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Research method

Progress in the field of molecular astrophysics will require a wide ranging and coherent program in molecular astrophysics combining laboratory studies, quantum mechanical calculations, and astronomical modeling. All of these scientific disciplines are incorporated within this network. Here we summarize the innovative, state-of-the-art techniques and methods employed by the network teams in these 4 science areas as well as the methods used to provide easy access to this wealth of data for the scientific community through the web.

Laboratory studies on chemical reaction: The CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus, was pioneered by the group of Rowe (Rennes) for the study of ion-molecule and neutral-neutral reactions at temperatures as low as 10 K. The technique is currently being extended to study reactions of PAHs and energy transfer processes and product branching ratios. Further developments will focus on discharge and pulsed laser techniques for the production of excess concentrations of atomic radicals, the construction of nozzles for the use of H$_2$ as carrier gas for energy transfer studies and the implementation of additional detection techniques such as REMPI. These techniques are well complemented by new methods developed at Bordeaux to measure product branching ratios for reactions of atomic carbon at intermediate to low temperatures. The ion-trap methods (Toulouse and Leiden) - where ions can be stored for long times at low temperatures and pressures - are uniquely suited to study slow processes (eg., radiative association reactions) as well as the reactivity of photofragment ions such as dehydrogenated PAHs and (carbon) clusters. Molecular beam techniques (Bordeaux and Perugia) - using pulsed supersonic beams of neutral species and laser spectroscopy for the detection of reagents and products - are unique in their capabilities to identify neutral-neutral reaction product(s). New laser ablation techniques will be included to produce intense beams of refractory elements (C-atoms, metals). By varying the relative orientation of the two sources, very low collision energies, appropriate for interstellar space, can be attained.

Quantum mechanical studies: The Oxford group has been a pioneer in applying quantum scattering methods as well as simpler capture models to calculations of rate constants for reactions controlled by long-range potentials between reagents (eg., between ions and molecules and between pairs of radicals). A feature of future work will be the use of ab initio potentials, especially where chemical bonding contributes to the potential in the crucial, transition state, region. The Göttingen and Göteborg groups have developed a complementary approach - the statistical adiabatic channel model (SACM)- to calculate the same quantities for reactions occurring over attractive potential energy surfaces, which has been tested, with impressive success, against the results of trajectory calculations for several types of long-range potentials (ion-dipole, dipole-dipole, etc.). These theoretical approaches are complementary to the experimental approaches to reaction kinetics. Collisional rate coefficients can be obtained through theoretical studies, involving both the derivation of the potential energy surface via quantum chemistry and the dynamical treatment of the collisions. The network gathers experts in Basel, Bordeaux, Grenoble, Madrid, Meudon, Rennes and Warsaw which together cover the required expertize in the computation of intermolecular potentials. Besides standard ab-initio codes (ALCHEMY, MOLPRO, GAUSSIAN, ...), new techniques will be developed, in particular for the determination of bending and inversion motions. These potential surfaces will be used in dynamical calculations of rotational and rovibrational excitation of molecules through collisions with H, He, and H$_2$, using either full close-coupling calculations or less time-consuming treatments with some degrees of freedom frozen. Semi-classical techniques will be used when the relative energy between the pertuber and the target is much larger than the energy transfer resulting from the collision. The groups in Meudon, Grenoble, Rennes, Bordeaux and Durham have very complementary and unique expertize and together cover all facets of this field. Experimental studies will be specifically carried out to validate the theoretical calculations. These experiments involve a heavy experimental set-up specific to each collisional system. The group of Nijmegen has a well known expertise in the field.

Astronomical models: The physico-chemical models are coupled to excitation calculations and radiative transfer codes to predict molecular line intensities to be compared with observations. Collisional excitation rates required to compute the line emission come from quantum chemical calculations. The line radiative transfer codes treat simultaneously the statistical equilibrium between the level populations and the line transfer in the comoving frame using accelerated lambda iteration numerical schemes. The astronomical teams in the network (Orsay IAS, Grenoble, Groningen, Meudon, Madrid, UMIST) in the proposed network have world-leading, complementary expertise in various aspects of astrophysical modeling. Obviously the models will greatly benefit from the new chemical and collisional studies that will be performed within the network.

