Target Structure Induced Suppression of the Ionization Cross Section for Very Low Energy Antiproton-Hydrogen Collisions

Low energy antiprotons have been used previously to give benchmark data for theories of atomic collisions. Here we present measurements of the cross section for single, nondissociative ionization of molecular hydrogen for impact of antiprotons with kinetic energies in the range 2–11 keV, i.e., in the velocity interval of 0.3–0.65 a.u. We find a cross section which is proportional to the projectile velocity, which is quite unlike the behavior of corresponding atomic cross sections, and which has never previously been observed experimentally.

Figure 1

Experimental data and theoretical calculations of the cross section for single ionization by antiproton impact on atomic hydrogen, molecular hydrogen and helium as a function of the projectile laboratory velocity. For atomic hydrogen, we show as dashed-dotted black curves calculations by Lühr and Saenz [12] and by Igarashi et al. (green dash-dot-dot) [11]. Also shown as a brown solid curve is the low-velocity calculation of the total cross section for ionization by Cohen [4]. In the case of the molecular hydrogen target, we show the experimental data of the present work (▴ run in 2010, ▵ run in 2009) as well as previous data obtained by our group, (○), published in Hvelplund et al. [9]. The straight, solid line is a linear fit to these data below a velocity of 1 a.u. The data points marked ▪ indicate the sum of the cross sections for nondissociative and dissociative ionization of molecular hydrogen by Hvelplund et al. [9]. The long-dashed dark blue curve shows a calculation of this cross section by Lühr and Saenz [3]. The vertical line indicates the projectile energy above which more than 90% of the $D_2{}^{+}$ ions created and more than 90% of the projectiles deflected in the collisions are collected by our apparatus. For the helium target, we show experimental data by Hvelplund et al. (□) [9] and by Knudsen et al. (▾) [1]. The short-dashed, nearly horizontal line is a straight-line fit to these data below 1 a.u. to guide the eye.

Figure 2

An example of a TOF spectrum as observed in these measurements. It was obtained for an impact energy of 10.6 keV. The peak is $D_2{}^{+}$ ions. The two vertical lines indicate the TOF interval where $D^{+}$ ions are expected. One channel is 2.44 nsec. Zero TOF is in channel $-70$.