Role of a Neighbor Ion in the Fragmentation Dynamics of Covalent Molecules

International audienceFragmentation of molecular nitrogen dimers (N2)2 induced by collision with low energy 135 keV Ar 9+ ions is studied to evidence the influence of a molecular environment on the fragmentation dynamics of N2 cations. Following the capture of three or four electrons from the dimer, the 3-body N + 2 + N m+ + N n+ (with (m,n)=(1,1) or (1,2)) fragmentation channels provide clean experimental cases where molecular fragmentation may occur in the presence of a neighbor molecular cation. The effect of the environment on the fragmentation dynamics within the dimer is investigated through the comparison of the KER spectra for these 3-body channels and for isolated N (m+n)+ 2 monomer cations. The corresponding KER spectra exhibit energy shifts of the order of 10 eV, attributed to the deformation of the N m+ + N n+ potential energy curves in the presence of the neighboring N + 2 cation. Weakly bound systems offer a unique tool to investigate the transition from gas phase isolated atoms/molecules to the condensed phase. Over the past decades, large amount of experimental and theoretical work has been conducted to understand the properties of clusters spanning a large range of sizes and constituents. Rare gas dimers are of particular interest as they offer a simple system consisting of two neighbor quasi-independent atoms where the electrons remain localized on each atom resulting in a very low electron mobility across the dimer [1]. This specific property has enabled access to site sensitive decay processes by retaining the memory of the initial capture processes [2]. Low electron mobility also leads to specific de-excitation processes enabled by the presence of the neighbor atom. Among these, Interatomic Coulomb Decay (ICD) has been thoroughly studied as the resulting emission of a low energy electron may be responsible for extensive radiation damage in matter [3,4,5,6]. In clusters of complex molecules, the surrounding molecules may act as a protective environment by dissipating the transferred excitation energy among the degrees of freedom available in the cluster. As an example, in amino acids clusters, it has been shown that preferential breaking of intermolecular hydrogen bonds significantly prevents intramolecular fragmentation to occur [7]. Recently, it has also been demonstrated that ion irradiation of fullerene or polycyclic aromatic hydrocarbon (PAH) clusters can drive the formation of new chemical bonds leading to the formation of larger covalent molecules [8,9]. By contrast, dimers formed by two diatomic molecules are small clusters of intermediate complexity for which the availability of detailed experimental data is still scarce. We consider here dimers of the simple diatomic N 2 molecule which constitutes a model system of polyatomic complex containing both covalent intramolecular bonds and van der Waals intermolecular bonds. The first evidence of (N 2) 2 dimers at 77 K temperature was found using infrared spectroscopy more than 40 years ago [10,11]. More recently, quantum chemistry calculations have predicted the existence of several isomeric states of the ground state of (N 2) 2 that correspond to different geometrical alignment of the constitutive molecules with respect to the dimer axis [12,13]. In the present work, slow highly charged ions and (N 2) 2 targets are used to produce (N 2) q+ 2 ionized dimers. A COLTRIMS (COLd Target Recoil Ion Momentum Spec-troscopy) setup allows to measure in coincidence the time of arrival and positions of the positively charged fragments resulting from the collision between the (N 2) 2 dimer target and Ar 9+ projectile ions. The (N 2) 2 target is produced using the supersonic expansion of N 2 through a 30 µm nozzle at a pressure of 25 bars at room temperature. The proportion of (N 2) 2 dimers inside the gas jet is estimated to be a few percent. A continuous beam of Ar 9+ projectiles is extracted from the electron cyclotron resonance ion source of the ARIBE-GANIL facility with energy of 135 keV and is collimated by a 600 µm diameter aperture located in front of the spectrometer. The target and projectile beams cross at right angle at the center of the spectrometer where a homogeneous perpendicular electric field (40 V/cm) ensures 4π collection of singly charged ions up to 24 eV on a 80 mm diameter micro-channel plate (MCP) coupled to delay-line anode. The final charge state of the projectile ions is determined using an electrostatic parallel plates analyzer located afte