Misure ottiche su atmosfere planetarie

in laboratorio

Marcel Snels1, Stefania Stefani2

2) Institute for Space Astrophysics and Planetology (IAPS)

1) Institute of Atmospheric Sciences and Climate (ISAC)

La ricerca dei pianeti extrasolari in Italia, 5-7 nov. 2014, RomaEXO-IT

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

OUTLINE

High Pressure-High Temperature gas cell Cavity Ring Down Multi Pass gas cell

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

Experimental setup

FT-IR Specification:Detectors: DTGS 350-10000 cm-1

MCT 850-12000 cm-1

InGaAs 9000-12.800 cm-1

Si 9000-25000 cm-1

Sources: MIR 100-8000 cm-1

NIR/VIS 3000-25000cm-1

Beam splitter: KBr 380-10000 cm-1

CaF2 4000-50000 cm-1

Resolution (10-0.07) cm-1

Optical parameters Pressure up to 300 Bar Temperature up to 310

°C Optical path: l ≈ 2 cm

High Pressure High Temperature Gas cell

Abs

orba

nce(

cm-1)

CO2 Collision Induced Absorption (CIA)

p=40 bar

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

Study of the continuum

Venus’ atmospheric windows

Cavity ring down apparatus

Wavemeter, etalon

DFB laserOptical isolator

Beam splitter

Acousto opticswitch

collimatorBeam expander

Cavity ring down cell

Fotodiode

Assembling the high pressure Cavity Ring Down cell

Properties: Optical path 5.4 km; Volume : 3.8 L, Pressure rating 124 bar @ 37oC; 78 bar @ 200 oC

Laboratory measurements on CO2 [1]

Loss rate = 1.93*10-6+1.17(5)*10-8 D+ 5.47(14)*10-10 D2

[1] Carbon dioxide absorption at high densities in the 1.18 m nightside transparency window of Venus M. Snels, S.Stefani, G.Piccioni and B. Bzard(Journal of Quantitative Spectroscopy and Radiative Transfer, 2013)

Quadratic fitWe 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)*10-10 cm-1Amagat-2

Refer to B. Bézard et al. (Icarus, 2011 and JOURNAL OF GEOPHYSICAL RESEARCH, 2009) Continuum: 7(2)*10-10 cm-1Amagat-2

Far wings: 2*10-10 cm-1Amagat-2

Multipass cell for FTIR

This apparatus is characterized by variable optical path from 2.5 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)

Multi pass gas cell @ intermediate pressure and high temperature

Optical Layout Max path length: 30 mP.L. variable from 2.5 to 30 mPressureFrom 1 to 10 barTemperatureFrom 200 to 400K

H2 @ room T 2 bar and 15m of Optical Path

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

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.