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A Database of Optical Constants of Cosmic
Dust Analogs
Cornelia Jager a; , Vladimir B. Il'in c , Thomas Henning b;a ,
Harald Mutschke a , Dirk Fabian a , Dmitry A. Semenov a;c , and
Nikolai V. Voshchinnikov c
a Astrophysical Institute and University Observatory, Friedrich Schiller University,
D-07745 Jena, Germany
b Max Planck Institute for Astronomy, D-69117 Heidelberg, Germany
c Astronomical Institute of St.Petersburg State University, 198504 St.Petersburg,
Russia
Abstract
We describe the current state and future of the WWW Jena-Petersburg Database
of Optical Constants (JPDOC) that also contains references to papers and links to
internet resources related to measurements or calculations of the optical constants of
materials of astronomical interest. The most important part of the JPDOC are data
measured in broad wavelength ranges and partly at low temperatures in the Jena
Laboratory. To demonstrate the use of these data, we show as examples infrared
refractive indices of crystalline and amorphous magnesium silicates, spinel, and
hydrogenated amorphous carbon (HAC) and calculate the absorption cross-sections
of small particles composed of these materials.
Key words: spectroscopy, optical constants, databases, light scattering
1 Introduction
Nanometer- and micrometer-sized solid particles are distributed in the interstellar
medium and play an important role for astrophysical processes such as star and
planet formation. These particles show a rich chemistry and mineralogy as has been
revealed by spectroscopic astronomical observations in the last decades. Many new
observational data have been measured in the last years, e.g. by the Infrared Space
corresponding author: conny@astro.uni-jena.de
Preprint submitted to Elsevier Science 30 July 2002

Observatory in 1995{1998, and the interpretation of these spectroscopic data is still
in progress. This requires the comparison with data of \analog materials" delivered
by spectroscopical laboratories.
Various terrestrial analogs of cosmic solids have been studied in chemical and phys-
ical laboratories. However, many of these experiments neither took into account the
specics of cosmic dust materials (composition, lattice structure, processing, etc.),
nor covered the wavelength intervals of the current astrophysical interests. Note also
that these data are mainly in the form of graphics in papers, and free World Wide
Web (WWW) resources on the optical constants are generally limited by several
collections of refractive indices for a few materials.
Since 1992, the Astrophysical Institute and University Observatory (AIU) Jena op-
erates a Chemical and Spectroscopical Laboratory with the goal to study optical
properties of analog materials of cosmic dust in the wavelength range from the
ultraviolet to the far infrared. During this period, a compilation of optical con-
stants (i.e. the complex refractive index m = n + k i or the complex dielectric
function " = m 2 ) of such materials has been created. In collaboration with the
Astronomical Institute of St.Petersburg University, this collection was expanded
into an internet database that has been made available for the public in 1998 at
http://www.astro.uni-jena.de/Users/database/entry.html or http://www.astro.spbu.
ru/JPDOC/entry.html.
In this paper we describe the current state and future of the database and give
several examples of the data it contains and their possible applications.
2 Electronic database
2.1 Current state
The JPDOC provides access to references to the papers, data les and links to the
internet resources related to measurements and calculations of optical constants
in the wavelength interval from X-rays to the radio domain. The materials being
considered are:
amorphous/glassy/crystalline silicates of dierent kinds;
silicon, SiO, crystalline/fused SiO 2 ;
metals: Fe, Mg and others;
oxides: FeO, Fe 2 O 3 , Fe 3 O 4 , MgO, Al 2 O 3 , MgAl 2 O 4 ;
suldes: FeS, MgS, SiS 2 ;
carbides: SiC, FeC, TiC;
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carbonaceous species: diamonds, graphite, coals, kerogens, HAC, glassy/amor-
phous carbon, PAHs and so on;
organics: tholin, \organic refractory", etc.;
ices: H 2 O, CO, CO 2 , NH 3 , HCN, etc. and their mixtures;
FeSi, CaCO 3 and others.
The database contains more than 1000 references to the papers, reports, disserta-
tions where the refractive index, re ectivity, transmittance, etc. were derived. It
also gives references to useful books and reviews on the subject. Data accessible via
the JPDOC are mainly those measured in the laboratory of the AIU Jena supple-
mented with data freely available in the internet. The database also provides links
to internet collections of optical data les and personal WWW pages with relevant
software.
