Network meeting 2008

Apetrei Cristina

Electronic spectroscopy of metal-containing species and their relevance for astrophysics

The major source of heavy elements in the interstellar medium is the loss of mass from the evolved stars. The envelope of the carbon-rich star IRC+10216 has been studied to a large extent over the past years leading to the astronomical detection of metal-bearing halide species such as: NaCl, AlCl, KCl and AlF [1,2] and the metal cyanides and isocyanides MgCN [3], NaCN [4], MgNC [5] and AlNC [6]. The detection of these species in the circumstellar shells is evidence that not all refractory elements are locked onto the dust grains but a considerable fraction must remain in the gas-phase. One of the most common metallic element in IRC+10216 is aluminum and our research in Basel has been focused on aluminum-carbon containing species such as AlCCH [7] and AlC2 [8]. Their gas-phase electronic spectra were recorded for the first time using a resonant 2-photon 2-color ionization scheme and laser induced fluorescence for high resolution spectra. AlCCH and AlC2 were produced using laser ablation of an aluminum rod in the presence of acetylene/He or Ar mixtures. Based on the analysis of the observed rotational structure and the theoretical data, the AlCCH spectrum is assigned to the singlet sigma to singlet pi electronic transition of linear AlCCH and AlC2, doublet A1 to doublet B2 electronic excitation of the T-shaped isomer. The inferred rotational constants may provide the basis for a search of the AlCCH and AlC2 millimeter-wave spectrum in the laboratory and in space. [1] L. M. Ziurys, A. J. Apponi and T. G. Phillips, Astrophysical Journal, 1994, 433, 729-732. [2] J. Cernicharo and M. Guelin, Astronomy and Astrophysics, 1987, 183, L10-L12. [3] L. M. Ziurys, A. J. Apponi, M. Guelin and J. Cernicharo, Astrophysical Journal, 1995, 445, L47-L50. [4] B. E. Turner, T. C. Steimle and L. Meerts, Astrophysical Journal, 1994, 426, L97-L100. [5] K. Kawaguchi, E. Kagi, T. Hirano, S. Takano and S. Saito, Astrophysical Journal, 1993, 406, L39-L42. [6] L. M. Ziurys, C. Savage, J. L. Highberger, A. J. Apponi, M. Guelin and J. Cernicharo, Astrophysical Journal, 2002, 564, L45-L48. [7] C. Apetrei, H. Ding and J. P. Maier, Physical Chemistry Chemical Physics, 2007, 9, 3897-3901. [8] E. Chasovskikh, E. B. Jochnowitz, E. Kim, J. P. Maier and I. Navizet, Journal of Physical Chemistry A, 2007, 111, 11986-11989.

Balucani Nadia

Progress in crossed beam studies of astronomically relevant neutral-neutral reactions

Frankcombe Terry

Statistical modelling of NH+/ND+ + H2/HD/D2 branching ratios

The fraction of interstellar ammonia that is deuterated exceeds the universal deuterium fraction. Whereas this is usually rationalised via exchange reactions with deuterium-containing cations, in this work we study deuterium fractionation along a proposed gas-phase ammonia formation path. Specifically we study the deuterium substituted variants of the NH+ + H2 reaction, for which the primary product is NH2+ + H. We combine high quality ab initio electronic structure calculations, potential energy surface fitting, adiabatic capture theory, extensive exact rovibrational state calculations and detailed angular-momentum-resolved state counting analysis to determine statistical branching fractions for all of the NH+ + H2 isotopologues. It is determined that in all cases where both hydrogen and deuterium are present, the NHD+ product is preferred. We thus provide evidence for an alternative explanation of the observed deuterated ammonia fractionation. In the process we reconcile two apparently contradictory experimental studies. POSTER: Adiabatic capture theory and interstellar nitrogen chemistry. The adiabatic capture centrifugal sudden approximation (ACCSA) has been applied to a number of nitrogen-containing gas-phase reactions at interstellar conditions, including the N + NH, C + NO and N + CN reactions. The resultant thermal rate constants are in agreement with available rate constants from quasi-classical trajectory calculations. For N + NH these are significantly larger than the available experimentally derived rate. The calculated rate constants could only be approximately described with simple Arrhenius-like forms. Subtle quantum effects are evident in the initial-rotational-state-resolved cross sections and rate constants.

