The following text is an extract from the proposal which has been submitted to the DFG in Dec. 1999

1. Astrophysical background
1.1 Interstellar and circumstellar matter

The space between the stars is not empty, but is filled with a tenuous interstellar medium (ISM) which in the Galaxy is characterized by an average density of 1 hydrogen nucleus per cm3. This medium consists of distinct phases with very different thermodynamical properties (see Hollenbach and Thronson 1987). Most of the total mass of the galactic interstellar gas, which amounts to 6x109 solar masses, is contained in cool and "dense" clouds. There are two types of such clouds: molecular clouds containing mostly molecular hydrogen, and clouds of atomic hydrogen. Molecular clouds are characterized by kinetic temperatures of 10-15 K and gas densities between a few hundred and 107 hydrogen molecules per cm3 in their dense cores. Hot molecular cores associated with star formation can have temperatures up to 200 K leading to the release of grain mantle components into the gas phase. Compared with quiescent molecular clouds, diffuse atomic hydrogen clouds have higher average temperatures (80 K) and lower densities (50 hydrogen atoms per cm3). Outflows from evolved stars and protoplanetary disks are characterized by even higher densities and temperatures up to the grain formation / destruction temperatures.

The interstellar medium is an integral component of the various classes of normal and active galaxies. In one of the most important evolutionary processes in the universe, the molecular cloud phase of the ISM is transformed into stars which in their later stages enrich the gas with heavy elements. A fraction of these heavy elements has condensed into submicron- and micron-sized solid particles (cosmic "dust"). Molecular line and continuum radiation observed at infrared, millimeter and radio wavelengths has frequently be used to determine the physical conditions (temperature, particle density, column density, magnetic fields) in the interstellar medium (Genzel 1991). The infrared bands caused by aromatic hydrocarbons have been recently recognized to be an important indicator for star formation at higher red-shifts.

Atoms, molecules, and grains are more than just probes of the interstellar medium, but act as active players; they provide heating and cooling through energetic photoelectrons, fine structure line emission, and gas-grain collisions. The degree of ionization in molecular clouds and protoplanetary accretion disks and the coupling of the magnetic field with matter is determined by the shielding effect of dust grains and the depletion of metals. Radiation pressure acting on grains and gas causes molecular outflows from evolved stars and may even prevent the formation of extremely massive stars. Opacity-limited fragmentation may be decisive for the determination of the lower mass end of the initial mass function of stars. The chemical evolution of protoplanetary disks resembles the chemistry in the early solar nebula, which finally determines the composition and structure of the planets.


Table 1: Types of molecular processes (after van Dishoeck 1999)
Chemical processes

Ion-molecule reactions

X+ + YZ ® XY+ + Z

Charge transfer

X+ + YZ ® X + YZ+

Neutral-neutral reaction

X + YZ ® XY + Z

Radiative association

X + Y ® (XY)* ® XY + h n

Grain surface formation

X + Y + g ® XY + g

Associative detachment

X + Y ® XY + e


XY + hn ® X + Y

Dissociative recombination

XY+ + e ® X + Y

Collisional dissociation

XY + M ® X + Y + M

Heating and cooling processes

Photoelectric heating

Grain/PAH + hn ® grain/PAH + + e*

Cosmic ray heating

H2 + cosmic ray ® H2+ + e*

CO line cooling

CO(J) + coll. ® CO(J*) ® CO(J’) + hn

Oi line cooling

O(3P2) + coll. ® O(3P1) ® O(3P2) + hn

Cii line cooling

C+(3P1/2) + coll. ® C+(3P3/2) ® C+(3P1/2) + hn

Gas grain heating/cooling

gas + grain ® gas’ + grain’


A comprehensive description of the ISM has to include a wide variety of basic physical and chemical processes that determine the evolution of the medium and the observable properties (van Dishoeck 1997, Hartquist and Williams 1998, Ehrenfreund et al. 1999). The typical chemical processes and the interactions of the matter with radiation are summarized in Table 1. Their importance for diffusive and molecular clouds has been thoroughly discussed by van Dishoeck (van Dishoeck 1998).

The atoms, molecules, the dust particles and the radiation field interact with each other in an extremely complex manner. Concerning the chemical processes one usually distinguishes between neutral and ionic reactions and reactions which occur on grain surfaces. Ion-molecule reactions have dominated for long time the reaction models developed for describing molecular clouds and protoplanetary accretion disks (Herbst and Klemperer 1973, Black and Dalgarno 1973) since many of them have no barriers (Gerlich 1993, Smith M.A. 1993). A very important reaction mechanism at the low temperatures and densities of the ISM is radiative association, a challenge for laboratory experiments (Gerlich and Horning 1992).

