Low-mass secondary-ion ejection from molecular solids by MeV heavy ions: Radial velocity distributions.

Abstract

Secondary ions sputtered in individual MeV ion impacts are analyzed in a high resolution time-of-flight mass spectrometer. The initial radial velocity distributions of low mass (up to m/z\ensuremath{\approxeq}300 u) positive and negative secondary ions, sputtered from carbon-containing molecular solids (polymers, bioorganic molecules, and fullerene targets) are investigated. The first (〈v〉) and second (〈${\mathit{v}}^{2}$〉) moments of the velocity distributions vary systematically with the atomic composition of secondary ions of the type ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}^{+}$, ${\mathrm{C}}_{\mathit{n}}$${\mathrm{F}}_{\mathit{m}}^{+}$, ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}$${\mathrm{F}}^{+}$, and ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}$${\mathrm{O}}^{+}$. Positive ions formed from extensive fragmentation-rearrangement of the original molecular structure (e.g., ${\mathrm{C}}_{\mathit{n}}^{+}$) tend to be ejected towards the MeV ion trajectory (positive mean radial velocity) and to have the largest 〈${\mathit{v}}^{2}$〉. Saturated species (e.g., ${\mathrm{C}}_{\mathit{nX}2\mathit{n}}$, X=F, H) tend to have smaller 〈${\mathit{v}}^{2}$〉 and negative 〈v〉. These effects are weaker as the stopping power of the primary ions is decreased and are not observed for negative ions. The observed effects demonstrate a correlation between chemical composition of an ion and its formation and ejection processes. The chemical transformations and the processes leading to ion formation and ejection are functions of both the density and the gradient of the deposited energy at a particular position from the track center. This interconnection results in a regular dependence of the properties of ejected ions (e.g., momentum received) on their chemical composition. The correlation of the momentum imparted to the fragment ions with the geometry of impact indicates that such species are predominantly ejected in a nonevaporative process. \textcopyright{} 1996 The American Physical Society.