Gerlich Dieter

Experimental studies of gas phase reactions involving hydrogen atoms

Gonzalez Manuel

Radiative transfer revisited for emission lines in Photon Dominated Regions

Transfer in lines controls the gas cooling of photon dominated regions (PDR) provides many of the observational constraints that are available for their modelling. The interpretation of infrared and radio observations by the new generation of instruments, such as Herschel, requires sophisticated line radiative-transfer methods. The effect of dust emission on the excitation of molecular species in molecular regions is investigated in detail to explicitly show the origin of various approximations used in the literature. Applications to H_{2}O is emphasised. The standard 1D radiative transfer equation is written as a function of the space variable (as opposed to the usual optical depth). This permits to simultaneuosly consider all pumping contributions to a multi-level species in a non-uniform slab of dust and gas. This treatment is included in the Meudon PDR Code (available at Infrared emission from hot grains at the edge of the PDR may penetrate deep inside the cloud, providing an efficient radiation source to excite some species at a location where cold grains no longer emit. This leads to non-negligible differences with classical escape probability methods for some lines, e.g. water. Cooling efficiency does not follow directly from line emissivities. The infrared pumping contribution leads to a higher excitation that enhances collisional de-excitation and reduces cooling efficiency.

Gusdorf Antoine

molecular emission in regions of stellar formation

Recent observations show that young stars being formed eject matter at several ten of kilometers per second, in the form of "jets" that impact the matter whose collapse is at the origin of the formation of the star. The supersonic impact between this jet and the parent interstellar cloud of the star generates a shock front, whose structure depends on its velocity as well as on the physical properties of the gas in which it propagates. This shock wave modifies the chemical composition of the gas, partially or totally dissociating the molecular hydrogen, and enables chemical reactions to occur, favoring the formation of characteristics molecules. In this talk, I'll briefly show how to model such regions of shocks, and I will mainly show how we use two of these "shock tracing" molecules (H2, SiO, and CO) to constraint the physical parameters of the pre-shock region.

Herbst Eric

The Future of Astrochemistry (invited)

In the last 30 years, molecular astronomy has made huge strides. More than 150 molecules have been discovered in the gas-phase in the interstellar and circumstellar media using high-resolution molecular spectroscopy. Lower resolution studies indicate the presence of polycyclic aromatic hydrocarbons as well as dust particles of silicates and carbonaceous material, with icy mantles in cold regions. In addition to this wealth of interesting molecular data, molecules have been used as probes of their environments in two senses. First, molecular spectra can be used to characterize the ``current'' physical conditions and motions of interstellar sources. Secondly, models, in which the chemistry of formation and depletion is simulated, can also tell us about the history of these sources. Both approaches have been used to yield much information on the interstellar medium in our galaxy and others, especially those portions involved in the formation of stars and planets. But our knowledge of the molecular universe is still limited. In my talk, I will discuss current outstanding problems and how the new generation of telescopes coupled with chemical modeling will lead to their solution.

Kania Patrik

Millimeter wave spectroscopy of titanium monoxide and titanium dioxide

Leonid Shirkov

Dynamics of nuclear motion in cyanoacetylene molecule and its complexes with helium by state-of-the-art quantum mechanical methods.

