What happens when you detonate a chemical bond? Attosecond laser technique produces films on dissociation of chemical bonds – sciencedaily
On fine summer days, the sunlight all around us shatters, breaking the bonds. Chemical bonds.
Ultraviolet light breaks the bonds between the DNA atoms of our skin cells, potentially causing cancer. UV light also breaks oxygen bonds, ultimately creating ozone, and cleaves hydrogen from other molecules to leave free radicals behind that can damage tissue.
University of California, Berkeley, chemists using some of the shortest laser pulses available – a quintillionth of a second – have now been able to solve the step-by-step process leading to the explosion of a chemical bond, essentially making a movie of the event. They can follow indecisively bouncing electrons in various states of the molecule before the bond breaks, and the atoms split apart.
The technique, reported last week in the newspaper Science, will help chemists understand and potentially manipulate chemical reactions stimulated by light, known as photochemical reactions. Chemists and biologists, in particular, want to understand how large molecules manage to absorb light energy without breaking bonds, as happens when molecules in the eye absorb light, giving us vision, or molecules plants absorb light for photosynthesis.
“Think of a molecule, rhodopsin, in the eye,” said first author Yuki Kobayashi, a doctoral student at UC Berkeley. “When light hits the retina, rhodopsin absorbs visible light, and we can see things because the conformation of rhodopsin binding changes.”
In fact, when light energy is absorbed, a bond in rhodopsin twists, instead of breaking, triggering other reactions that result in 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 light absorption leads to twisting after the molecule goes through an excited state called avoided crossing. or conical intersection.
To avoid breaking a bond in DNA, “you want to redirect the energy of the dissociation to vibrational heat. For rhodopsin, you want to redirect the energy of the vibration to a cis-trans isomerization, a twist. “said Kobayashi. “These redirects of chemical reactions are happening all around us, but we haven’t seen the actual time before.”
Fast laser pulses
Attosecond lasers – an attosecond is a billionth of a billionth of a second – have been around for about a decade and are used by scientists to probe very fast reactions. Since most chemical reactions occur quickly, these fast-pulsing lasers are essential for “seeing” how the electrons that form the chemical bond behave when the bond breaks and / or reforms.
Leone, professor of chemistry and physics, is an experimenter who also uses theoretical tools and is a pioneer in the use of attosecond lasers to probe chemical reactions. He has six of these extreme x-ray and ultraviolet lasers (collectively, XUV) in his lab at UC Berkeley.
Working with one of the simplest molecules, iodine monobromide (IBr) – which is an iodine atom bonded to a bromine atom – the UC Berkeley team hit the molecules with a pulse of 8 femtoseconds of visible light to excite one of their outermost electrons. , then probed them with attosecond laser pulses.
By pulsing the XUV attosecond laser at 1.5 femtosecond intervals (one femtosecond equals 1,000 attoseconds), much like the use of strobe light, the researchers were able to detect the steps leading to the breakdown of molecules. The high-energy XUV laser was able to explore states of energy excited with respect to the internal electrons of the molecule, which do not normally participate in chemical reactions.
“You kind of make a movie about the paths of the electron as it approaches the intersection and the likelihood of it going one path or another,” Leone said. “These tools that we’re developing allow you to look at solids, gases, and liquids, but you need shorter timescales (provided by an attosecond laser). Otherwise, you just see the beginning and the end. , and you don’t know what else happened in between. “
Experience has clearly shown that the outer electrons of IBr, when excited, suddenly see a variety of states or places they might be and explore many of them before deciding which way to go. In this single molecule, however, all paths lead to the sedimentation of the electron on the iodine or on the bromine and the two atoms separate.
In the larger molecules, which the team hopes to explore soon, the excited electrons would have more choices, some where the energy either twists, like with rhodopsin, or molecular vibrates without the molecules breaking down.
“In biology, it turns out that evolution has selected things that are extremely efficient at absorbing energy and not breaking a bond,” Leone said. “When something’s wrong with your chemistry, that’s when you see diseases popping up.”
The other co-authors of the article were Kristina Chang of UC Berkeley and Tao Zeng of Carleton University in Ottawa, Canada. Leone, John R. Thomas Chair in Physical Chemistry, and Neumark, Professor of Chemistry at UC Berkeley, are also researchers at the Lawrence Berkeley National Laboratory.