Based on new experimental evidence (Smith 1997, Chasting et al. 1998, Rowe et al. 1999, Kaiser et al. 1998) it has been shown that also neutral-neutral reactions can have large enough rate coefficients at low temperatures that they may be important. Very relevant for understanding the evolution of interstellar matter is the process of isotopic fractionation occurring predominantly at the low temperatures of interstellar clouds but also in protoplanetary accretion disks (Wootten 1987, Millar et al. 1989, Aikawa and Herbst 1999).

Not much quantitative knowledge exists on the role of grain surfaces in the chemical balance of the ISM. Although there is no experimental proof, it is commonly assumed that H2 is formed via catalytic reactions on dust particles (Hollenbach and Salpeter 1971). Also for the formation of hydrogenated species such as H2O, NH3 and CH4 the importance of grain surface reactions has been postulated (e.g. Tielens and Hagen 1982, d'Hendecourt et al. 1985, Tielens and Allamandola 1987, Herbst 1993).

So far, more than 100 interstellar molecules have been identified including neutral and ionic radicals with unsaturated bonds and complex polycyclic aromatic hydrocarbons. Many more will be found since the wide temperature range, the low densities as well as the lack of containing "walls" make molecular and diffuse interstellar clouds to very special chemical factories. One also has to consider that all species and processes involved are strongly influenced by the environmental radiation fields, energetic cosmic particles, turbulent motions, and shock fronts produced by stellar winds and supernova explosions. In the denser regions of protoplanetary accretion disks also ternary reactions can become important (e.g. Willacy et al. 1998). Finally, the freeze-out of molecules on cold grains and desorption processes have to be considered both in dense and cold molecular cloud cores and the outer regions of protoplanetary disks.

The lifecycle of dust grains is closely connected to that of stars (e.g. Millar and Williams 1993, Dorschner and Henning 1995, Henning 1999). Interstellar grains form in the cooling outflows from evolved stars and supernovae. The C/O ratio determines if carbonaceous grains and carbides (C/O >  1) or oxides including silicates (C/O <  1) prevail (Cherchneff 1998, Gail and Sedlmayr 1998). Cases exist where silicates are stored in circumstellar disks and carbonaceous matter exists in the envelope region. Sputtering and grain-grain collisions in interstellar shocks lead to the disintegration of interstellar grains and the production of very small grains (VSGs) (Jones et al. 1996). VSGs are stochastically heated by the interstellar and circumstellar radiation field. Molecular ice mantles composed mainly of H2O, CO, and CO2 are formed in molecular clouds and circumstellar envelopes (Schutte 1999). Grain-grain collisions with low relative velocities are the process leading from micron-sized particles immersed in protoplanetary disks to kilometer-sized planetesimals (Beckwith et al. 2000).

Table 2: Carriers of infrared emission components.
(after Tielens et al. 1999)


IR emission component

of C atoms


Abundance of C locked up (ppm)



UIR bands




PAH clusters






25 µm




Small grains





Classic grains

8 > 100 µm


≥ 100



The ubiquitous unidentified infrared bands (UIRs) are widely attributed to polycyclic aromatic hydrocarbon (PAHs) molecules (Leger and Puget 1984, Allamandola et al. 1985) although it has been shown by laboratory emission experiments that small PAHs do not match with the astrophysical observations (Schlemmer et al. 1994, Cook et al. 1996). PAHs can either accompany grain formation or are produced by the erosion of carbonaceous grains in the diffuse interstellar medium (Tielens 1997). In addition, carbon-chain molecules exist both in the diffuse ISM and molecular clouds (Irvine et al.1987, Thaddeus 1994). Both carbon-chain molecules and PAHs have been proposed as carriers of the mysterious diffuse interstellar bands (DIBs) (Tielens and Snow 1995, Tulej et al. 1998, Romanini et al. 1999, Maier et al. 1998). The size of the carriers of infrared emission components are summarized in Table 2. In addition, Table 3 gives abundance estimates for large interstellar molecules. Spectroscopic evidence exists for amorphous and crystalline silicates, carbonaceous grains, carbides and to a lesser degree for sulfides and other metal oxides (Henning 1998, 1999).

Table 3: Abundances of large molecules.
(after Tielens et al. 1999).