Theoretical description and computational modeling of the spectroscopic and collisional processes involving cyanoacetylene HC3N in the interstellar environment is our main scientific object. Modeling of the interstellar medium requires the knowledge of the rate constants for rovibrational (de)excitation of molecules with the most abundant species such as helium. Theoretical study of the spectroscopy of the cyanoacetylene molecule is based on the Watson’s isomorphic Hamiltonian describing the rovibrational motions in linear polyatomic molecules, including the anharmonicity effects, as well as the Corriolis coupling of the total angular momentum of the molecule with the vibration angular momenta corresponding to degenerate vibrational modes. The potential energy surface for the intramolecular dynamics is computed with CCSD(T) method in cc-pqvz basis. For the stretching modes the anharmonicity is very well described by 4th order polynomial, although the bending modes were found to be sufficiently anharmonic and required use of potential function in the form, describing the dissociation limit, like extended Rydberg potential with the set of parameters to fit ab-initio values. Rayleigh-Ritz variational procedure was used for solving the Schrödinger equation in pure harmonic basis as well as in the mixed morse-harmonic basis. The eigenvalues of the Hamiltonian matrix, converted to the banded form, were computed using the modified Davidson algorithm for large, sparse, real-symmetric matrices. This approach gave good correspondence with the experimental values for the states without rotation and small value of the rotational number, although excited vibrational states are a still a computational challenge. Then, we considered the rovibrational (de)excitation of HC3N in collisions with He. A Hamiltonian describing the nuclear motions in the collisional complexes was already derived, including the coupling of the vibrational angular momentum of the molecule with the total angular momentum of the collisional complex. Potential energy surface for He-HC3N was calculated by symmetry-adapted perturbation theory including its dependence of the intramolecular degrees of freedom and then, it was fitted to functional form using non-linear square optimization algorithm of Levenberg-Marquard. Finally, with the new scattering codes treating the rovibrational dynamics of He-HC3N, the rate constants for the rovibrational (de)excitation was written. The values of the rate constants were calculated. Applications of the computed potential surface to high-resolution spectroscopy of the He-HC3N are also considered. As the next step, we will research dynamics of nuclear motion in HC3N on several potential energy surfaces coupled by spin-orbit and non-adiabatic effects, like the Renner-Teller effect.

Lewen Frank

Terahertz High Resolution Molecular Spectroscopy in Cologne

Broadband scanning spectroscopy in the terahertz region of the electromagnetic spectrum has been used continuously in Cologne for more than 15 years. A large variety of transient species with astrophysical interest have been observed in laboratory. Reliable and precise spectra have been measured to calculate and predict spectra with high accuracies even in the upper THz range. Most of these data are available via our Cologne Database for molecular spectroscopy ( The most critical part of a THz high resolution spectrometer is the development of the frequency source, since the radiation source should be as powerful as possible, monochromatic and broadband tunable. Different methods have been successfully established in Cologne: I) Spectrometers with fundamental frequency sources like phase locked Russian Backward wave Oscillators in the frequency range from 180 – 1000GHz and millimeter- / microwave synthesizers with cascaded power transistor amplifier in the range from 10 -70 GHz II) Frequency multiplication of fundamental frequency sources with nonlinear electronic devices, like Schottky Barrier Diodes and newly developed GaAs/GaAl Superlattice Diodes (SL), operating up to 3THz. We will present an overview of the terahertz capabilities in the Cologne laboratories and we will show some details of the phase stabilization systems and the frequency multipliers specifically developed for very precise molecular line measurements. Spectra taken in Sub-Doppler mode can be as precise as 1 kHz. Selected samples of molecular spectra are presented to demonstrate the capabilities of the various types of spectrometers.

Ni chuimin Roisin

Carbon Anions in Photon Dominated Regions

All molecular clouds in the Universe are, to a greater or lesser extent, bathed in ultraviolet photons. These photons are capable of ionisation and dissociation and give rise to chemical processes which both synthesize and destroy molecules. These effects occur within regions of interstellar clouds called Photon-Dominated Regions (PDRs). In recent years, molecules of increasing chemical complexity, including small hydrocarbon chains, have been detected in the harsh environment of these PDRs and computational models have been unable to explain the high abundances observed. Combining a physical model of a PDR region with a chemical kinetic model of hydrocarbon and Polycyclic Aromatic Hydrocarbon (PAH) formation may explain the higher abundances of carbon chains observed in PDR regions. PAH degradation can provide a path for the synthesis of simpler hydrocarbon chain molecules. Also, the recent detection of the hydrocarbon anion C6H-, in the interstellar medium has led us to investigate the synthesis of hydrocarbon anions in a variety of interstellar and circumstellar clouds. We find that the anion/neutral abundance can be quite large, on the order of a few percent, once the neutral has more than five carbon atoms. Detailed modelling shows that column densities of C6H- observed in IRC+10216 and TMC-1 can be reproduced. We also explore anion ratios in PDRs such as the Horsehead Nebula.

