What happens when you explode a chemical bond?

During the beautiful summer days, the sunlight around us breaks breaking the links. Chemical bonds.

Ultraviolet light breaks the bonds between the atoms of our skin cells' DNA and can cause cancer. Ultraviolet rays also break the bonds with oxygen, eventually creating ozone, and dissociate the hydrogen from other molecules to leave behind free radicals that can damage the cells. tissue.

The University of California at Berkeley chemists using some of the shortest laser pulses available – one-fifteenth billionth of a second – have now been able to solve the step-by-step process leading to the explosion of a chemical bond. Event. They can follow the electrons that bounce undecidedly in various states of the molecule before the bond breaks and the atoms separate.

The technique, reported last week in the newspaper Science, will help chemists understand and possibly manipulate light-stimulated chemical reactions, called photochemical reactions. Chemists and biologists, in particular, wish to understand how large molecules absorb the energy of light without breaking any bonds, as occurs when the molecules of the eye absorb light, giving us vision, or that plant molecules absorb light for photosynthesis.

"Think of a molecule, rhodopsin, in the eyes," said first author Yuki Kobayashi, a PhD student at UC Berkeley. "When light hits the retina, rhodopsin absorbs visible light and we can see things because the conformation link of rhodopsin changes."

In fact, when the light energy is absorbed, a link in the rhodopsin deforms, instead of breaking, triggering other reactions that cause the perception of light. The technique developed by Kobayashi and his colleagues at UC Berkeley, Professors Stephen Leone and Daniel Neumark, could be used to study in detail how this absorption of light entails a twisting after the passage of the molecule into a excited state, called avoided crossing or conical intersection.

To avoid breaking a link in the DNA, "you want to redirect the energy of dissociation to the simple fact of heating the vibrations." For rhodopsin, you want to redirect energy from vibration to cis-trans isomerization, twist, "Kobayashi said. "These reorientations of chemical reactions occur ubiquitously around us, but we have not yet seen the moment."

Fast laser pulses

The attosecond lasers – an attosecond equals one billionth of a billionth of a second – have been around for a decade and are used by scientists to study very fast reactions. Since most chemical reactions occur rapidly, these fast-pulse lasers are essential for "seeing" the behavior of electrons forming the chemical bond when the bond breaks and / or reform.

Leone, professor of chemistry and physics, is an experimenter who also uses theoretical tools and who is a pioneer in the use of attosecond lasers to detect chemical reactions. He owns six of these extreme X-ray and ultraviolet lasers (collectively, XUV) in his UC Berkeley lab.

Working with one of the simplest molecules, iodine monobromide (IBr) – an iodine atom bonded to a bromine atom – the UC Berkeley team hit molecules with a visible light pulse of 8 femtoseconds to excite one of their outermost electrons, then probed with attosecond laser pulses.

By pulsing the attosecond XUV laser at programmed intervals of 1.5 femtoseconds (a femtosecond equals 1000 attoseconds), much like using a strobe light, researchers could detect the steps leading to fragmentation of molecules. The high energy XUV laser was able to explore the excited states of energy relative to the internal electrons of the molecule, which normally do not participate in chemical reactions.

"You're sort of making a movie about the electron tracks when it's approaching the waypoint and about the likelihood that it goes by one way or another," Leone said. . "These tools we develop allow you to examine solids, gases and liquids, but you need shorter time scales (provided by an attosecond laser) .Otherwise, you only see the beginning and end, and you do not know what else happened between the two. "

The experiment has clearly shown that the external electrons of IBr, once excited, suddenly see a variety of states or places where they might find themselves and explore many before deciding the path to be continued. In this simple molecule, however, all paths lead to the sedimentation of the electron on iodine or bromine and on the theft of two atoms.

In larger molecules, which the team hopes to explore soon, excited electrons would have more choice, in some cases the energy would move in the opposite direction, as with rhodopsin, or in molecular vibration without that the molecules do not separate.

"In biology, it turned out that evolution had selected extremely effective elements to absorb energy and not break a link," Leone said. "When something is wrong in your chemistry, it's when you see the diseases multiply."