ECAMP 15

The study of the interactions between ion beams and biomolecules such as nucleobases, nucleosides, amino acids or peptides is a relevant topic for hadrontherapy applications. After interaction with the energetic ions, the biomolecular target could be ionized and excited. Molecular fragmentation is one of the relaxation process for such ionized/excited states. To better quantify the fragmentation channels and the collision processes, it is interesting to measure the differential cross-sections of the created fragments in both kinetic energy (KE) and emission angle. The kinetic energy of the emitted fragment gives, indeed, information on the electronic state of the transient molecular ions prior dissociation. To do so, a Velocity Map Imaging (VMI) spectrometer initially designed to measure ion-induced electron emission from biologically relevant molecules [1] has been adapted to study fragmentation dynamics.
In the experimental set-up, the pulsed projectile ion beam crossed perpendicularly a continuous target beam produced from an effusive cell. The fragments formed in the interaction volume are velocity focused by a multi-electrode VMI spectrometer onto the 2D position sensitive detector (PSD) mounted after a flight tube. The time-of-flight (ToF) arrival signals are used to distinguish the fragments according to their m/q ratio with m and q, the mass and the charge state of fragment respectively. The detector is pulsed in order to image only a specific m/q specie determined by its ToF. An inverse Abel transform is then performed to the image to reconstruct the initial velocity vector and deduce both kinetic energy spectra and emission angle. The mass resolution of the spectrometer depends, in part, on the flight tube length. We have therefore designed a longer flight tube to improve the resolution, which allows us to access to both the kinetic energy and emission angle for many fragments of complex molecules that we could not separate before.
In this contribution, the new design of the VMI spectrometer will be presented. Moreover, the different fragmentation pathways of Uracil and Adenine upon 120keV-Ar8+ collision will be discussed based on the KE spectra measured. The influence of the target beam initial velocity on the images of fragments that leave with a low kinetic energy will also be discussed. Indeed, due to Maxwell-Boltzmann velocity distribution of the target beam, the images are shifted and distorted, presenting a more elliptical shape. A deconvolution method using a Wiener filter has been implemented on the experimental images to remove the influence of the target initial speed with rather satisfactory results.

[1] Nicolas Sens, Michal Ryszka, Jean-Christophe Poully, Alain Méry, Jean-Yves Chesnel, and Violaine Vizcaino. A velocity map imaging spectrometer for measuring absolute differential cross sections for ion-induced electron emission from molecules. Rev. Sci. Instrum. 93, 085103 (2022)5