Parker David

Velocity Map Imaging studies of Molecular Dynamics

The velocity map imaging technique measures the speed and angular distributions of nascent state-selected atoms or molecules arising from photodissociation, photoionization, reactive scattering, and also inelastic scattering, which is currently studied in Nijmegen as part of the Molecular Universe network. In this talk, I will describe the concept and applications of velocity map imaging and its related method spatial map imaging and how it is particularly powerful for studying inelastic scattering.

Pilleri Paolo

Search for Corannulene (C20H10) in the Red Rectangle

Polycyclic Aromatic Hydrocarbons (PAHs) are widely accepted as an important component of interstellar and circumstellar matter. PAHs are thought to be responsible for the mid-infrared emission features called Aromatic Infrared Bands (AIBs) in the 3-13 µm wavelength range. From these AIBs it is difficult to identify individual PAHs, since they are associated to specific chemical bonds that are common to the whole class and not to the entire molecular structure. To provide an unambiguous identification of an individual PAH, one would like to explore the mm wavelength domain, where the transitions are associated to the rotation of the entire molecular structure, providing a spectral fingerprint of a specific molecule. Unfortunately, most PAHs have no (or a very small) permanent dipole moment, precluding their definitive radio-astronomical identification. An exception to this is the non planar corannulene molecule (C20H10), with a measured Dipole moment of 2.07 Debye [1]. Following high resolution microwave spectroscopic measurements and detailed Monte Carlo simulations of the emission of interstellar PAHs, we present the results of a first observational campaign at the IRAM 30m telescope toward the Red Rectangle (RR) on several high-J rotational transitions of C20H10. We also searched for several other, smaller hydrocarbon molecules (e. g. C2H, C3H2, ...). No C20H10 or hydrocarbon lines were detected at any frequency, with a typical rms level of T=5 mK. Combining these limits with the emission model results in an upper limit for the fractional abundance of carbon locked in corannulene compared to all carbon in PAHs of about 10e-5. [1] K. Balbridge et al., Theor. Chemi Acc. 1997, 97,67-71

Scifoni Emanuele

Rotational excitation of interstellar NH3 and ND2H by He and H2 molecules

From its first detection in the interstellar medium, in 1968, as a first polyatomic molecule, ammonia has been the object of many studies through both theoretical and experimental means. In fact, its large abundance there (also in a rich variety of isotopical forms), its inversion-splitting occurrence in the range of 1 cm-1 and its typical collisions-dominated population of levels, as a symmetric top molecule, have made it a very good probe of various temperatures and gas densities. New calculations, including the ammonia inversion degree of freedom as an average on the potential energy surface, are reported as an attempt to overcome the longstanding gap between theoretical and experimental results about rotational excitation of NH$_3$ and its deuterated isotopomers by He and H$_2$ collisions in interstellar medium conditions.

Sims Ian

Experimental measurements of low temperature reactive and inelastic rate coefficients for atom – H2 collisions

Over the past 15 years the use of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) technique coupled with pulsed laser photochemical kinetics methods has revolutionised the field of low temperature kinetics in the gas phase. Radical-radical, radical-unsaturated molecule and even radical-saturated molecule reactions have been shown to be rapid down to the temperatures of those coldest of environments, dense interstellar clouds (10—20 K), and the results have had a major impact in astrochemistry and planetology, as well as proving an exacting test for theory. We have recently implemented a new approach designed to enable us to study gas-phase reaction kinetics and energy transfer at temperatures lower than previously achieved, using H2 as the collision partner. We have employed specially designed liquid nitrogen cooled nozzles coupled with either He or H2 buffer gas to yield flow temperatures of 20 K, 10 K and 7 K. We have coupled these new Laval nozzles with VUV laser photochemical techniques in order to study a variety of processes involving reactions or energy transfer processes. Results for collision-induced fine structure transitions for C(3PJ) in collision with He and H2 will be reported, as well as reactions of atomic species with H2 (including S(1D) + H2). All of these processes are of fundamental interest and are of relevance to the chemistry of dense interstellar clouds and/or the atmospheric chemistry of the outer planets.

