Electron Capture and Loss by Moving Ions in Dense Media

Several experiments have shown an anomaly in the stopping power which appears to be greater for incident heavy particles in condensed media than in gases. It is suggested that this anomaly is characteristic of incident ions, i.e., of particles that are not completely stripped of orbital electrons, and it seems to be due to the reaction of the perturbed medium upon the moving ion having charge Ze, which reaction results in a field F=(1/Ze)(dW/dz) acting against the motion of the ion (dW/dz is the energy loss per cm). Although F is relatively weak in gases, it is substantial in condensed media and may cause the spontaneous emission of electrons carried by the ion (autoionization), thus increasing the ions' effective charge. The higher effective charge accounts for the higher stopping power in condensed media. It has been determined that when an incident helium atom passing through a liquid undergoes a collision, thus emerging in a singly ionized and excited state, it cannot exist in such a state since it becomes instantly doubly ionized. However, the same type of collision occurring in a gas leaves the atom singly ionized. The effective charge of the incident particle depends upon the values φ₀, φ₁, and φ₂, representing the probability of the particle's being neutral, singly ionized, and doubly ionized, respectively, where ${{φ}}₀:{{φ}}₁:{{φ}}₂={{σ}}₁₀{{σ}}₂₁:{{σ}}₀₁{{σ}}₂₁:{{σ}}₀₁{{σ}}₁₂$ (${{σ}}if$ represents the cross section for electron loss or capture; $i$ and $f$ represent the charge before and after the event, respectively). It is shown that ${{σ}}if$ is higher for a condensed medium than for a gas when $i<f$ and lower when $i>f$. ${{σ}}if$ has been calculated for He passing through argon gas at 400 kev, and gives roughly ${{φ}}₀:{{φ}}₁:{{φ}}₂=0.05:0.75:0.20$; thus, an average square charge is ${〈{{ε}}²〉}Av=1.55{e}²$. If autoionization is taken into account, ${{σ}}if$ for He in liquid argon gives roughly ${{φ}}₀:{{φ}}₁:{{φ}}₂=0.04:0.70:0.26$, and thus ${〈{{ε}}²〉}Av=1.74{e}²$.