(H atoms)-1

Abundance of C locked up (ppm)


The Universe




Diffuse ISM
Molecular clouds




Diffuse ISM
(Identification uncertain)




1.2 Observations

We are now in the golden age of observational astronomy. Observations over the whole range of the electromagnetic spectrum both from the ground and space have provided a wealth of ISM data of a class and quality that is unprecedented. Millimeter line and infrared spectroscopy have been decisive for the determination of the physical and chemical conditions in the ISM (Sandford 1996, van Dishoeck and Hogerheijde 1999). The Infrared Space Observatory ISO recently led to a truly revolutionary step in our understanding of the interstellar medium by opening the spectral range from 2.4 µm to 200 µm for spectroscopy. Detection includes (i) the ubiquitous nature of the UIR bands, (ii) the widespread presence of water in warm dense gas, (iii) the detection of a whole set of H2 ro-vibrational and pure rotational lines, (iv) a complete inventory of molecular ices, (v) the detection of the sharp signatures of crystalline Mg-rich silicates at mid-infrared wavelengths in various environments, and (vi) the detection of long carbon chain molecules in interstellar space. Early results of the ISO mission have been summarized in 1996 in a special issue of Astronomy and Astrophysics (Vol. 315) and are more completely covered by the proceedings of the Paris ISO meeting (Cox and Kessler 1999). New infrared missions such as SIRTF, FIRST and the NGST, which will allow an even better and deeper infrared view into the universe, are already scheduled. The American-German airborne observatory SOFIA will succeed the highly successful Kuiper Airborne Observatory (Haas et al. 1995) in 2002/2003.
A wealth of high-quality millimeter and sub-millimeter data comes from single-dish telescopes such as the IRAM 30-m telescope, the CSO, HHT, and the JCMT. Bolometer and line arrays make the deep mapping of larger regions possible. Interferometers such as the PdB, OVRO, and BIMA interferometers enable high spatial resolution, thereby triggering the investigation of molecular outflows from AGB stars and protoplanetary disks (e.g. Dutrey et al. 1997, Guilloteau and Dutrey 1998). The American-European project Alma of a large millimeter interferometer in the Atacama desert will lead to a breakthrough in our understanding of the chemistry of planet-forming circumstellar disks, but also extragalactic systems.


1.3 Challenges for Laboratory Astrophysics

In order to exploit the truly unique observational data, a dedicated research effort is required covering the physics and chemistry of molecules, clusters, nanoparticles and grains under the extreme conditions of interstellar space. Worldwide, there are several related activities in this interdisciplinary field of research as mentioned in the preface. This field which is briefly called laboratory astrophysics (Ehrenfreund et al. 1999), encompasses various domains as schematically indicated in the figure on the next page. Activities include systematic empirical laboratory investigations as well as basic research where fundamental questions are answered either by experiment or basic theory. The results from these efforts can be either used directly for understanding and predicting observational facts or they are utilized as parameters in large scale models. There is general agreement that most of the present models describing physical and chemical processes in the ISM, treat only parts of the problem and that they use parameters and concepts often not based on reliable data.

Because of the wide area of fundamental research needed for astronomy, a major aim is to address those topics which seem likely to yield the greatest scientific return. There are still many open questions in gas-phase reactions at low temperatures and at low densities. Processes of specific interest include exothermic ion-molecule reactions which are hindered by a small barrier (Gerlich 1993, Smith D. 1993) or the formation of fast products by exothermic reactions which significantly affect chemical networks. Other open questions concern the formation of H2, the balance between characteristic molecules (e.g. SO and CS), and the formation of typical products (e.g. C7 or ammonia). Critical quantities are rates for recombination of electrons with cold ions (H3+), or the photostability of large molecules such as PAHs.

1.3 Challenges for Laboratory Astrophysics
Laboratory Astrophysics

In astrochemistry, most poorly understood is probably the interaction of gas with grain surfaces. The parameters used in models for sticking and desorption of molecules on grains or for chemical reactions occurring on the surfaces or in ice layers, are often taken from crude measurements or simple models. Also the formation and growth of the carbonaceous and silicatic solids from the gas phase is poorly understood. Grain processes which have to be simulated include sputtering by energetic particles and radiation, collisional destruction and growth, as well as crystal/amorphous interconversion. Thermal annealing and crystallization assisted by the presence of OH groups is a special field of interest after the ISO detection of crystalline silicates in rather different cosmic environments.

The structural, spectral, and scattering properties of micron- and submicron-sized solid grains and grain aggregates have to be known if the information contained in observed spectral energy distributions, intensity and polarization maps should be completely exploited. The influence of such factors as temperature, finite size, shape, and morphology on optical properties of cosmic dust analogues have to be investigated. Without the knowledge of the spectral properties of gas phase molecules the mystery of the carriers of the Diffuse Interstellar Bands and the Unidentified Infrared Bands will remain unsolved.

Special challenges are related to the role of hydrogen, the most abundant molecule in the Universe. Our knowledge on this molecule has significantly augmented in recent years as became apparent on the international conference H2 in Space (Combes and Pineau des Forets 1999), however, it also became obvious that many more laboratory studies are needed for astrophysical interpretations. This concerns the formation of H2 and the ortho / para ratio under the conditions prevailing in space as well as the energy balance of the ISM (Flower et al. 1999). Other important quantities are the H/D ratio, a measure of galactic evolutions or the amount of dark matter in the form of H2 (Valentijn and van der Werf 1999).

