Soft fragmentation of carbon monoxide by slow highly charged ions

The fragmentation pattern and corresponding kinetic energy release distributions from carbon monoxide after collisions with slow, highly charged ions (${\text{C}}^{6+}$ at $v=0.8\text{a}\text{.u}\text{.}$ and ${\text{Ar}}^{11+}$ at $v=0.39\text{a}\text{.u}\text{.}$) are reported. Correlated measurements of the projectile final charge state and all charged molecular fragments were obtained using a coincidence time-of-flight technique coupled to projectile charge state separation. Fragmentation branching ratios for the dominant electron capture processes in these collisions are similar to those resulting from ionization by fast highly charged projectiles. This result can help simplify simulations of astrophysical or artificial plasma environments, as the branching ratio may be considered essentially constant for both electron capture and ionization as long as the relevant impact parameter is large.

Figure 1

(Color online) Fragmentation branching ratios from the transient excited states of ${\text{CO}}^{+}$ (top) and ${\text{CO}}^{2+}$ (bottom) as a function of $\frac{q}{v}$. Fragmentation channels are denoted by ${\text{CO}}^{+}$ $(\square )$, ${\text{C}}^{+}+\text{O}$ $(△)$, $\text{C}+{\text{O}}^{+}$ $(○)$, ${\text{C}}^{+}+{\text{O}}^{+}$ $(■)$, ${\text{CO}}^{2+}$ $(▽)$, ${\text{C}}^{2+}+\text{O}$ $(◇)$, and $\text{C}+{\text{O}}^{2+}$ $(★)$. Besides the present results at $q/v=7.5$ $\left({\text{C}}^{6+}\right)$ and 28 $\left({\text{Ar}}^{11+}\right)$, the other projectiles include 1 MeV/u ${\text{F}}^{4+}$, ${\text{F}}^{9+}$, ${\text{B}}^{5+}$, and ${\text{H}}^{+}$, and 25 keV ${\text{H}}^{+}$. Except for the 25 keV ${\text{H}}^{+}$ impact, in which the fragmentation branching ratio shown here includes both capture and ionization, the results from the previously reported measurements [17, 19, 20] represent direct ionization of the CO target.

Figure 2

(Color online) The probability for double electron capture (DC) to populate the projectile principle quantum states $n$ and $n^{\prime }$ according to EOBM calculations of ${\text{Ar}}^{11+}+\text{CO}$ collisions at $v=0.39$. The probabilities are plotted versus the energy of the $\left(n,n^{\prime }\right)$ state. The string of numbers in the legend indicates which of the 10 non-$K$-shell electrons are captured. For example, in the top panel, the two least tightly bound electrons are captured from CO. The EOBM estimates that this occurs 78% of the time. Colors are associated with a given $n$: $n=4$ (green), 5 (blue), 6 (red), 7 (black).

Figure 3

(Color online) ${\text{C}}^{+}+{\text{O}}^{+}$ KER distributions resulting from collisions with a range of projectile $\frac{q}{v}$. The high velocity results, in which electrons are removed by ionization, are from Ref. [19], while the ${\text{O}}^{7+}$ results are from Tarisien et al. [27].