State-of-the-art models for photo-dissociation regions - where the chemistry is driven by photo-reactions with inclusion of radiative as well as heating and cooling processes - are applicable to surfaces of proto-planetary disks and the inner surfaces of proto-stellar condensations once the protostar turns on. These will be extended to take into account the propagation of the molecular dissociation front into the surroundings. The decoupling between ion/neutral species plays a key role in the dynamical and chemical evolution and in the excitation of molecules in C-type shocks as well as in ambipolar diffusion in star forming regions. It is critical for these models to include the latest rates for computation of the ionisation state of interstellar species (molecules, PAHs, carbon clusters and grains) in these environments. Existing chemical models of star forming dark clouds will be updated to focus on chemical evolution driven by ambipolar diffusion and free-fall collapse.

Molecular spectroscopy: The identification of lines in astrophysical spectra requires direct comparison with spectroscopic measurements of known species in controlled laboratory experiments. The laboratory spectroscopy groups in Basel, Lille, Orsay LPPM, and Cologne have all expertise and highly sophisticated apparatus necessary to cope with the challenge of studying molecules of astrophysical relevance. Spectrometers of high spectral accuracy and sensitivity are available which cover most of the significant spectral range. The groups have complementary facilities and techniques of producing, characterizing and identifying astrophysical molecules by means of laboratory spectroscopy. The spectrometers of the Lille group will study the millimeter and sub-millimeter range of small (deuterated) species in the ground and vibrationally excited states and of larger species (eg., PAHs) in their low-lying ro-vibrational spectra. A new experimental set-up will include a laser desorption device. In order to select the best candidates, the same spectra are first investigated by FIR Fourier transform spectroscopy in Orsay LPPM at low resolution. High resolution (up to 0.002 cm$^{-1}$) thermal emission spectra of astrophysically relevant molecules (eg., H$_2$O, NH$_3$, HCN) have also been recorded using a Fourier transform spectrometer with 4K bolometers in the far-IR (40-500 cm$^{-1}$) by the Orsay LPPM team. This technique will be extended by using more energetic sources towards the study of rotational emission spectra of free radical (OH, NH, NH$_2$). In addition, this spectrometer can be adapted to the synchroton beams available at Orsay for sensitive absorption spectroscopy. Recently, the first far-IR absorption spectra of naphthalene have been recorded. Orsay LPPM will develop new techniques to optimize the concentration of transient species, before recording their spectra in the far-IR or sub-millimeter in the collaborating laboratories of the network. The expertise of the Basel group lies in the measurement of the electronic spectra of astrophysically relevant molecules, radicals and ions, in particular carbon chains and their ions. A number of novel experimental approaches have been developed in Basel to measure the electronic transitions of bare carbon chains and large polyacetylene chains (up to 28 C-atoms). Ultra high-resolution spectroscopy at Cologne focuses on stable and reactive species such as CO and its isotopomeres, light hydrides (e.g. SH, NH, PH, CH), large polyatomic molecules (HCN, HC$_x$N, $x=3\dots 7$), radicals and ions (CO$^+$, SO$^+$, SH$^+$) in the frequency range from 50 GHz up to 2 THz. The line position can be determined in best cases with a precision of 500 Hz by using the sub-Doppler mode of the spectrometer. Pure carbon chain molecules (e.g. C$_8$, C$_{10}$) can be produced by using a UV-excimer laser ablation source. Rotationally resolved spectra of vibrational modes can be studied in all detail, after they have been characterized by matrix isolated spectra, obtained by the Basel group.

Databases and Web Interfaces: The Cologne Database for Molecular Spectroscopy (CDMS) contains predictions of spectroscopic transition frequencies and intensities with their uncertainties, concerning atoms and molecules of interest to the astronomical community. The frequency range currently covers the radio frequency to far-infrared regions. The BASECOL database for collisional ro-vibrational excitation of molecules by various colliders provides excitation rate coefficients, full information on the chain of errors of the data, and full bibliographical information. This collaborative project between Meudon, Grenoble and Bordeaux will be made available at the European level. The UMIST database (Manchester) provides the fundamental set of reaction rate data and related software for use in chemical kinetic modeling of astronomical regions. The database will be extended to include photodissociation and photoionisation cross-sections, the chemistries of minor elements such as Mg and Li, and the synthesis of carbon chains and rings. In addition the current web site (www.rate99.co.uk) will be improved to give more information on individual reactions and to allow a facility in which users can post new data, as well as comment on current data. The CASSIS project (Toulouse) will combine suitably calibrated and corrected molecular data from intelligent access of appropriate molecular databases with astrophysical models in order to provide the user with tools for the analysis of astrophysical spectra with high spectroscopic content. All of these data bases are already available and provide the community with downloadable data.

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