Relaxation dynamics of astro-PAHs by coupling a cryogenic electrostatic storage ring to VUV synchrotron radiation – SynPAHcool

The SynPAHcool project aims to study the relaxation dynamics of polycyclic aromatic hydrocarbons (PAHs) of astrophysical interest following the absorption of a VUV photon and in extreme isolation conditions similar to those of the interstellar medium (ISM). More specifically, we want to focus on the competition between dissociation and radiative cooling which plays a crucial role in the survival of these species in ISM.
In order to study this competition in the laboratory, a cryogenic electrostatic storage ring (T<30K), called Polar Mini-Ring, will be built, coupled, and interfaced with the VUV DESIRS beamline at SOLEIL synchrotron: a challenge and a first. This device will provide unique measures of relaxation rates (dissociation, radiative cooling, and possibly isomerization) for PAH cations irradiated by VUV photons of variable energy and on an unprecedented temporal range from microsecond to seconds. The three partners, ILM (Lyon), IRAP (Toulouse), and SOLEIL (Saint-Aubin) will pool their scientific expertise and technological know-how to build this new experimental device, carry out a series of preparatory experiments and three campaigns on the DESIRS beamline.
Our current knowledge of PAH dissociation rates is based upon heterogeneous data from different experimental devices (time of flight or ion traps) whose very different time ranges probe very different internal energy ranges. Extrapolating these measurements from one range to another shows very significant differences. An electrostatic storage ring, such as Polar Mini-Ring, will solve this problem because of its very wide temporal dynamics ranging from microsecond to seconds. On the low internal energy side, dissociation competes with radiative emission cooling, which, for PAH cations, may result in recurrent fluorescence in the few milliseconds range and via infrared emission up to several seconds. To accurately quantify the rates of these processes as a function of internal energy, it is crucial to start from cooled ions and to work in the single-photon absorption regime with a tuneable photon energy between 7 and 14 eV. The DESIRS line is therefore particularly well suited to irradiate PAH cations that will be stored and cooled in Polar Mini-Ring. The construction of a combined shutter/chopper device will allow the VUV synchrotron radiation time structure to match the one of the cation storage in Polar Mini-Ring with variable duty cycle irradiation sequences to achieve our scientific objectives.
Preliminary experiments will be conducted to commission Polar Mini-Ring and to prepare the campaigns for DESIRS. Their scientific objective will be to evidence the process of isomerization in the case of the H loss fragment of the methyl-pyrene cation and to move to the study of PAH of larger sizes relevant to astrophysics, containing about 80 carbon atoms. They will use the PIRENEA cryogenic trapping device. The final goal of the project will be achieved in the last year with measurement campaigns with Polar Mini-Ring coupled to DESIRS at SOLEIL.