poster at From Dust to Planetesimals 2006

Experimental Infrared Spectroscopy of Dust Grains in Aerosol:
Modeling of forsterite spectra
Harald
1
Mutschke ,
Akemi
1
Tamanai ,
and Michiel
2
Min
1. Astrophysical Institute and University Observatory, Friedrich-Schiller-University Jena, Schillergaesschen 3, 07745 Jena, Germany
2. Astronomical institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
Correspondence to: [email protected]
Abstract
We investigate the ability of the statistical light-scattering model with a distribution of form factors (DFF) and different sets of optical constants to
reproduce our laboratory-measured infrared extinction spectra of crystalline Mg2SiO4 (forsterite) particles in Aerosol and in KBr/PE pellets.
Experimental Set-up for Aerosol spectroscopy
Samples
We equipped our FTIR Spectrometer (BRUKER 113v) with a multi-path gas cell in order to measure the extinction of a cloud of silicate dust
carried by the gas filling the cell (Fig.1). A brush powder disperser (Fig.2) was used for suspending particles in the gas (dry N2). In order to
produce a high concentration of small-sized particles in the aerosol, we placed an impactor between the dust flow generator and the cell (Fig. 3).
Forsterite (crystalline Mg2SiO4)
FTIR Spectrometer
Lamp
Multi-path gas cell
•Alfa Aesar Johnson, 99% metal, Sedimentation,
d<1.0µm, irregular shape (AJsed.)
• Marusu Yuuyaku,
d<0.2µm, ellipsoidal shape (Marusu)
• Sol-gel (Jäger et al. 2003), Sedimentation,
d<1.0µm, irregular shape (Sol-gel)
0.28µ
µm
0.54µ
µm
Impactor
1.2µ
µm
IR Detector
Dust flow
generator
Carrier gas N2
AJsed.
Cross section
Front
Fig. 2 Function of Dust flow generator
Fig. 1 Extinction measurement set-up
Experimental Results
(RBG1000, Feststoffpartikel Dosierer, Palas GmbH)
<15-30µm range>
11.06µ
µm
11.05µ
µm
10.83µ
µm
AJsed.(Aero)
Marusu (Aero)
Sol-gel (Aero)
AJsed.(KBr)
Marusu (KBr)
Sol-gel (KBr)
AJsed.
Sol-gel
Marusu
0.5
Q (λ
λ )/ Q (Peak)
1
Aerosol (µ
µm)
Marusu
ext
0.4
0.3
ext
Qext(λ
λ )/ Qext(Peak)
Crystalline Mg2SiO4
1.5
0.7
0.6
AJsed.
0.5
0.2
Sol-gel
0.1
0
0
9
9.5
10
10.5
11
11.5
12
12.5
Sol-gel
Fig. 4 Grain shape of Forsterite particles (by TEM)
Fig. 3 Function of Impactor
<10µm range>
∆λ=0.23µ
µm
∆λ
Marusu
13
9
9.5
Wavelength (µ
µm)
10
10.5
11
11.5
12
9.81
KBr (µ
µm)
∆λ (µm)
(µ )
9.97
0.16
10.14 10.41
0.27
10.83 11.07
0.24
11.88 11.88
9.86 10.02
0
0.16
11.06 11.23
0.17
11.88 11.89
9.87
9.95
0.01
0.08
11.05 11.19
0.14
11.86 11.88
0.02
12.5
Wavelength (µ
µm)
Fig.5 Normalized Qext vs. wavelength of 3 different forsterite
Fig. 6 Comparison between the aerosol (solid lines) and KBr
samples measured in the aerosol experiments .
(dotted lines) measurements.
Tab.1 The peak positions of crystalline
forsterite for 3 different samples from the
aerosol and KBr measurements.
Fig. 7 Comparison between the aerosol (solid lines) and KBr
(dashed lines) measurements for two of the samples.
