Fully Correlated Electronic Dynamics for Antiproton Impact Ionization of Helium

We present total cross sections for single and double ionization of helium by antiproton impact over a wide range of impact energies from $10\mathrm{keV}/\mathrm{amu}$ to $1\mathrm{MeV}/\mathrm{amu}$. A nonperturbative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the ionized electrons. Excellent agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections at high impact energies, whereas for lower impact energies, some discrepancies with experiment are found. At an impact energy of 1 MeV we also find that the double-to-single ionization ratio is twice as large for antiproton impact as for proton impact, confirming a long-standing unexpected experimental measurement.

Figure 1

(a) Cross sections for single ionization of helium by antiproton impact. Filled squares: TDCC calculations; filled diamonds: calculations of Schultz and Krstić [5]; filled and open circles: experimental measurements of 4, 15. (b) Cross sections for double ionization of helium by antiproton impact. Filled squares: TDCC calculations; crosses: calculations of Diaz et al. [10]; filled circles: experimental measurements of 4.

Figure 2

Evolution of the ionization probability, $\mathcal{P}(E,b=0.5a_0)$, for a 50 keV antiproton collision with a helium atom as a function of the impacting ion distance. Upper panel: single ionization probability summed over all partial waves. Lower panel: double ionization probability. The helium atom is located at the origin of the collision system ($d_0=0$).

Figure 3

Weighted probabilities for single (upper) and double (lower) ionization of helium by antiproton (red solid line) and proton (black dashed line) impact at 1 MeV impact energy.