1. Misure ottiche su atmosfere planetarie in laboratorio
La ricerca dei pianeti extrasolari in Italia, 5-7 nov. 2014, Roma
EXO-IT
Misure ottiche su atmosfere planetarie in laboratorio
Marcel Snels1, Stefania Stefani2
1) Institute of Atmospheric Sciences and Climate (ISAC)
2) Institute for Space Astrophysics and Planetology (IAPS)

2. OUTLINE Rationale of the work Experimental setup:
Measurements of CO2 transmittance using a venusian real vertical profile
Collision Induced Absorption (CIA) bands
Study of the continuum
Study of the weak absorption
Conclusions
High Pressure-High Temperature gas cell
Cavity Ring Down
Multi Pass gas cell

3. Rationale of the work Several databases are available:
HITRAN (HIgh resolution TRANsmission), (Rothman LS et al et al. 2009)
HITEMP (HIgh TEMPerature), (Rothman LS et al et al. 2010)
CDSD (Carbon Dioxide Spectroscopic Databank), (Tashkun S.A.; et al 2006)
Laboratory measurements of the optical properties of gases at planetary conditions is of great importance to support radiative transfer modelling relevant for remote sensing instruments and retrieved chemical-physical products

4. Experimental setup FT-IR Specification: Detectors: DTGS 350-10000 cm-1
MCT cm-1
InGaAs cm-1
Si cm-1
Sources:
MIR cm-1
NIR/VIS cm-1
Beam splitter:
KBr cm-1
CaF cm-1
Resolution ( ) cm-1

5. High Pressure High Temperature Gas cell
Optical parameters
Pressure up to 300 Bar
Temperature up to 310 °C
Optical path: l ≈ 2 cm

6. *All data can be downloaded from http:\\ exact.iaps.inaf.it
**Thanks to this data now we have a theoretical model which reproduces our spectra better than 80%
** Measurements and modeling of high pressure pure CO2 spectra in central and wing regions from 600 to 9000 cm-1. H.Tran et al (JQSRT 2011)
*All data can be downloaded from
exact.iaps.inaf.it
*Experimental CO2 absorption coefficients at high pressure and high temperature .S.Stefani et al (JQSRT, 2012)
1–28
Absorbance(cm-1)

7. CO2 Collision Induced Absorption (CIA)
p=40 bar

8. CO2 Collision Induced Absorption (CIA)
p=40 bar

9. Study of the continuum
Continuum: set of very weak absorption which consist of different effects, such as CIA, far wings, line mixing and so on. In order to distinguish each contribution it's necessary to work at high densities with very long optical paths.
Our experimental setup does not allow to reach large optical path, because the gas cell is only 2 cm long.
To overcome this limitation, the Cavity Ring Down (CRD) spectroscopy can be used

10. Venus’ atmospheric windows

11. Cavity ring down apparatus
Acousto optic
switch
DFB laser
Optical
isolator
Beam
splitter
Wavemeter, etalon
Fotodiode
Beam expander
collimator
Cavity ring down cell

12. Assembling the high pressure Cavity Ring Down cell
Properties: Optical path 5.4 km; Volume : 3.8 L,
Pressure rating oC; oC

13. Laboratory measurements on CO2 [1]
Loss rate = 1.93* (5)*10-8 D+ 5.47(14)*10-10 D2
We have three different contributions:
Constant due to the loss of the mirrors: 1.93*10-6 (cm-1)
Linear contribution due to Ryleigh scattering :
1.17(5)*10-8 cm-1Amagat-1 (calculated 1.23(8)* 10-8 cm-1Amagat-1)
Quadratic contribution due to the wings + CIA + continuum:
5.47(14)* cm-1Amagat-2
Refer to B. Bézard et al. (Icarus, 2011 and JOURNAL OF GEOPHYSICAL RESEARCH, 2009)
Continuum: 7(2)* cm-1Amagat-2
Far wings: 2* cm-1Amagat-2
Quadratic fit
[1] Carbon dioxide absorption at high densities in the 1.18 mm nightside transparency window of Venus M. Snels, S.Stefani, G.Piccioni and B. Bzard
(Journal of Quantitative Spectroscopy and Radiative Transfer, 2013)

14. Multipass cell for FTIR
This apparatus is characterized by variable optical path from up to 30 m, a pressure up to 10 bar and temperature from -30°C up to +100°C to study:
Collision induced absorption in CO2, H2 ecc.
Line broadening in weak bands
Weak absorption features in planetary atmospheres (impurities,
less abundant isotopic species, gas mixtures, weak bands)

15. White cell optics

16. Multi pass gas cell @ intermediate pressure and high temperature
Optical Layout
Max path length: 30 m
P.L. variable from 2.5 to 30 m
Pressure
From 1 to 10 bar
Temperature
From 200 to 400K

17. H2 @ room T 5 bar and 15m of Optical Path

18. Conclusions
The FTIR spectrometer in combination with heatable/coolable high pressure absorption cells allows measurements for a large class of planetary atmospheres covering a large spectral range.
CRD is a powerful technique for studying weak absorptions in planetary atmospheres up to high pressures and can be performed at different wavelengths by using narrow band tunable lasers, which are availabe throughout the NIR.
Collision induced phenomena, important in high pressure atmospheres can be studied for pure gases and mixtures.