The field of laboratory astrophysics is at a point where a comprehensive effort such as our proposed FGLA shall have a major impact by concentrating in a coherent way experimental and theoretical expertises and by providing an attractive discussion forum for modelers and astronomers.

2 Scientific concept of the research group

2.1 Introduction

The overview given in the last section demonstrates that laboratory astrophysics is a wide interdisciplinary field. It is obvious that a Forschergruppe, like our FGLA must concentrate on specific aims which are selected according to the challenges of modern astronomy, the scientific experience of the researchers and the potential of the involved laboratories. As a fundamental concept for our proposal we have selected six major fields which are depicted in the following graph. Some more topics will be integrated by external cooperation (see Section 2.3.3).

Concept of FGLA

The included numbers refer to the projects TP1 to TP10 of this proposal. Two of them deal with basic theory, one with modeling. All other contributions are experimental efforts. The relations between the groups which are indicated by thin lines, will be detailed in 3.5 of the individual projects. Basic theory is not only needed for understanding experimental results on a profound level but an important aspect for laboratory astrophysics is that theoretical descriptions must help to generalize the results for efficiently integrating them into models of the ISM. In modeling, very sophisticated and realistic descriptions can be made today due to advances in computer power. We think that this subfield is important to be integrated into our FGLA since it is an important link to the astrophysical community. Experiments on gas phase collisions are dealt with in three projects. The availability of radicals (H and C atoms) will open up a new important area in ISM chemistry. The physics and chemistry of grains, solids, and surfaces is another important subject of the FGLA. Optical properties of interstellar matter in its various forms and under different excitation schemes link together the three fields: laboratory research, astrophysical modeling, and observations. Also spectroscopic studies and light scattering are included. These six selected fields are the basis of our joint activities and will be discussed in some detail on the following pages.

2.2 Basic theory

For simple systems, such as electron-atom recombination or absorption and emission of photons, the treatment from first principles is standard nowadays. For treating collision dynamics highly sophisticated quantum chemical methods have been developed if only a few atoms are involved. Larger molecules of interstellar relevance can be calculated to spectroscopic precision (Botschwina et al 1995, Botschwina and Hey 1999). For larger systems, Hartree-Fock methods (Szabo and Ostlund 1982), density functional methods (Parr and Yang 1989) or tight-binding approaches (Gorringe et al 1997) are used. Also interest in surface related phenomena is growing and dynamical processes on grains are being handled by more and more detailed approaches. Here, density functional approaches are a good compromise because they still allow for a realistic treatment of the electronic states.

Because for large systems not all degrees of freedom can be treated explicitly, methods of dissipative quantum dynamics such as the path integral method (Weiss 1999) or the density matrix method (Blum 1996) have to be used. Presently, this field is developing towards dynamical simulations of relevant subspaces, e.g. using efficient algorithms for propagating a wavepacket in time by an integration of the time-dependent Schrödinger equation. It has been demonstrated that the use of non-Hermitian Hamiltonians in these schemes allows for an approximate modeling of dissipative surroundings. Alternatively, density matrix approaches can be used, with the advantage that coupling to a dissipative surrounding can be included more easily. For any of these dynamic simulation schemes, it remains crucial that a relevant low-dimensional subspace has to be identified beforehand, containing e.g. a single reaction coordinate and a dissipative coupling to further degrees of freedom treated as a heat bath in thermal equilibrium.

All above mentioned theoretical methods are of interest for our FGLA but cannot be persuaded within the two theoretical projects; however, we will concentrate on large systems which are of special interest for understanding the role of interstellar dust. The more "simple" gas phase reaction dynamics (e.g. state specific cross sections measured in TP 4) will be treated with the help of external cooperations. TP1 shall contribute to our understanding of the elementary steps which control the dynamical evolution of complex interstellar matter. The method applied will be a density-functional based tight-binding method, combining a precision close to more sophisticated ab initio methods with an outstanding numerical performance. Interaction potentials, geometries, formation energies and vibrational properties will be calculated for a variety of molecular structures. Besides these gas phase processes it is intended to treat the interaction of molecules with cold surfaces.

In TP2, the interaction between a nanoparticle and a molecule on its surface will be investigated within the reduced density matrix formalism. Dissipative quantum dynamics will be used to calculate the competition between electron transfer and fluorescence. The work is closely related to the single molecule spectroscopy project TP6. Both projects shall investigate the influence of the nanoparticle size and of the type of adsorbed aromatic hydrocarbons on the electron transfer rate.

