Dissociation dynamics and ionization processes in slow ion-molecule collisions

Abstract

When an isolated molecule interacts with an external perturbation created by energetic charged ions, excitation of the molecule may occur. If the perturbation strength is enough, removal of a number of electrons from the molecule via different ionization processes can occur, resulting in multiple ionization of the molecule. The so-formed multiply ionized molecular ion is generally unstable and subsequently dissociates into its fragment ions. These fragment ions move apart in a continuum with finite kinetic energies and constitute a dynamic many-body coulombically interacting system. The perturbation strength of the projectile ion interacting with the target is generally characterized by the projectile charge state (q) and its velocity (v), both are in atomic units (a.u.). For v < 1 a.u. and v > 1 a.u., the impact of projectile is considered slow and fast, respectively. In the ion-molecule collision, for a given charge state and velocity of the projectile ion, the energy transferred to the molecule depends on the collision impact parameter (b). Experimentally, there is no control over the impact parameter of collision, and therefore it limits the information on energy transfer that can be obtained in an experiment. The energy transferred to a molecule under charged particle impact is shared among its electronic and nuclear degrees of freedom, resulting in the excitation of the target molecule. Subsequently, the fate of so-formed excited molecule is decided by the properties of its excited states. This work investigates the multiply ionized molecular ions which are formed under charged particle impact, and eventually dissociate into their fragment ions. The main focus of this work is to address the question such as how the projectile properties affect the kinematics of dissociating multiply ionized molecular ions created under the impact of slow (v < 1 a.u.) projectile ions. To address this, systematic experiments are performed with projectile ions of various charge states, and with the simplest projectile, the proton (H + ), of different velocities. The second part of this work addresses how the underlying ionization process, such as the direct and capture processes influence the dissociation dynamics of formed molecular ions. Investigations are carried out using ions beams from an electron beam ion source (EBIS) and fragments are analyzed using an ion-momentum spectrometer (IMS). A post collision charge state analyzer is especially designed and implemented. Coupled with the existing IMS, it is used to separate direct and capture ionization processes. Our investigation shows that in slow impact regime, the projectile charge state, q, influences the probability of accessing different excited electronic states of the transient molecular ion. We also demonstrate that despite being a structureless projectile, a slight variation in the impact velocity of the proton affects the kinematics in three-body dissociation of CO2^3+ molecular ion in slow impact regime. With the separation of ionization processes, we find that the different order of capture processes significantly influence the branching ratio of various formed fragment ions, while the KER distributions are not significantly affected.