Coulomb over-the-barrier Monte Carlo simulation to probe ion-dimer collision dynamics

We present a combined theoretical and experimental study of primary and postcollision mechanisms involved when colliding low-energy multiply charged ions with van der Waals dimers. The collision dynamics is investigated using a classical calculation based on the Coulombic Over-the-Barrier Model adapted to rare gas dimer targets. Despite its simplicity, the model predictions are found in very good agreement with experimental results obtained using COLd Target Recoil Ion Momentum Spectroscopy, both for the relative yields of the different relaxation processes and for the associated transverse momentum exchange distributions between the projectile and the target. This agreement shows to what extent van der Waals dimers can be assimilated to independent atoms.

Figure 1

(a) Schematic view of the interaction between a projectile ${\mathrm{A}}^{q+}$ and a target B with $N_B$ shared electrons involved on the way in ($N_B=2$ in this example), as the projectile is approaching the target, and where 0 to $N_B$ electrons may be captured by the projectile on the way out, as the projectile is leaving the target. (b–f) Schematic representation of the potential barrier evolution on the way in and on the way out of the collision. On the way in, the approach of the projectile towards the target lowers the potential barrier height between them (b). At a critical internuclear distance called the sharing radius $R_i^{\mathrm{in}}$ for each electron $i$, the potential barrier may become lower than the electron binding energy on the target (c), causing this electron to occupy a molecular state induced by the proximity of the projectile and the target. The maximum number of electrons shared will be reached at the impact parameter distance $\left|\right|\vec{b}\left|\right|$ where the way in stage ends and the way out stage starts (d). On the way out, the projectile will leave the target causing an increase of the potential barrier height. For each shared electron $i$, at a given distance called capture radius $R_i^{\mathrm{out}}$, the model will determine the probability for this active electron to be captured by the projectile or to be recaptured by the target (e, f).

Figure 2

Schematic view of the interaction between a projectile ${\mathrm{A}}^{q+}$ and a target B with $N_B$ electrons involved on the way in ($N_B=3$ in this example) and where 0 to $N_B$ electrons can be captured by the projectile on the way out. The transverse momentum exchange, due to a Coulomb repulsion along the interaction between the projectile and the target, is calculated for all successive steps in each of the three (way in, transition step, and way out) stages of the collision.

Figure 3

Representation of the collision between a projectile ${\mathrm{A}}^{q+}$ and a dimer target B-C in a three-dimensional Cartesian coordinate system ($x,y,z$) (a) and in the transverse plan ($x,y$) (b). ($x_B$, $y_B$, $z_B$) are the coordinates of the center B of the molecule, ${\theta }_B$ is the angle of the molecular axis with respect to the projectile beam axis ($z$ axis), ${\varphi }_B$ is the projected angle of the molecular axis in the transverse plane with respect to the $x$ axis, and ($x_A$, $y_A$) are the coordinates of the projectile in the transverse plane.

Figure 4

Relative yield of the different fragmentation channels obtained experimentally (white) and using the MC-COBM calculations. MC-COBM results are given for two different scenarios: when considering no direct production of dissociative states with one neutral fragment (light gray) and when considering equiprobable populations of dissociative states and transient nondissociative states (gray). The contributions of the initial capture channels are indicated when relevant.

Figure 5

Differential cross section (DCS) in transverse momentum exchange obtained experimentally (thick black line) and using the MC-COBM for ${\mathrm{Ar}}^{9+}$ projectiles colliding with Ar dimers (a) and Ne dimers (b). The MC-COBM distributions, previously normalized to the experimental data, are given for three different charge screening parameters: $S=1$ (blue dashed line), $S=1/2$ [green line (gray)]. and $S=0$ [red line (dark gray)]. For $S=1/2$, the distribution prior convolution with the apparatus response function is also given as a dotted green (gray) line.

Figure 6

Differential cross section (DCS) in transverse momentum exchange obtained experimentally (thick black line) and using the MC-COBM for ${\mathrm{Xe}}^{20+}$ projectiles colliding with Ar dimers (a) and Ne dimers (b). The MC-COBM distributions, previously normalized to the experimental data, are given for three different charge screening parameters: $S=1$ (blue dashed line), $S=1/2$ [green line (gray)], and $S=0$ [red line (dark gray)]. For $S=1/2$, the distribution prior convolution with the apparatus response function is also given as a dotted green (or gray) line.