Useli bacchitta Francesca

Electronic spectra of PAH+: searching for possible carriers of the 4430 Å DIB

Polycyclic Aromatic Hydrocarbons cations (PAH+) seem to be one of the most promising candidates for the identification of the diffuse interstellar bands (DIBs). We used the PIRENEA experimental set-up to measure the visible spectra of dehydrogenated derivatives of coronene (C24Hp+; p = 0, 10, 11) and of methylene-pyrene cation (CH2-pyr+). All these species have been proposed as possible carriers of the strongest DIB, the 4430 Å band [1, 2, 3]. We obtained their electronic spectra by recording with an OPO tunable laser the photo-dissociation yield of the species isolated in the cold ion trap of PIRENEA as a function of laser wavelength. Experimental results are presented and their astrophysical implications are discussed. References [1] Duley, ApJ 643, (2006) L21 [2] Duley, ApJ 639, (2006) L59 [3] Léger et al., A&A 293, (1995) L53-L56

Valiron Pierre

Constraining the spin ratio of H2 in dark clouds by collisional modeling

The ortho/para ratio of molecular hydrogen is expected to differ from the 3/1 value in star formation regions and to slowly decay to thermodynamical equilibrium values through chemical interconversion processes involving mainly collisions with residual ions. The ortho/para H2 ratio might thus be related to the age of the cloud, and could constrain chemical evolution models. It is also expected to influence the efficiency of deuteration in cold cores. However the direct measurement of the ortho/para H2 ratio is very difficult. Observations of electronic H2 transitions in absorption have been obtained with the satellite FUSE towards diffuse and translucent molecular clouds and shocked regions. Infrared observations of rotational transitions have been also recently performed with ISO and more recently Spitzer, and a few maps of the ortho/para H2 ratio were deduced in denser molecular regions. We will show that the strong dependence of inelastic rates involving selected transitions of some molecules can also provide useful constraints of the ortho/para ratio of H2 in dark clouds, where no direct measurement is possible. The discussion will focus upon the anomalous absorption of H2CO and low-lying rotational lines of HC3N for which the relevant microphysics data is now available. Possible generalization to NH3 and HCN lines will be sketched.

Walters Adam

Spectral data for Herschel, ALMA and SOFIA

Unambiguous identification of observed molecular spectral lines is critical for the scientific success of near-future FIR/Sub-mm astrophysical instruments. With the launch of the Herschel Space Observatory scheduled within a year and first science observations with the international interferometer ALMA and the airborne Stratospheric observatory SOFIA starting around 2010, it is therefore important to continuously evaluate existing spectroscopic data and to carry out new measurements and analysis when needed. Priorities for new data will be discussed. For HIFI/HSO the major need is to cover the new spectral region extending up to nearly 2 THz. It will be shown how predictions extrapolated from low frequency laboratory data may not be sufficiently reliable for correct interpretation of astrophysical observations. For ALMA the increased sensitivity will require for example the spectra of isotopologues of known species if new less-abundant species are to be efficiently identified. A rapid overview of the two major web-based public databases (JPL [1] and CDMS [2]) that cover the spectral region considered is available from the CESR Website [3] and concerns species already detected in space, including isotopologues, vibrationally excited states etc. We are contributing to the improvement in available spectroscopic data, and several examples will be given. New laboratory measurements at Cologne include a study up to 900 GHz of diethyl ether. New measurements at JPL include the following species: C3H8 [5]; HCO+, DCO+, H13CO+, D13CO+ [6]; CH3D, [7]; H13COOH, DCOOH, HCOOD [8] and CH2NH. At Lille we measured, for example, for the first time the rotational spectrum of the CS+ radical cation [9]. References [1] [2] [3] [4] I. Medvedev, M. Winnewisser, F. DeLucia, et al., Astrophys. J. Suppl. Ser. 148, 593-597 (2003) [5] B.J. Drouin, J.C. Pearson, A. Walters, V. Lattanzi, J. Mol. Spec. 240, 227-237 (2006) [6] V. Lattanzi, A. Walters, B.J. Drouin, J.C. Pearson, Astrophys. J. 662, 771-778 (2007) [7] V. Lattanzi, A. Walters, B.J. Drouin, J.C. Pearson, J. Quant. Spectrosc. & Rad. Transfer 109, 580-586 (2008) [8] V. Lattanzi, A. Walters, B.J. Drouin, J.C. Pearson, Astrophys. J. Supp Ser.176, 536 (2008) [9] S. Bailleux, A. Walters, E. Grigorova, L. Margulès, Astrophys. J 679 920 (2008)