The rst version of the JPDOC was described in [1]. In the following years (1999{
2001) only minor improvements were made. All the time the site was rather well
visited { on average about 5 visitors a day and all together over 5000 visits for 3
years { and many visitors found the database helpful. That encouraged us to make
essential updates in 2002. We increased by about 25% the number of references to
papers (not only to the recent ones) in physical and astronomical journals as well
as to books and reviews. Further, we opened access to more data from the Jena
Laboratory, connected new pages presenting recently calculated low-temperature
Rosseland and Planck mean opacities, included new materials (FeSi, CaCO 3 , etc.),
gave more links to other internet resources (incl. Ioe Institute site, Database of
Optical Properties, etc.), and presented some more graphical illustrations and in-
formation about the physical properties of the materials.
2.2 Future plans
We intend to continue including new data and collecting references and links to
resources on the subject. Collaboration is planned with the Physical Institute of the
St.Petersburg University, Ioe Physical Institute and the St.Petersburg Institute of
Precise Mechanics and Optics. If it is successful, original data and bibliography for
the materials { interesting not only for astronomical but some other applications {
will be involved as well.
Some changes of the general design of the database are planned too, but main eorts
will be directed at extension of the JPDOC. The next step will be the creation of
the Database of Optical Properties (DOP) of scatterer models which will include
the JPDOC as a part.
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2.3 Database of Optical Properties
In astrophysics, the optical constants are mainly used to calculate the optical prop-
erties of scatterers, i.e. cross-sections, scattering matrix, etc. For many applications,
it is necessary to understand general trends in the data. This understanding can
often be gained from consideration of results obtained by simplied scattering mod-
els. Optical properties derived from such models have been discussed in some books
(see, e.g., [2,3]). In principle they can be calculated by using various light-scattering
tools which are freely available via the internet [4]. Nevertheless, there is a denite
necessity of a WWW database devoted to systematic consideration of the optical
properties of various model scatterers and related topics. As of this writing, our
database will include:
original codes realizing various methods to calculate the optical properties of
homogeneous and inhomogeneous, spherical and nonspherical particles;
review(s) of exact and approximate methods of light scattering theory (incl. dis-
cussion of their applicability ranges);
a review on the eective medium theory (EMT) and computer programs to mix
the optical constants for composite particles according to dierent EMT rules;
a database of several thousands references to papers on various aspects of light
scattering theory and its applications;
a graphics library illustrating light scattering by particles of dierent size/shape/
structure (a part of the data will be in tabular form to serve as benchmarks);
a special tool to calculate on-line selected optical characteristics of dierent scat-
terers (homogeneous and core-mantle spheres, innite cylinders, spheroids, etc.);
a self-training algorithm of determination of the optical properties of randomly
oriented fractal-like clusters of spherical particles based on an articial neural
network (perceptron) (see [5] for more details);
a collection of links to related internet resources.
The work on all the parts of the DOP is either in progress or has been nished.
It is undertaken by several persons from dierent institutes of former Soviet Union
countries under support of an INTAS grant. The DOP will be (partly) available via
the Internet [6] upon its completion scheduled at the end of 2002. Because of the
large volume due to the graphics library, the whole database may be available only
on CDs.
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Table 1
Summary of data measured in the Jena laboratory, which are currently available
from the JPDOC.
compound composition cryst. state spectral range data sets at 10K
silicates (Mg,Fe)SiO 3
, glassy UV/VIS/IR 10 1
(Mg,Fe) 2 SiO 4
MgSiO 3 , cryst. IR 2 1
(Mg,Fe) 2
SiO 4
MgSi x O y amorph. UV/VIS/IR 5 1
(Ca,Al,Mg,Fe)Si x O y amorph. IR 13
suldes (Mg,Fe)S cryst. IR 5 1
SiS 2 cryst. IR 1
oxides (Mg,Fe)O cryst. UV/VIS/IR 6 1
Al 2
O 3
amorph. IR 2
(Mg,Al)O x cryst. IR 8
carbon a-C:H amorph. UV/VIS/IR 6
3 Examples of data contained in the JPDOC
Most of the materials studied in Jena are synthetic compounds prepared especially
for the purpose of spectroscopic investigation. They include silicates in both amor-
phous and crystalline state, oxides of magnesium, iron, and aluminum, suldes, and
carbon in dierent forms. Chemical and physical analytical methods were gener-
ally applied to conrm the homogeneity, composition, and crystal structure of the
products prior to the spectroscopic measurements. Further, some natural crystals
(oxides and silicates) have been included in the studies. If necessary, data have been
determined for the dierent crystallographic axes. For part of the compounds, data
are available at cryogenic temperatures. A summary of the data currently available
is given in Table 1. In the following we give some examples of the data and their
possible applications.