2.3 Modeling

Large-scale modeling of the interstellar and circumstellar medium is extremely important if one wants to understand the different phase transitions including the formation of stars in molecular cloud cores and of planets in circumstellar disks which are driven by gravity, radiation forces, gas pressure, and magnetic fields. Such calculations must be based on a detailed knowledge of microscopic processes such as ionization, chemical reactions, and grain evolution. The numerical modeling is critical for a better understanding of the influence of the elementary reaction steps which control the dynamical evolution and to filter out the dominant key processes.

It would go far beyond the possibilities of the FGLA to contribute to this field in a systematic way. Therefore, we selected in TP3 the numerical modeling of chemical reactions in protoplanetary disks as the numerical modeling project of the research group. There are at least five different reasons for this choice: (1) The chemistry in protoplanetary disks is largely unexplored (see description in TP3) in strong contrast to a lot of work which has been done for molecular clouds and outflows from AGB stars (e.g. Herbst 1995, Millar 1998, van Dishoeck 1998, and references therein); (2) The complex interplay between the dynamics and the chemistry in the disks makes these systems extremely interesting for a long-term program; (3) The physical conditions in the disks require knowledge of surface reactions, neutral-neutral reactions, molecule-ion chemistry, and ternary reactions at different temperature regimes; (4) Millimeter and submillimeter interferometry with new projects such as ALMA on the horizon will deliver high-quality astronomical data which can test the predictions of the models; (5) We can build on our experience of the large-scale modeling of disks (Klahr et al. 1999; Steinacker and Henning 1999; Bell et al. 1998) and the treatment of chemical processes in disks (see TP3). A close collaboration between this astrophysical project and the theoretical projects (TP1 and TP2) as well as the experimental projects (TP4 and TP5) will foster the integration of the different subgroups. The numerical modeling project will provide information on what is important to measure and what is the required accuracy of the laboratory data.

Here, we should note that we will establish connections to the theory group of the AIU in Jena, which developed the necessary continuum and line transfer codes for an interpretation of astronomical data (e.g. Ossenkopf 1997, Wolf et al. 1999). We will also continue with our established contacts to other theory groups which perform large-scale modeling of protoplanetary accretion disks and molecular cloud cores (E. Herbst, T. Millar, E. van Dishoeck, W. Kley, H.W. Yorke). These collaborations will be essential for establishing a strong position of the FGLA in the astronomical community.

2.4 Gas phase collisions

Gas phase studies appear to be in a mature state compared with other areas of astrophysical chemistry, and many laboratory results have been included in model calculations (Aikawa & Herbst 1999, Terzieva & Herbst 1998). Nonetheless, there are still many uncertainties and unsolved problems, some of which are related to the fact that the temperature range of interstellar interest became only accessible to experiments in recent years by using a new flow-reactor technique (for a review see: Smith 1994, and references therein) or cooled ion traps (Gerlich 1992, Gerlich 1993). Since rate coefficients often depend dramatically on temperature there is a great need for more measurements carried out under astrophysically relevant conditions

Some aspects of molecular dynamics at very low energies are very specific to the interstellar environment. On the one hand, it is evident that small barriers or endothermicities in bimolecular reactions play an important role at low temperature collisions. On the other hand, excitation of internal degrees of freedom in a reactant which is otherwise cold has a reverse effect when released in such a collision. Usually these effects are simply neglected. However, there are known cases where these effects play a significant role in the formation or alteration of interstellar molecules. This topic will be one major aspect of TP4.

One very important process in molecule formation at the very low densities and temperatures of molecular clouds is the radiative association. In this process, two species collide and stick together aided by emission of a photon. The success of such an encounter is determined by the competition between (1) the lifetime of the collision complex with respect to redissociation as compared to (2) its lifetime with respect to the emission of a photon. For small molecules, this competition is strongly in favor of the redissociation (generally > 106 / 1). But although only every millionth collision might lead to success, radiative association between ions and neutrals became measurable due to the very high sensitivity of ion trapping techniques combined with single ion detection. However, only very few systems have been studied so far (Gerlich 1993, Gerlich 1994, Sorgenfrei and Gerlich 1993). Future studies also in the FGLA will be concerned with associative reactions with CO, the second most abundant molecule in the ISM, and the question at which size of a molecular ion a target molecule "sticks" to it, i.e. association at every collision occurs.

Isotope exchange reactions are an example for a reaction which is slightly endothermic in one direction due to the difference in zero point energy of the corresponding reactants and products. This phenomenon leads to isotopic fractionation in the astrophysical environment. The prediction of reliable barrier heights for such systems is important because differences to observational abundances can be used as tracers to the temporal and spatial evolution of the corresponding astrophysical objects (Walmsley et al. 1999). Several laboratory experiments along these lines have been made in one participating group (e.g. Gerlich 1994), a typical example is the system C2H2+ + HD and other isotopic variants. Here, isotopic fractionation leads to an enhancement of C2HD+. Further experiments for other key reactions, e.g. H3+ + H2 and isotopic variants, will be performed in TP4.

