Fragmentation dynamics of molecules studied with an new electrostatic storage ring. – ANNEAU

This project aims at studying experimentally the relaxation dynamics of molecular systems over a broad range of time with a control on the type and the amount of excitation energy. The molecules under focus go from small molecules such as HDO up to system with much larger size such as polypeptides. Regarding the excitation and the dissociation of small molecules, quantum effects play an important role and the individual electronic levels have to be taken into account. For molecules of bigger size, the excitation levels are distributed among some bands of energy and the relaxation involves a large number of rovibrational states. Thus, in contrary to the small molecules, statistical processes are more appropriate for modeling the energy relaxation and fragmentation of large molecules. The goal of this project is to study the transition between the statistical and the non-statistical quantum aspects by varying the size of the system. As an example, the benzene molecule up to large PAH such as the coronene will be studied first. Then, other systems of interest will go from dipeptide up to long peptide chains (hundred peptides). The fast processes in a time range of ns-µs as well as the intrinsically slow statistical processes in the range of ms-s will be studied as a function of the excitation energy of the system. To achieve this goal, both the amount of energy deposited into the system as well as its electronic and nuclear nature will be controlled.

This project couples innovating technologies in the areas of ion trapping, excitation by ion impact and coincidence particle detections. It will allow us to provide information on both the fast and slow relaxation dynamics under energy control. It is based essentially on the construction of an inventive electrostatic ring, which is cryogenic and of reasonable size. The molecules will be first cooled down during a sufficiently long storage time to ensure a well-controlled narrow internal energy distribution. The stored molecules will be then excited by interaction with a projectile ion beam, which will be superimposed to the molecular beam along a straight section of the ring. As the stored molecules have a kinetic energy on the order of keV, the detection will be performed in reverse kinematics. Neutral and charged fragments and electrons originating from the collision will be detected in coincidence. By using imaging detectors, the kinetic energies of both the electrons and the fragments will be measured. With these precise measurements of all observables relative to a single event, we will have access to important parameters such as the temperature of the system, the energy resonances, etc.

This original experimental set up, unique in the world, will open a new field of investigation for organic molecules and molecules of biological interests due to its complementarities to the existing methods in the field of mass spectrometry, such as photon-induced-dissociation (PID) or electron-capture-dissociation (ECD). By using the ion impact method, the excitation of the molecules is performed in a femto second time scale and over a broad range of energy (0.1-300 eV). By varying the mass and the kinetic energy of the projectile ions, the dominant excitation mechanism will be selected to be either of electronic or nuclear nature. By itself, the electrostatic ring represents a true breakthrough due to its simplicity of conception, its compactness and its moderate cost in comparison to the analog existing instruments. The use of electrostatic field pushes forward the upper limit in m/q ratio allowing one to study small as well large monocharged molecules. Ultimately, some original scientific issues will be accessible due to the innovative characteristics of the apparatus. This is the case as an example of topics such as the charge transfer between anions and protonated biomolecules or between multicharged ions (Xe30+) and strongly negatively charged biomolecules.