Theoretical physicist Uwe Thumm and his colleagues Feng He and Andreas Becker
not only work with some of the smallest molecules in the universe, but they
now have found a way to control the motion of the molecules' building blocks,
electrons and nuclei.
Thumm is a professor of physics at Kansas
State University, Feng He is a research associate at the K-State physics
department, and Becker is a professor at the University of Colorado in Boulder.
The collaborators have found a way to steer the movement of electrons in a hydrogen
molecule using ultrafast laser pulses. These pulses are so short that their
duration is measured in attoseconds -- that's one billionth of a billionth of
a second.
In a recent research paper, the three collaborators explained how attosecond
laser pulses can be used to direct the motion of an electron inside a hydrogen
molecule, and what the measurable consequences of this control over the electron
would be. The paper appears this month in Vol. 101 of The Physical Review Letters.
As theoretical physicists, Thumm and his colleagues do not perform experiments,
but instead simulate the outcome of present and future experiments by developing
mathematical models. These models explain the nature of atoms, molecules, light
and their interactions in terms of mathematical equations that are solved with
the help of powerful computers.
The researchers' model describes experiments that are currently being performed
at various laboratories worldwide, including the J.R. Macdonald Laboratory at
K-State.
For the past few years, Thumm and his colleagues studied what happens with
the hydrogen molecular ion when it interacts with short laser pulses. They used
hydrogen because it's the simplest molecule, although they have now extended
their research toward the imaging and control of the much faster moving electrons.
The hydrogen molecular ion has two protons and just one electron that "glues"
them together. A few years ago, by performing computer simulations, they found
that laser pulses can control the motion of the protons by setting them in motion
or slowing them down.
The researchers use a first ultrafast laser to pump the molecule with infrared
pulses. The protons vibrate and move apart slowly, but the electron still tries
to hang on. The second part of their model uses the laser to probe the particles
with a second delayed light pulse to see what happens when the electron fails
to keep the protons glued together. The infrared laser pulses create an electric
field that puts a force on the electron. Eventually, Thumm said, the electron
has to choose which proton it will stick with.
Thumm and his colleagues were surprised to find that for certain laser pulses
the electron can move in the opposite direction from what they anticipated.
"Our naive expectation was that the electron would follow the laser electric
force," Thumm said. "That's what other simulations predicted, and
they agree with classical physics and common intuition."
For instance, if you're pulling on a shopping cart, the cart will move in the
direction of the force -- in this case, toward you. But at the quantum level,
the rules are different.
The researchers found that sometimes the electron moves in the direction of
the force, but sometimes not. Thumm, He and Becker found that the electron picks
the proton on the left or the one of the right depending on the intensity of
the laser pulse. Knowing which intensity will make the electron move to the
left or the right gives physicists the ability to steer the electrons by setting
the laser pulse to a specific intensity.
Thumm said this finding is not only a contribution to basic physics research,
but it also could help chemists better understand and possibly control chemical
reactions.
"We would like to see a 'molecular movie' that shows the redistribution
of electrons in time -- within attoseconds -- during a chemical reaction,"
he said. "It would promote our understanding of basic processes that eventually
enable life: electrons bind atoms to simple molecules, such as the hydrogen
molecule or water. Through many chemical reactions, these simple molecules react
with each other and eventually form huge bio-molecules that make life, as we
know it, possible."
One possible commercial application of the finding, Thumm said, could be helping
companies become more efficient in producing a desired chemical compound while
minimizing unwanted byproducts in the reaction.
