Manipulating Atomic Fragmentation Processes by Controlling the Projectile Coherence

We have measured the scattering angle dependence of cross sections for ionization in $p+{\mathrm{H}}_2$ collisions for a fixed projectile energy loss. Depending on the projectile coherence, interference due to indistinguishable diffraction of the projectile from the two atomic centers was either present or absent in the data. This shows that, due to the fundamentals of quantum mechanics, the preparation of the beam must be included in theoretical calculations. The results have far-reaching implications on formal atomic scattering theory because this critical aspect has been overlooked for several decades.

Figure 1

Double differential cross sections for ionization of ${\mathrm{H}}_2$ by ion impact as a function of scattering angle for a projectile energy loss of 30 eV. The closed symbols show the data for a large slit-target distance $L$ corresponding to a transverse coherence length $\Delta r$ larger than the internuclear separation $D$, the open symbols show the data for small $L$ corresponding to $\Delta r<D$. The crosses are data for ionization of atomic hydrogen [25, 26]. The solid curve is a molecular 3-body distorted wave (M3DW) calculation [24], which assumes a completely coherent projectile beam. The dashed curve is a second Born approximation with Coulomb waves (SBA-C) calculation for ionization of atomic hydrogen [26, 28].

Figure 2

Ratios between the double differential cross sections for ionization of ${\mathrm{H}}_2$ for a large slit-target distance (closed symbols in Fig. 1) and for a small distance (open symbols in Fig. 1). The solid curve shows the ratio between the double differential cross sections calculated for ionization of ${\mathrm{H}}_2$ using the M3DW model and for ionization of atomic hydrogen calculated using the SBA-C model.