Theoretical approach
In order to reproduce the measured spectra in a theoretical simulation, a statistical model for the shape distribution of the particulate samples is required.
We apply the “Distribution of Form Factors” (DFF, Min et al. 2006) approach, where the extinction cross section of an ensemble of particles small compared to the wavelength is represented by:
< Cext > 2π
P ( L)
=
* ∫ Im
dL
V
λ 0 1 / (ε − 1) + L
1
with ε(λ) being the dielectric function of the material, V the volume of a particle, and
P(L) the probability for the form factor L (0<=L<=1). For crystalline Mg2SiO4, the dielectric function ε(λ) is anisotropic.
The cross sections obtained for the three crystal axes have to be averaged.
We have fitted form factor distributions P(L) to our experimental spectra using the available sets of optical constants for crystalline Mg2SiO4 (Servoin & Piriou 1972, Sogawa et al. 2006, Suto et al. 2006).
Results of the simulation
Fig.8 The aerosol spectra of the Marusu (left) and the AJ (right) samples compared to the extinction cross section spectra
calculated from different optical data. The arrows indicate major insufficiencies of the fit.
• In order to reproduce the measured aerosol spectra in the
10µm band, we had to assume different form factor
distributions (DFFs) for the crystal axes (see Fig.9).
• Even with these DFFs we have not succeeded in reproducing
the 10µm and 23µm ranges simultaneously (see arrows in
Figs.8,10). This may point to insufficiencies in the ε(λ).
• The agreement of measured KBr/PE pellet spectra with the
simulation using the same DFFs is reasonably good but
reveals the same problem (Fig.10)
• The fit with the data by Servoin & Piriou (1973) is less
good than with the new data of the Kyoto group.
Fig.9 The DFFs obatined by the fits to the 10µm band regions
measured in aerosol. The dashed and solid curves have been used
for the contributions of the x and the y,z crystal axes, resp.
Fig.10 The KBr/PE spectra of the Marusu and AJ samples (black curves) compared to the extinction cross section spectra calculated
using the same form factor distributions as for the Aerosol measurements. The arrows indicate major insufficiencies of the fit.
References:
Jäger, C., Dorschner, J., Mutschke, H., Posch, Th.., and Henning, Th., 2003, “Steps toward interstellar silicate mineralogy VII. Spectral properties and crystallization behaviour of
magnesium silicates produced by the sol-gel method,” A&A, 408, 193
Min, M., Hovenier, J.W., Dominik, C., de Koter, A., Yurkin, M.A., 2006, “Absorption and scattering properties of arbitrarily shaped particles in the Rayleigh domain. A rapid
computational method and a theoretical foundation for the statistical approach”, J. Quant. Spectr. Rad. Transfer, 97, 161
Servoin, J.L. and Piriou B., 1973, “Infrared Reflectivity and Raman Scattering of Mg2SiO4 Single Crystal,” phys.stat.sol (b), 55, 677-686
Sogawa, H., Koike, C., Chihara, H., Suto, H., Tachibana, S., Tsuchiyama, A., and Kozasa, T., 2006, “Infrared reflection spectra of forsterite crystal”, A&A 451, 357
Suto, H., Sogawa, H., Tachibana, S., Koike, C., Karoji, H., Tsuchiyama, A., et al., 2006, “Low-temperature single crystal reflection spectra of forsterite”, MNRAS 370, 1599
Tamanai, A., Mutschke, H., Blum, J., and Meeus, G., 2006, “The 10µm infrared band of silicate dust: A laboratory study comparing the aerosol and KBr pellet techniques”, ApJ
Letters, 648, L147
Acknowledgement:
Our project has been supported by Deutsche Forschungsgemeinschaft (DFG) under the grant MU 1164/5-3-4.
We express our gratitude to Prof. C. Koike for providing us the Marusu sample and Dr. C. Jäger for supplying the
sol-gel samples for our experiment. We are grateful to W. Teuschel for his technical support of our experimental
devices and G. Born for her assistance with sample preparations.