When light knocks an electron out of a molecule, the remaining electrons don't just sit still: they rearrange, like passengers in a train car when one gets off at a stop. In ordinary molecules, this rearrangement is barely noticeable, but in chiral molecules, mirror images like left and right hands, it can make the orbitals rotate. This is exactly the kind of rotation that has now been seen for the first time in an experiment.
Physicists zapped epichlorohydrin molecules with tunable ultraviolet light and tracked the angles at which electrons flew out. In the region of the Cooper minimum, a special energy where the signal usually weakens, the angular distribution began to oscillate. Moreover, only two ionization channels oscillated, and the predictions of standard methods (Hartree–Fock and density functional theory) did not match the experiment either in the number of channels or in amplitude.
It was only when electron correlation, their mutual influence on each other, was taken into account that the observations could be explained. It turned out that during ionization, different states of the cation mix, and this mixing acts like a turn of the steering wheel: the orbital from which the electron departed changes its orientation. For chiral molecules, such a turn is especially strong because they lack the symmetry that would forbid mixing.
This experiment is the first direct proof of a long-standing theoretical prediction. Now that the effect has been captured in numbers, it can be used as a fine tool: from the angular distribution of photoelectrons, one can determine how electrons interact within a molecule, and even distinguish between left- and right-handed forms of chiral substances.
Chiral molecules are like gloves: you can't put a left glove on your right hand. In nature, often only one of the two mirror forms is found, and this determines, for example, a substance's smell.