Wehres Nadine

A combined experimental / observational study of the outflows in the Red Rectangle proto-planetary nebula

We present recent results on observational data for the Red Rectangle proto-planetary nebula. Observations were carried out in February this year using the New Technology Telescope (NTT) at La Silla Observatory in Chile. The ESO Multi Mode Instrument (EMMI) has been used for long slit spectroscopy between 550 nm and 650 nm at a resolution of 5000 and 2600. The observations concern the ongoing chemistry in the outflows of the nebula. The nebula is studied spectroscopically and spatially from close to the central star to further out the nebula. The spatial steps were set at 3'', 5'', 7'', 16'' and 21''. The obtained spectra give a comparison of the chemistry at certain distances from the central star which goes together with different temperature and density regimes. The nebula itself is an object of special interest for many reasons. In the optical regime a so-called extended red emission (ERE) [1] is observed that stretches over a range of about 250 nm, between 550 nm and 800 nm. The featureless emission bump is superimposed with narrow emission features, which are supposed to be created by molecular fluorescence [2], [3]. These sharp and narrow emission features were found to vary with distance from the central star. Specifically, the peak position shifts to the red and the width decreases with offset from the star [4], [5], [6]. This behaviour is very suggestive for molecular carriers which are rotationally and vibrationally cooling down with distance from the central star. The new observations aim at putting certain constraints on the carriers and to conclude on their nature. Furthermore Sarre [5] and Fossey [6] independently concluded that some of the features may be related to some of the diffuse interstellar bands (DIBs), which are features seen in absorption in reddened diffuse interstellar clouds. With large off-sets from the central star of the Red Rectangle these emission features approach the features of the diffuse interstellar bands and an overlap in energy and lineshape is possible. Up to now, most of the research only focuses on the stronger bands whereas the aim of the present study is an overall mapping of all emission features. Preliminary results are presented that are compared to ongoing experimental work. References [1] Witt et al., ApJ, (2006), 636, 303 [2] Schmidt et al., ApJ, (1980), 239, L133 [3] Warren-Smith et al., Nature, (1981), 292, 317 [4] Hans Van Winckel et al., 2002, A&A, 390, 147 [5] Sarre, Nature, (1991), 351, 356 [6] Fossey, 1990 PhD thesis, University College, London

Wiesenfeld Laurent

Collisional excitation of molecules

Yang Chung-hsin

Intermolecular interaction probed by inelastic scattering

The intermolecular interactions of ammonia with molecular hydrogen, and water with molecular hydrogen, have far-reaching consequences in the spectroscopy and chemistry of interstellar space. Our groups as part of the EU Molecular Universe network are charged with the task of understanding the energy transfer processes taking place during collisions of NH2D and H2O with H2. The conditions of interstellar space can be simulated in our laboratory by forming collimated molecular beams of NH2D or H2O, crossing them with a beam of noble gas, and applying the technique velocity map imaging to record the angular distribution of each new rotational state that was populated by collisions between molecules in the two beams. The angular distribution and the total signal strength yield detailed insight into the potential energy surfaces governing the intermolecular interactions. We compare our data with the latest predictions from ab initio theory.