Velocity and Charge State Dependences of Molecular Dissociation Induced by Slow Multicharged Ions

Post collision analyzer to study charge-exchange processes in ion-molecule collisions

We have designed an electrostatic charge state analyzer for ion beams having energies in the range of 5-20 keV/q. It is primarily built to investigate the different ionization processes involved in the slow (v < 1 a.u.) impact of highly charged ions on molecules. The analyzer is a cylindrical deflector analyzer (CDA) based on a pair of concentric cylindrical sectors of radii 110.2 and 95 mm, subtending an angle of 127° at its center. Additionally, an Einzel lens and a quadrupole deflector are deployed to focus and steer the ion beam. The compact design of the analyzer permits easy integration with an ion momentum spectrometer used for studying the fragmentation of the target molecules. The characterization of the CDA including its calibration and its transmission function is carried out using an ion beam delivered from an electron beam ion source. To check the performance of the setup, we have carried out experiments comprising the impact of Ar¹⁶⁺ projectiles on CO₂ target molecules at an energy of 18 keV/q. With the help of the CDA, different charge exchange ionization processes, such as single capture, double capture, and triple capture of electrons by the projectile ion have been separated efficiently. The possibility of a modification in the geometry of CDA is discussed to further improve its performance.

Proton migration in hydrocarbons induced by slow highly charged ion impact

Different from most of the previous studies using light or photons, we use highly charged ions as projectiles to activate proton migration in the smallest saturated and unsaturated hydrocarbon molecules, i.e., CH₄ and C₂H₂. The H₃ ⁺ formation channel (H₃ ⁺ + CH⁺) and isomerization channel (C⁺ + CH₂ ⁺), serving as indicators of proton migration, are observed in the fragmentation of CH₄ and C₂H₂ dications. Corresponding kinematical information, i.e., kinetic energy release, is for the first time obtained in the collisions with highly charged ions. In particular, for the C⁺ + CH₂ ⁺ channel, a new pathway is identified, which is tentatively attributed to the isomerization on high-lying states of acetylene dication. The kinetic energy release spectra for other two-body breakup channels are also determined and precursor dication states could thus be identified.

Charge driven fragmentation of nucleobases

We studied multiple ionization of single nucleobases by means of slow highly charged ions (Xe(q+), q=5-25). The products of the subsequent fragmentation were studied using high resolution coincidence time-of-flight spectrometry. We observed a strong dependence of the fragment kinetic energies on the initial charge state of the intermediate parent ions as well as on the initial chemical environment of the respective fragment ions within the parent molecule. The data allow us to shed light on the charge distribution within the molecule as well as on the fragmentation dynamics of these intermediate size systems.