In the cold environment of molecular clouds, it can not be assumed that all internal degrees of freedom (spin-orbit levels, rotation, vibration) of molecules are thermally equilibrated. First observations of transitions of spin-orbit levels occurred in the 1980s and consisted of forbidden infrared transitions of neutral atomic oxygen, Oi, and singly ionized carbon, Cii. The basic processes of collisional and radiative excitation (and relaxation) of fine structure states is still very poorly understood, i.e. rarely treated experimentally or theoretically. First results are dealing with the fine structure relaxation of Al by Ar between 30 and 300 K (Le Picard et al. 1998). Relaxation of ArII has been studied in one participating group using a special trap arrangement (Haufler 1997, Schlemmer 1998) which allows the overlap of the trap volume with a molecular beam. Radiative lifetimes in the tens of seconds regime have been determined with this technique. The work in TP4 will deal with the important question how these radiating levels are populated via collisions and also try to determine lifetimes for spin orbit excited C+.

Another system relevant to the ISM is the formation of ammonia which is started by the initial reaction N+ + H2 → NH+ + H, followed by a chain of hydrogen abstraction reactions. The endothermicity is of the same order of magnitude as the smallest rotational excitation and the fine structure energy of the three spin-orbit states of N+ (3P0,1,2). In TP4, new technical approaches will be developed to find out which energy form increases the formation probability of NH+ and thus also of ammonia.

Collisions with atomic species, especially H atoms dominate in the ISM environment. However, due to experimental difficulties only very few laboratory studies deal with this important topic of gas phase chemistry (McEwan et al. 1999). TP5 and TP7 will specialize in collisions of ions with atomic targets (H, C, N, O) at low temperatures. In general, rate coefficients for neutral-neutral reactions are much smaller than for the ion-molecule case. However, more recent investigations show that especially in low temperature collisions with radicals rate coefficients can be of the same order of magnitude as compared to ionic systems (Chasting et al. 1998). The latter class of reactions will not be studied within the FGLA. This and several other very important topics in gas phase chemistry can be included to the FGLA thanks to the well established cooperation within the: TMR network, connections to the TSR in Heidelberg) and other facilities such as the free electron laser in Amsterdam or the infrared detector.

2.5 Carbon structures

Carbon belongs to the most abundant non-volatile elements in the universe (see e.g. the review by Henning and Salama 1998). Due to its unique ability to form three types of hybridized orbitals, carbon can form a large number of molecules with a variety of isomers. It is not surprising, that many of them (unsaturated chain-like hydrocarbons, polyynes, PAHs, etc.) have already been identified in the interstellar medium (e.g. in Thaddeus 1999). Also bare carbon molecules have been discovered in the atmosphere of carbon rich stars (Hinkle et al. 1988, Bernath et al. 1989). Small carbon particles are the most probable carriers of the interstellar extinction hump at 217 nm (Fitzpatrick and Massa 1986) and also of the 3.4 µm absorption band of the diffuse interstellar medium (Pendleton and Chiar 1997).

The relation between the carbonaceous species in space, their history and evolution is, however, not understood yet. This is mainly due to a lack in basic physical and chemical data on the formation dynamics as well as on the spectroscopic properties of the carbon structures. Although complex chemical networks have been developed to describe the formation of carbonaceous molecules and ions in different regions of space, these depend critically on rate coefficients which are poorly known for the relevant physical conditions. For example, the laboratory data suggesting the   anion (Tulej et al. 1998) as a carrier of several diffuse interstellar bands (DIBs) opened the questions about the formation mechanisms of such ionic species in the interstellar medium. Ruffle and coworkers (1999) calculated the abundance of   using an extended reaction network, the results depend, however, on rate coefficients of several key reactions (radiative electron attachment, photodetachment, and reactions with hydrogen and carbon atoms) that have not been experimentally investigated till now. Compared to the rich spectroscopic experimental work on carbon molecules (especially in the last decade), the information about reaction dynamics of carbon species is quite limited.

In the FGLA, the carbon-containing molecules will be studied in several projects. The FGLA intends to contribute to a detailed knowledge of the reaction dynamics of carbon species that is substantial for explanation of their growth and destruction in the interstellar medium. Reactions between carbon species will be studied in TP7. The growth of bare carbon structures will be investigated in a wide temperature range between 10 and 1000 K. The observation of isomerization of the reaction products in reactions of type Cm+ + Cn is the main aim of TP7.

