Light chemistry

For two atoms to react they must first collide. The use of light to control collisions between ultracold atoms provides a potentially useful tool for studying chemical reactions.

The use of laser light to trap and manipulate atoms and molecules is invaluable in the study of how atoms and molecules interact. Recent improvements in our ability to sculpt the shape and temporal variation of laser light-fields has led to unprecedented control over these interactions. Writing in Physical Review Letters¹, Matthew Wright and colleagues describe a system capable of varying the light field in a magneto-optical atom-trap over nanosecond timescales and of the use of a technique known as frequency-chirping — rapidly increasing the frequency of the light emitted in individual laser pulses — to control collisions between ultracold rubidium atoms held in the trap.

The development of techniques to control the outcome of chemical reactions is one of the central problems of chemistry. It has long been hoped that the coherent, monochromatic nature of laser light could provide a ready means to favour certain chemical reactions — such as the forming or breaking of specific chemical bonds — over others by tuning the light so that it resonates with a desired molecular mode². However, owing to the speed with which energy injected by a laser can dissipate in a system containing many atoms, achieving efficient control over such processes has been difficult to realize.

In previous work³, Vala et al. found that by using a frequency-chirped picosecond laser pulse they could rapidly promote ground-state caesium atoms up to an excited state in which they could form Cs₂ molecules. In their latest work¹, Wright et al. use similar techniques to induce collisions between ultracold rubidium atoms. They find that by exciting the atoms held in a magneto-optical trap with a nanosecond laser pulse that is frequency-chirped, they are able to more efficiently promote those atoms to a state that causes them to attract each other, in comparison with a non-chirped pulse. This in turn caused a tenfold increase in the rate of collisions between atoms.

In future experiments, the authors plan to explore the effects of changing both the chirp and the magnitude of successive laser pulses. With further development they hope that this will lead to the ability to form and study the formation of molecules from ultracold atoms in the ground state.