Filming molecules in action

A team of researchers managed to record a ‘movie’ of what happens when chiral molecules interact with light at one million billionth of a second. 

Chiral molecules are molecules that are mirror images of each other but cannot be superimposed — just like the right and left hands. Their chemical and physical properties are identical, but differ only in their interaction with other entities that also have a handedness, like polarized light.  

This bulk property was first observed over two centuries ago, but where exactly it comes from on the atomic level has been a longstanding mystery. 

Using ultrashort lasers to tap into the timescale at which the quantum mechanical processes that govern chirality occur, a team of researchers from Canada, France, Spain, Germany and Saudi Arabia probed the dynamics of electrons to directly observe what makes chiral molecules different.

When an intense, ultrashort laser pulse is focused onto molecules in a gas, an electron gets knocked off and accelerated in the electric field of the laser. When the field changes direction, the electron reverses and collides and recombines with the parent ion again. 

The energy the electron gains in this process is then emitted as photons. This phenomenon is known as High-Harmonic Generation (HHG). The emitted higher frequency harmonics encode and amplify information about the recombining system. 

The team adapted HHG by using elliptically polarized laser pulses shone on two chiral compounds to record a ‘movie’ with ‘frames’ — harmonic emissions — only a tenth of a femtosecond apart.  By repeating the process on left-handed and right-handed chiral and achiral molecules at this high time resolution, they were able to detect a slight difference in the electron dynamics that accounts for the difference in their behaviour.  

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