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 workshop

Consider the following problems in astrochemical photodissociation. Capabilities and challenges of laboratory experiments Theoretical methods

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

Speakers and links to their slideshows