Photodissociation in Astrochemistry, 2 - 4 February 2015
This focussed workshop sought to clarify the outstanding questions and priorities regarding photodissociation in the forthcoming era of astrochemistry, and evaluate the present and future capabilities of spectroscopists and chemical physicists. About 40 participants with expertise ranging from laser physics, quantum chemistry and the modelling of interstellar clouds and planetary atmospheres attended the workshop.
Here we provide the workshop program, its original objective, a summary of its results, some review material concerning photodissociation in astrochemistry, and copies of most of the presented slideshows.
Objectives of the workshopConsider the following problems in astrochemical photodissociation.
- Which molecules are present in clouds, disks, and planet atmospheres exposed to intense UV radiation; how do they photodissociate: through lines or continuum; how do they (observably) influence their surroundings?
- What are the important ranges of temperature, density, extinction magnitudes, and UV wavelengths?
- What is the most useful kind of data? A completely assigned and modelled spectrum, raw experimental cross sections and linelists, superb accuracy over a small wavelength range (e.g., Ly alpha) or lower accuracy over a much larger range? Importance of self- and mutual shielding and tabulation of shielding functions; isotope selective processes?
- Photolysis in the gas phase vs ices; small versus large molecules (PAHs)
- Contributions to public databases:
- The generation of UV radiation: synchrotrons, lasers, plasmas, or electron excitation.
- The quantitative measurement of cross sections and dissociation: photoabsorption, fluorescence, or the detection of dissociation products and their excitation states; branching ratios to products.
- The synthesis and study of radical species and ions, as well as ices.
- Calculation of electronic structure and dipole moments
- Nuclear dynamics of excited states, modelling the branching ratio of ionisation, dissociation products, and fluorescence.
- More empirical theoretical models with increasing complexity: competition between dissociation and internal conversion for polyatomic molecules.
Summary of the workshop results
The chemists and molecular physicists present highlighted a wide variety of experiments, varying from absolute cross section measurements across a wide wavelength range obtained using synchrotrons to detailed analysis of the product branching ratios and product excitation at a specific laser wavelength. Ab initio quantum chemistry and nuclear dynamics calculations are powerful methods to investigate photodissociation processes for small to medium-sized molecules, including radicals and ions.
The astrochemical modellers demonstrated the sensitivities in their models to photoabsorption and destruction of molecules and their isotopologues, including simple ones like H2 and N2, which have, or would, benefit from accurate constraint of basic photoprocesses. At the other extreme are the photofragmentation processes of large PAH species and their re-arrangement to C60.
A newly developing topic is the photoabsorption/desorption of molecular ices and the similarities and differences to gas-phase photodissociation. Ice spectra are often blue shifted and sometimes there is no correspondence with any gas-phase band. However, integrated cross sections are generally not too different from those in the gas-phase species, with some notable exceptions.
The branching ratio to particular gas-phase photodissociation fragments and internal excitation was shown to be a significant and observable effect in atmospheres and disks. Some difficult but successful laboratory quantifications of these ratios were presented.
It became clear that the providers of photodissociation/ionisation cross sections, and rates were willing and able to continue and extend their experimental studies or calculations if they were given a clear objective, viz á viz a particular molecule, process, or wavelength. An initial "wish list" was put together at the meeting (see slides van Dishoeck), but more work is needed to refine this.
Several useful databases of photo-molecular rates and cross sections were discussed: The Leiden photodissociation/ionisation database, The University of Georgia Molecular Opacity Project, The MPI-Mainz UV/VIS Spectral Atlas, the CACID astrochemical ice database, VAMDC, AtMoCiad (Atomic and Molecular Cross section for Ionization and Aurora Database), and the KIDA and UMIST/UDFA rate databases.
The talks and discussions highlighted that there are many opportunities to improve our understanding of astronomical systems with additional work on photodissociation. Hopefully this workshop has encouraged laboratory and theoretical physical chemists to continue or adapt their research to astrochemical problems, and will lead astrochemists to identify specific needs in their models for the wider community to address.
Some review papers on the subject of astrochemical photodissociation
- Ewine van Dishoeck's slideshow
- Photodissociation of molecules in astrochemistry, van Dishoeck and Visser (2015)
- Three milieux for interstellar chemistry: gas, dust, and ice, Herbst (2014)
- Astrochemistry of dust, ice and gas: introduction and overview, van Dishoeck (2014)
- Chemical Reviews: Astrochemistry (2013)
- The molecular Universe, Tielens (2013)
- Exoplanet atmospheres, Seager and Drake (2010)
- Photoprocesses in protoplanetary disks, van Dishoeck et al. (2006)
Speakers and links to their slideshows
- Alan Heays (Leiden Observatory)
- Ewine van Dishoeck (Leiden Observatory)
- Mike Ashfold (University of Bristol)
- Gerrit Groenenboom (Radboud Universiteit, Nijmegen)
- Phillip Stancil (University of Georgia)
- David Parker (Radboud Universiteit, Nijmegen)
- Marc van Hemert (Leiden Institute of Chemistry)
- Pablo Castellanos Nash (Leiden Observatory)
- Paul Seakins (University of Leeds)
- Evelyne Roueff (LERMA, Observatoire de Paris)
- Markus Röllig (University of Cologne)
- Wing Fai Thi (MPI for Extraterrestrial Physics)
- Carla Maria Coppola (University of Bari Aldo Moro)
- Olivia Venot (KU Leuven)
- Antonio García Muñoz (European Space Agency)
- Cheuk-Yiu Ng (University of California, Davis)
- Jordy Bouwman (Radboud Universiteit, Nijmegen)
- Alan Heays (Leiden Observatory)
- Jim Lyons (Arizona State University)
- Ruud Visser (European Southern Observatory)
- Catherine Walsh (Leiden Observatory)
- Inga Kamp (University of Groningen)
- Xiaohu Li (Leiden Observatory)
- Nigel Mason (The Open University, Milton Keynes)
- Valentine Wakelam (Laboratoire d'Astrophysique de Bordeaux)
- Guillermo Muñoz Caro (Centro de Astrobiología, Madrid)
- Anita Dawes / Nigel Mason (The Open University, Milton Keynes)
Vacuum ultraviolet photoabsorption spectroscopy of astrochemical ice analogues
- Guilherme Almeida (PUC-RJ, Rio de Janeiro)
- Mingli Niu (VU University, Amsterdam)
- Rowin Meijerink (Leiden Observatory)
- Wim Ubachs (VU University, Amsterdam)
- Panayotis Lavvas (CNRS, Reims)
- Sergio Pilling (Universidade do Vale do Paraiba, Brazil)
Photodissociation induced by broadband soft X-rays in the surface of the outer solar system moons
- Gaël Cessateur (BIRA-IASB, Belgium)
Photoabsorption in Jovian moons and cometary atmospheres