Will an electron exiting a molecule through a quantum tunnel behave differently depending on whether the molecule is left- or right-handed?
Chemists have adopted the terms “left-handed” and “right-handed” from anatomy to designate molecules with a certain form of asymmetry. Look at your hands, palms up, to investigate the notion of chirality. They are clearly mirror reflections of one another. However, no matter how hard we try, they will not entirely overlap. Such “chiral” objects may be found at all sizes in nature, from galaxies to molecules.
We encounter chirality every day, not just when we pick up an item or put on our shoes, but also when we eat or breathe: our taste and smell can discriminate between two mirror versions of a chiral molecule. In fact, our bodies are so sensitive to chirality that a molecule may be both a medication and a poison. Chirality is so critical in pharmacology, where chiral molecules account for 90 percent of produced medications.
Chiral molecules have unique symmetry features that make them ideal candidates for studying basic physics processes. Recently, chirality was employed by research teams headed by Prof. Yann Mairesse of CNRS / Bordeaux University and Prof. Nirit Dudovich of the Weizmann Institute’s Department of Physics of Complex Systems to throw fresh insight on one of the most exciting quantum phenomena: the tunnelling process.
Tunneling is a phenomena in which quantum particles traverse apparently insurmountable physical boundaries. Because this motion is disallowed in classical physics, establishing an intuitive image of its dynamics is very challenging. The researchers used an intense laser field to construct a tunnel in chiral molecules. “An energy barrier naturally binds the electrons of the molecules around the nucleus,” Mairesse adds. “Consider the electrons to be air trapped within an inflating balloon. Even though there is no hole in the balloon, the powerful laser beams may decrease its thickness enough for some air to tunnel through it.”
Mairesse, Dudovich, and their colleagues set out to investigate an as-yet undiscovered element of tunnelling: the instant when a chiral molecule collides with a chiral light field, and how this short interaction impacts electron tunnelling. “We were quite interested to investigate the relationship between chirality and tunnelling. We were interested in learning more about what tunnelling might look like in these specific conditions “Dudovich explains.
An electron may leave an atom or molecule in a few hundred attoseconds. Many of the mechanisms researched in Mairesse and Dudovich’s laboratories have such short time durations. The following question was posed by the two teams: How does the chirality of a molecule effect electron escape?
“We spun the barrier around the chiral molecules using a laser beam that spins in time,” Mairesse explains. “To continue with the balloon metaphor, if the laser field rotates horizontally, the air should escape the balloon on the horizontal plane, following the laser field’s direction. We discovered that if the balloon is chiral, the air leaves the balloon going towards the floor or the ceiling, depending on the laser’s rotation direction. In other words, the electrons emerge from the chiral tunnel with a recollection of the barrier’s rotation orientation. This is similar to the action of a corkscrew, but on a nanometer and attosecond scale.”
The two teams revealed that the chirality of the molecule influences the possibility of an electron tunnelling, the phase at which the electron tunnels out, and the time of the tunnelling event. These fascinating findings pave the way for future research that will leverage the unique symmetry features of chiral molecules to examine the fastest processes in light-matter interaction.
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