3.1 Crystalline silicates
Silicate minerals of the olivine and pyroxene classes have been shown to be
present in out ows of evolved stars as well as in comets and protoplanetary
disks. The positions of the infrared emission bands produced by these minerals
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are diagnostic for the crystal structure as well as for the chemical composition,
especially the iron content. Comparison of the laboratory data with observed
features can constrain the conditions in these environments which have led to
the formation or processing of the dust grains.
Fig. 1. Upper panel: Imaginary part of the refractive index for crystalline
forsterite (Mg 2 SiO 4 ) in the three dierent crystallographic directions. Lower panel:
Mass-normalized absorption cross section of prolate spheroidal forsterite particles
(averaged over all spatial orientations) with dierent axis ratios. The dots and aster-
isks below the spectra indicate positions of astronomically observed emission bands
(after [7]).
We have used the infrared optical constants of forsterite contained in the
database for calculating the absorption cross sections of spherical and non-
spherical particles with sizes small compared to the wavelength (see Fig. 1).
The calculations have been performed for prolate spheroidal shapes with the
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long spheroid axis corresponding to the crystallographic c direction. The spec-
tra show resonances due to surface modes which shift very strongly as a func-
tion of the aspect ratio of the particles. This eect is probably very important
for the identication of emission features in astronomical spectra [8,9]. Inter-
stellar polarization measurements and laboratory experiments on the growth
of silicate particles [10] support the presence of elongated grains in astrophys-
ical environments. Information about the grain shape may provide constraints
for the formation mechanism of crystalline silicate grains, i.e. the role of direct
condensation vs. processing of previously amorphous material.
3.2 Amorphous magnesium silicate
About 85-90 % of the dust condensing in the envelopes of oxygen-rich evolved
stars consist of amorphous magnesium or magnesium-iron silicates [7]. There-
fore, special attention is paid to the production and spectroscopic characteri-
zation of analog materials for this dust component. The comparison between
dierently produced magnesium silicates demonstrates that the amorphous
state of any magnesium silicate is not unique. There exist dierent possi-
bilities for the structural arrangement of subunits in the amorphous silicate
network, similar to the varying structures of amorphous carbon.
Fig. 2. Left panel: Imaginary part of the refractive index for amorphous Mg 0:7
SiO 2:7
(dotted line) and Mg 2:4 SiO 4:4 (solid line). Right panel: Absorption cross section
normalized to particle volume calculated for a continuous distribution of ellipsoidal
grain shapes (CDE [3], grain sizes small compared to the wavelength) composed of
the same materials.
Optical constants (n; k) of stoichiometric and nonstoichimetric magnesium
silicates with Mg/Si ratios from 0.7 to 2.4 produced by the sol-gel method
have been derived from re ection measurements by a combination of Kramers-
Kronig analysis and Lorentz-oscillator t method (see Fig. 2). The absorption
cross sections calculated for particles small compared to the wavelength show
that the Mg/Si ratio in uences the position and the width of the 10 and
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20 m bands. With increasing MgO content the 10 m band shifts to longer
wavelengths whereas the 20 m band becomes broadened and centered at
shorter wavelengths.
The astrophysical relevance of these sol-gel silicates was tested by comparison
of optically thin model spectra based on the new optical data with the dust
emissivity derived from ISO-SWS spectra of AGB stars in the range between
8{30 m. The emission spectrum of TY Dra, an evolved dust-forming star,
can excellently be reproduced by the models, suggesting that the dust grains
may indeed consist of pure amorphous Mg silicate [11].
3.3 Magnesium-aluminium oxide (spinel)
Magnesium-aluminium spinel (MgAl 2 O 4 ) has been considered as a primary
condensate in the out ows of oxygen-rich AGB stars and as a potential car-
rier of the 13 m emission band observed in the spectra of these stars [12].
