Helping to fulfill a long-held dream of chemical physicists, researchers have developed a technique for determining how the alignment of a molecule can affect the outcome of a collision between it and a target atom, potentially providing a powerful tool for studying how the orientation of a molecule can influence the course of a chemical reaction. The doughnut-shaped images above, obtained in recent experiments at Lawrence Berkeley Laboratory, provide information on the alignment of a helium hydride ion (HeH⁺) as it collides with a target of helium atoms and captures an electron from one of the target atoms to become a neutral HeH molecule. Three cases are shown: collisions producing HeH in its lowest energy state (left, corresponding to combining He and H in their lowest states), in an excited state (center, combinations of He and excited H atoms) , and a higher-energy excited state (right, combinations of excited He and H atoms). To fully understand these images, it is important to describe the details of the experiment producing these collisions.

In the experiment, an incoming beam of HeH⁺ strikes a helium target atom. Each collision takes place on the scattering plane (green). During the collision, the HeH⁺ captures an electron from the helium atom (blue), becoming an unstable HeH molecule (red) which subsequently breaks up into hydrogen and helium atoms. These atoms travel towards a detector (not shown).

Hydrogen atoms which strike the detector make up the dots which form the doughnut shape in each image below. The helium atoms are subtracted out from the images. The color of the dots indicates the number of hydrogen atoms recorded at that spot, with yellow meaning a high number of counts and red indicating a lower number of counts. The thin black horizontal line running through each image corresponds to the scattering plane, seen edge on, where the collision occurred.

This image shows the hydrogen atoms collected in the case when the collision produces neutral HeH in its lowest energy state. In the image, many hydrogen atoms are collected above and below the scattering plane. (In other words, there are many yellow dots above and below the black line.) This is evidence that the hydrogen nucleus associated with the HeH+ molecule during the collision tended to be above or below the scattering plane. Since the leftmost image is the case where the lowest energy state (ground state) of HeH is formed, the pattern indicates that this particular state is formed most often when the HeH+ is aligned nearly perpendicular to the scattering plane.

This next image shows the hydrogen atoms collected in the case in which the collision produces neutral HeH in an excited state. This is the only image of the three that does not provide information about the orientation of the HeH+. This is because the resulting HeH molecule actually stays together long enough to rotate many times before it splits into two atoms. This washes out the details of the molecule's alignment when it collided with the helium atom. Hence the resulting image shows no evidence of the clustering of H atom impacts seen in the other two images.

This final image shows the hydrogen atoms collected in the case in which the collision produces neutral HeH in an even higher energy state. One sees the same clustering of hydrogen as the first image but with less clarity because production of this final combination is less probable.