassini UVIS Model
at Titan
Joseph Ajello
JPL
Alan Heays
Leiden
Observatory
H IGH R ESOLUTION UV S PECTROSCOPY
FOR C ASSINI UVIS
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GOAL AND RESULT:
Obtain High Resolution Optically Thin Laboratory 20 and 100 eV
Spectra of N2 by Electron Impact (FWHM= 0.2Å) wavelength range
of 800 to 1350 Å to establish UV ab initio Emission Model Constants.
Identify ~800 N2 (~500) and N I,II (~300) features (same also under
16 features in Cassini UVIS Titan EUV Airglow Spectrum -FWHM 4Å).
Produce First Emission ab initio Quantum Model of Electron Excited
Induced UV Fluorescence of N2 of 9 electronic states (b 1Πu, b' 1Σu+,
c4' 1Σu+ c3 1Πu, o3 Πu, c4 1Πu, c5 1Πu, c5 1Σu+ c6 1Σu+  X 1Σg+ from 1215 eV (+ a 1Πg  X 1Σg+ Lyman-Birge-Hopfield system) for UVIS
model to ionization limit. All UV Emission states are perturbed (>20%
admixture) and weakly-to-strongly predissociated.
Determine from Laboratory Spectra Rotational Predissociation and
Emission Yields for complete & accurate N2 model.
COMPARISON HIGH RESOLUTION LAB
SPECTRUM & TITAN LOW RESOLUTION
UVIS LIMB SPECTRUM
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Lab Spectrum
0.2Å FWHM vs
Titan Airglow 4-5Å
FWHM
10-40 N2
Molecular Bands
or NI,II lines per
UVIS feature
16 UVIS features
vs 800 Lab
features (8001350 Å)
Quantum Model (Coupled-Schroedinger
Equation [CSE]) Based on High Resolution
Laboratory Spectra
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Observed 20 and 100
eV Lab Emission
(black) Compared to
CSE Model of N2
Excitation,
Predissociation, &
Fluorescence (red).
Output quantummechanical model
simulates electronimpact excitation and
fluorescence (grey =
each band)– to aid the
assignment of complex
and blended UVIS
spectrum.
Accurately Modeled
optical excitation (hi
energy)
Emission, Predissociation Cross
Sections Five (n=3,4,5) RydbergValence (12-15 eV) States of N2
state
Qex (10-19 cm2)
Qem(10-19 cm2)
Qpre (10-19 cm2)
c 1 S u+
155a
119a
36a
b' 1Su+
128b
20.4
107.6a,e
b 1P u
121b
6.9a
114.1a
c3 1P u
161b
~0.7
160.3a
o3 1Pu
75b
~0.4
74.6a
totals
640
148
492
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Average
Predissociation) yield
(Qpre/Qex) =77%, EUV
photon yield=23%,
175K Titan).
Five singlet-ungerade
states predissociate,
eject fast N-atoms
(>1 eV) of N(4So) +
N(2Do) & N(4So) +
N(2Po)