Solid carbonaceous particles show a wide range of structural properties depending sensitively on the condensation process and determining in turn their behavior (see also 2.2.6). The properties of carbon nanostructures will be investigated within the FGLA in several projects. In TP7, single isolated carbon nanoparticles will be studied that will be directly grown in a trapping apparatus. The charge state of the carbon nanostructures and its influence on the optical properties and reactivity of the surface of the grains will be one of the topics of TP5 and TP7. TP8 will focus on production of carbonaceous nanoparticles by using diverse gas aggregation and CVD sources. For characterization of the nanoparticles, electron microscopy and several spectroscopic methods will be applied. An important point of all these experiments is not only to characterize the produced nanostructures, but also to provide insight into formation mechanisms of the carbon nanoparticles under various conditions. The experiments aim to obtain information necessary for understanding the condensation processes of carbon in the ISM.

Studies on carbon-containing molecules will be carried out also by several other projects of the FGLA. TP4 and TP5 will study interactions of small hydrocarbons with H atoms. The analysis of isomerization (e.g. the linear and the ring structure of C3H2+) in radiative association is one of the topics of TP5. The experimental work on carbon-containing molecules within FGLA will be supported by the theoretical project TP1. Using the density-functional based tight-binding method, TP1 will provide precise calculations of geometries, formation energies and vibrational properties for the molecules experimentally investigated in TP4, 5 and 7.

The experiments of the FGLA on carbon structures will surely profit from external collaborations. Especially the spectroscopic work in the groups in Heidelberg (W. Krätschmer), Cologne (T. Giesen), and Basel (J.P. Maier) is of substantial relevance to our projects. For the structural investigations planned on carbonaceous grains, the same holds for the activities of the groups in Saclay (C. Reynaud, R. Papoular) and Naples (V. Mennella, L. Colangeli). Further, our studies will certainly benefit from the tight connections to combustion (R. Schlögl, Berlin and K. Siegmann, Zürich) and aerosol science (M. Schnaiter, Karlsruhe).

2.6 Grains and surfaces

Grains determine the thermal, dynamical, and ionization structure of the cool phases of the ISM. Molecular hydrogen, other molecules like CO2, and molecular ices form on grain surfaces. Examples for the importance of grains in astrophysical processes (see 2.1) are the coupling of the magnetic field to the dense molecular cloud cores and the radiation pressure on grains, which may limit the formation of massive stars. One of the main unsolved problems is the role of grains in the formation of planetesimals during the evolution of protoplanetary disks. The properties of small solid particles and their interaction with the gas phase as well as with electromagnetic radiation will be the subject of a number of projects both in Chemnitz and Jena. It is now generally assumed that grains play a significant role in interstellar chemistry in dense clouds. However, even for formation of H2, almost no experiments have been conducted under astrophysical conditions (e.g. Pirronello et al. 1999 and references therein). Based on the recent development of nanoparticle mass spectrometry (NPMS) in Chemnitz (Schlemmer et al. 1999), TP5 will deal with the formation of molecular hydrogen on a single isolated nanoparticle.

The investigation of structural and optical properties of grains as well as of growth processes has already been a strong research activity of the Jena group (Henning and Mutschke 1999, Schnaiter et al. 1998). This will be continued in the projects TP8, 9 and 10 of the FGLA. New aspects will be added, based on the concerted experience of the FGLA. Also the collaboration with F. Huisken (MPI für Strömungsforschung, Göttingen) including development of the production methods of carbonaceous (Schnaiter et al. 1999) and silicate nanoparticles will be of big importance.

A new and important research goal pursued in TP7 and TP8 is to understand the formation processes of particles from the gas phase. These condensation processes determine the solid-state structure of the condensed particles (especially for carbonaceous grains, see also 2.2.5) which in turn affect their optical properties (see below). TP7 will approach the problem by studying basic growth reactions, whereas TP8 will look at condensed structures and condensation conditions from a "macroscopic" point of view. In our view, important properties of grains which might effect their role in the ISM have been overlooked to date. In TP5 and TP7, we will look for the influence of the particles charge and quantum size effects on the optical properties and on the reactivity of a particle. Surface reactions will be quite extensively studied in TP5. The main goal of this projects is to experimentally analyze the interaction of an isolated nanoparticle with hydrogen atoms and to study their recombination on nanoparticle surfaces.

On the one hand the optical properties of small grains and grain agglomerates are a diagnostic instrument for the determination of the grain structure and on the other hand the basic input to astrophysical modeling. Although optical measurements will be used as diagnostic tools in all projects dealing with grains, the projects TP6, TP7, TP9, and TP10 are directly dedicated to luminescence, absorption, and scattering properties of solid particles. An important point is that all these projects will not only perform measurements of optical data but will correlate them to structural properties by using either optical measurement techniques of high spatial resolution or accompanying (electron)microscopy. This will result in new insights into the microphysical structures determining small-grain optical behavior which is urgently needed for the interpretation of new observational data (see 2.1.2).

