© 2009 sri international laboratory measurement of the co cameron bands and visible emissions...
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© 2009 SRI International
Laboratory measurement of the Laboratory measurement of the CO Cameron bands and visible CO Cameron bands and visible
emissions following EUV emissions following EUV photodissociation of COphotodissociation of CO22
Konstantinos S. Kalogerakis, Constantin Romanescu, Tom G. SlangerSRI International, Molecular Physics Laboratory, Menlo Park, CA 94025;
Long C. LeeSan Diego State Univ., Dept. Elect. & Comp. Engn., San Diego, CA 92182;
Musa Ahmed and Kevin R. WilsonLawrence Berkeley Lab., Div. Chem. Sci., Berkeley, CA 94720
© 2009 SRI International
MotivationMotivation
• CO2 is an important component of the atmospheres of terrestrial planets and comets:
Venus 96% CO2, 4% N2 90 atm.
Earth 80%N2, 20% O2, 350 ppm CO2 1 atm.
Mars 96% CO2, 4% N2 0.01 atm.
• The most prominent features in the dayglow of Mars are associated with EUV absorption by CO2;
• There are no spectral measurements in the visible;
• The study of the photodissociation of CO2 at energies below the first ionization potential can provide a marker for ground-based detection of CO2.
© 2009 SRI International
CO (a – X) Cameron bandsCO (a – X) Cameron bands
180 200 220 240 260 280 3000
1
2
3
4
5
6
7
8
9
10
CO (a - X) Cameron Bands
CO2
+ (B - X) 0 - 0
O(1S - 3P)Inte
nsity
, kR
/nm
Wavelength, nm
UV spectrum of the dayglow of Mars measured by SPICAM on Mars Express (Leblanc, F. et al., J. Geophys. Res. 111: E09S11, 2006)
CO (a 3) sources:• CO2 EUV dissociation;• CO2 and CO electron impact excitation;• CO2
+ - electron recombination.
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COCO22 photodissociation photodissociation
80 100 120 140 160 1800
20
40
60
80
100
1u
1g
IP
1u
1+
u
CO
2 ab
sorp
tion
cros
s-se
ctio
n, M
b
Wavelength, nm
1E8
1E9
1E10
1E11
1E12
80 100 120 140 160 180
H Lyman-
Sol
ar Ir
radi
ance
, pho
tons
. s-1. cm
2
Solar extreme ultraviolet irradiance (Woods, T.N. et al., Solar Phys. 177: 133-146, 1998)
CO2 absorption cross section (Chan, W.F. et al., Chem. Phys. 178: 401-413, 1993)
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COCO22 photodissociation products below the IP photodissociation products below the IP
0
10
11
12
13
14
h
5
5
5
CO2 (X 1+
g)
0
0
0
0
CO(e 3-, ) + O(3P)
CO(d 3, ) + O(3P)
CO(a' 3+, ) + O(3P)
CO2 IP
CO(a 3, ) + O(3P)Ene
rgy,
eV
© 2009 SRI International
Experimental set-upExperimental set-up~10 Torr ~40 mTorr ~5 mTorr low 10-5 Torr low 10 -7 Torr
1 mm 2 mm 2 mm 3 mmALS Gate Valve
Dry Scroll Pumpwith Roots Blower(200 l/s)
Turbomolecular Pump (550 l/s)
Turbomolecular Pump (550 l/s)
Dry ScrollPump (10 l/s)
Gas
Photodiode
PMT
Mon
och
rom
ator
Collection Optics
Light Baffles
© 2009 SRI International
CO excitation spectrumCO excitation spectrumALS excitation spectrum for CO2 + h; CO Cameron bands and triplet bands. Shown are vibrational level thresholds for CO(a,a’,d,e) in v = 1-10
10 11 12 13 14 15 160
2
4
6
8
10
12
14
16
18
Inte
nsi
ty,
arb
. u
nits
photon energy, eV
a'3+
e3-
Cameron bands210-220 nm
triplet bands > 500 nm
a3v = 0-4
CO2 I.P.
d3
1.0 1.5 2.0 2.5 3.0 3.5 4.00
20000
40000
60000
80000
100000
120000
Cameron bands
a'-a: Asundi bandsd-a: Triplet bandse-a: Herman bands
C(3P) + O(3P)
A 1
e 3-
d 3
a' 3+
a 3
X 1+
Po
ten
tial e
ne
rgy,
cm
-1
Internuclear distance, Å
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CO combined UV-VIS-NIR spectrumCO combined UV-VIS-NIR spectrum
Burke, M.L. et al., J. Phys. Chem. 1996: 100(1), 138-148.
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CO bands ratio – COCO bands ratio – CO22 pressure dependence pressure dependence
1E12 1E13 1E14 1E151
10
100
[vis
-IR
]/[C
amer
on]
inte
nsity
rat
io
CO2 number density, cm-3
Calculated intensity ratiofor [Asundi+Triplet]/[Cameron]bands, assuming1) only a' and d levels nascent
2) k(a,q) = 1.5E-11 cm3s-1
3) CO(a) rad. life. = 5 ms4) photon energy = 13.4 eV
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ConclusionsConclusions
A major CO2 photodissociative channel is represented by
CO* (a 3, a’ 3d 3ore 3-) + O(3P);
The upper triplet states cascade into the lower CO(a 3) state, generating the CO(a’-a, d-a, e-a) transitions in the visible and IR regions;
The CO(a 3) produces the well known CO(a-X) Cameron band radiation;
There have been no observations of the predicted visible/ IR emission in the dayglows of Venus or Mars because the proper instrumentation has not been flown.
© 2009 SRI International
AcknowledgementsAcknowledgements
Bill Olson, Gabe Hernandez, SRI InternationalSarah J. Ferrell, ALS BerkeleyALS Technical Team
FundingFundingThis work was supported by the NASA Outer Planets Research Program under grant NNX06AB82G.
The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231
© 2009 SRI International
© 2009 SRI International
© 2009 SRI International
COCO22 photodissociation - fluorescence spectrum photodissociation - fluorescence spectrum
200 300 600 700 8000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Energy = 13.4 eVGas mix: 6 mTorr CO
2 + 4 Torr He
Em
issi
on in
tens
ity, a
.u.
Wavelength, nm
ALS data