Therefore, in the Jena laboratory, a systematic study of the infrared properties
of Mg-Al oxides of both synthetic and natural origin was performed in order
to derive the optical constants of these materials. This led to the discovery of
two accompanying features in the astronomical spectra at larger wavelengths,
thereby strongly supporting the idea of spinel condensates in AGB star out-
ows (see Fig. 3, [13]). Recently, the experiments have been extended in the
direction of Ca-Al oxide minerals [14] and condensation studies of oxide grains
in low-pressure oxygen-rich atmospheres.
Fig. 3. Left panel: Imaginary part of the refractive index for synthetic and natural
magnesium-aluminium spinels. Right panel: Calculated normalized absorption spec-
tra for spherical particles small compared to the wavelength composed of natural
spinel (smooth solid line) and synthetic MgAl 2
O 4
(dotted line) in comparison to
the band prole of the newly discovered 32 m feature [13]. \Read leak" denotes
an instrumental artefact in the astronomical spectrum.
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3.4 Hydrogenated amorphous carbon
Amorphous carbonaceous materials can show a great diversity of optical prop-
erties due to the variability in their nanostructure. Especially in the infrared
range, the optical constants can dier by orders of magnitude according to the
conducting or insulating electrical behavior of the material. The amorphous-
carbon data contained in the database cover a wide range of these properties
as is illustrated by Fig. 4. The dierently pyrolized celluloses are represen-
tative for a suit of carbonaceous material ranging from strongly disordered
(mainly aliphatic, lower pyrolysis temperature) to graphitized (mainly aro-
matic, higher pyrolysis temperature) material.
Fig. 4. Complex refractive index of hydrogenated amorphous carbon prepared by
pyrolysis (annealing) of cellulose at dierent temperatures.
Especially interesting for astronomy is the calculation of the absorption and
scattering cross sections for particles small compared to the wavelength. Fig. 5
shows that particles composed of the strongly disordered material (400 o C) pro-
duce an absorption which is smaller by up to 3 orders of magnitude compared
to particles formed from the graphitized material (1000 o C). The absorption
cross section of carbonaceous particles in the far infrared ( > 100 m) can
be tted by a power law (C abs =V ) depending strongly on the inter-
nal structure of the carbon materials and on the particle shape. In the case
of spherical grains, the spectral index is considerably lower for the highly
disordered material than for the carbon material pyrolized at higher temper-
ature. With increasing graphitization due to a higher pyrolysis temperature
there is a gradual increase of [15].
Our calculations for dierent particle shapes show that there is no morpho-
logical eect on the spectral index of the low-temperature samples in contrast
to the more graphitic materials. For the latter materials we nd a signi-
cantly lower index in the case of broad shape distributions (CDE) compared
to spherical grain shapes. This is caused by percolation eects, present in the
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Fig. 5. Volume-normalized absorption cross section calculated for spherical grains
small compared to the wavelength (left panel) and a continuous distribution of
ellipsoidal grain shapes (CDE [3], right panel) from the optical data given in Fig. 4.
more graphitized samples which contain free charge carriers. We should note
that the results of the CDE calculations serve as an illustrative example. For
a more realistic calculation, one has to assume a special aggregate structure
and/or shape distribution of the individual particles [16]. For extreme values
of the refractive indices, however, computational methods for the calculation
of the absorption by aggregates or elongated particles become numerically
cumbersome and may even fail.
4 Summary and Outlook
We have presented recent developments in the Jena-Petersburg Database of
Optical Constants (JPDOC) and have demonstrated examples for the appli-
cation of optical constants contained for purposes of comparison with astro-
nomical spectroscopic observations.
The database will be continued to be improved. This will include measure-
ments on further analog materials of cosmic dust such as oxide and carbona-
ceous particles from gas condensation experiments, nanodiamonds, and others.
An extensive database will give better possibilities to achieve uniqueness in
the identication of astronomically observed bands and provide the possibility
to study grain size, shape and agglomeration eects in a realistic way.
The authors will highly acknowledge any contribution to the database such as
references, data les and links to be included in the database.
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Acknowledgements
We are grateful to all the people who sent us their comments and remarks on
the JPDOC. We also thank the German Research Foundation for supporting
our experimental work by several grants to the Research Group \Laboratory
Astrophysics". The development of the WWW database was supported by the
Volkswagen Foundation and the INTAS grant 99/652. V.I. acknowledges the
support by the RFBR grant 00-15-96607.
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