Ultraviolet light fragments the links between atoms within the DNA of skin cells, undoubtedly inflicting cancer on the human body. UV light also breaks down oxygen bonds, ultimately creating ozone, and slices hydrogen off other molecules to leave behind free radicals that can harm tissue.
At the University of California, Berkeley, chemists are using some of the shortest laser pulses available-one quintillionth of a second-have now been able to determine the step-by-step process leading to the exploding of a chemical bond, basically making a movie of the event.
They can follow electrons uncertainly bouncing around in numerous states within the molecule before the bond breaks, and the atoms go their separate ways.
The procedure, reported last week in the journal Science, will help chemists understand and theoretically manipulate chemical reactions excited by light, so-called photochemical reactions.
Chemists and biologists, in particular, are interested in understanding how large molecules manage to absorb light energy without breaking any bonds, as happens when molecules in the eye absorb
light, giving USA vision, or molecules in plants absorb light for photosynthesis.
"Think about a molecule, rhodopsin, in the eye," said first author Yuki Kobayashi, a UC Berkeley doctoral student. "When light hits the retina, rhodopsin absorbs the visible light, and we can see things because rhodopsin's bond's conformation changes."
In fact, once the energy is engaged, a bond in rhodopsin twists, instead of breaks, triggering other reactions that result in the perception of light. The technique Kobayashi and his UC Berkeley colleagues, professors Stephen Leone and Daniel Neumark, developed could be used to study in detail how this light absorption leads to twisting after the molecule passes through an excited state known as an avoided crossing or conical intersection.
To prevent the breaking of a bond in DNA, "you want to redirect the energy from dissociation to just being vibrationally hot. For rhodopsin, you want to redirect the energy from vibrating to a cis-trans isomerization, a twist," Kobayashi said. "These redirections of chemical reactions are happening ubiquitously around us, but we have not seen the actual moment of them before."
Attosecond lasers-a billionth of a billionth of a second-have been around for nearly a decade and are utilized by scientists to probe extremely brief reactions. Since most chemical reactions occur immediately, these fast-pulse lasers are key to "seeing" just how the electrons that form the chemical bond behave once the bond breaks and reforms.
Working with one of the less complicated molecules, iodine monobromide (IBr) the UC Berkeley team hit the molecules with an eight femtosecond pulse of observable light to stimulate one of their outermost electrons, then prodded them with attosecond laser pulses. Pulsing the attosecond XUV laser at timed intervals of 1.5 femtosecond the researchers could detect the phases leading to the breakup of the molecules.
"You are kind of making a movie of the pathways of the electron when it approaches the crossing and the probability of it going along one path or along another," Leone said. "These tools we are developing allow you to look at solids, gases and liquids, but you need the shorter time scales. Otherwise, you only see the beginning and the end, and you don't know what else happened in between."
The experiment showed clearly that the outer electrons of IBr, once provoked, suddenly see an array of states or places they could be and study several of them before determining which path to take.
In this rather meek molecule, however, all paths result in the electron deciding on either iodine or bromine and consequently, the two atoms flying apart.