2.7 Optical properties, spectroscopy, light scattering

Observed spectral lines are due to transitions between atomic and molecular states and usually well understood. Broad features can be due to complex molecules in the gas phase, clusters, or also molecules frozen out on solid-state material. Many of the observed spectral features are still not identified. One example is the mystery of the DIBs. Many, most probably molecular carriers are responsible for the plenty of these absorption lines (see 2.1). To date, the only molecule that shows compelling matches with the observed DIBs is the   anion studied in the group of J. Maier (Tulej et al. 1998). The carriers of the unidentified infrared bands (UIR) are also still unknown. However, the emission bands are attributed to polycyclic aromatic hydrocarbons (PAHs). Also a number of emission bands of circumstellar dust discovered in the last years have not been completely identified - the "crystalline silicate bands" (Molster et al. 1999), the 11 µm (Speck et al. 1997), 13 µm (Sloan et al. 1996), 21 µm (Kwok et al. 1995), and 30 µm features (Szczerba et al. 1999) of AGB and post-AGB stars.

The discrete absorption of radiation by gas phase molecules - in ranges from microwave to UV is well studied and close interactions with spectroscopic groups (R.J. Saykally, J.P. Maier, T. Giesen) will contribute to the know-how of the FGLA. The projects of the FGLA will also profit from the experiments on laser induced reactions (SPP Sternentstehung). However, there are many photon/gas-phase interactions, which are less well understood. Completely unknown, for example, is the radiative association spectrum of ion-molecule complexes. As molecules increase in size, they may become stable against photodestruction by VUV radiation due to efficient internal relaxation processes. In TP6, the spectroscopic properties of polycyclic aromatic hydrocarbons (PAHs) adsorbed on silicon nanoparticles will be studied using techniques of single-molecule confocal microscopy as well as magnetic resonance. The photostability of PAHs will be investigated and interactions between nanoparticles and PAHs will be described in detail.

The absorption/emission and scattering cross sections of the dust are crucial for the thermal and dynamical structure of star-forming regions, protoplanetary accretion disks, and circumstellar envelopes around evolved stars (Dorschner and Henning 1995). Therefore, related studies should comprise not only the efforts to identify the species causing observed features, but should rather perform systematic studies on the optical properties of small particles in dependence on their structure. The interaction of small particles with electromagnetic radiation is of interest in a wide wavelength region (from the VUV to millimeter wavelengths) and at material temperatures ranging from 10 K to the sublimation point. Questions such as the transition from dielectric to metallic behavior by increasing the percolation strength, the presence of hot spots, the question of quantum heating, and the luminescence of small particles deserve special attention (see, e.g., the contributions in Greenberg and Li 1997 and Ehrenfreund et al. 1999).

An important point is also the development of spectroscopic techniques allowing to study the laboratory analogs under conditions similar to those expected for their cosmic pendants. There is nearly a complete lack of experiments on isolated small particles. The matrix isolation technique here will provide new results on the optical properties of laboratory dust analogues (Schnaiter et al. 1996). In TP8, these will be related to the inner solid-state structure of gas-phase condensed nanoparticles, whereas in TP9 morphological influences on the infrared spectrum of submicron particles are studied. In the latter project, the introduction of new techniques like levitation in aerodynamic flows is one of the major goals. Several projects of the FGLA will apply the established trapping technique to isolate small particles and to investigate their optical properties. In TP7, carbon nanoparticles will be grown directly in the trapping apparatus and their blackbody radiation at high temperatures will be studied. The influence of the charge state of the nanoparticles and, in cooperation with TP5, the influence of hydrogenation of the particle surfaces are of interest.

In a variety of cosmic environments dust particles are expected to be very complex in structure. Their morphology might be described and understood by the concepts of fractal physics. However, they differ from spheres and, therefore, rather poor predictions for their optical properties can be given nowadays. Especially, the angle dependent light scattering depends sensitively on the shape and structure of the particles. The understanding of the scattering properties of complex grain structures is essential for the physics of these environments, e.g. by influencing radiation transport or for interpretation of observations, e.g. of dichroic extinction, which might trace magnetic fields and even particle-gas flows. Although scattering theory has considerably improved our understanding of aggregate optical properties (e.g. Henning and Stognienko 1996), experiments are restricted so far to analogue experiments in the microwave range (Gustafson 1995, Greenberg et al. 1961) or to very small aggregates (Bottiger et al. 1980). New possibilities will be opened by particle growth experiments in TP10. This project aims to study light scattering on individual aggregates which are produced in a natural agglomeration process (Wurm and Blum 1998).


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