Ultrafast laser spectroscopy tracks energy flow through molecules
James
E. Kloeppel, Physical Sciences Editor
(217) 244-1073; kloeppel@illinois.edu
6/20/02
CHAMPAIGN, Ill. —
Using an ultrafast laser spectroscopy technique, scientists at the University
of Illinois at Urbana-Champaign have tracked – and timed – the flow
of vibrational energy through certain molecules in their liquid state.
"To understand chemistry at the most fundamental level, we have to understand
the transfer of vibrational energy," said Dana Dlott, a professor of chemistry
at Illinois. "Lots of scientists can put energy into a molecule and watch
it drain away, but with our technique we can actually see where the energy goes."
The movement of vibrational energy within and between molecules plays a significant
role in nearly all condensed-phase chemical processes. "Vibrational energy
flow is a fundamental process in chemistry, and the one we know the least about,"
Dlott said. "Now that we have a tool that lets us watch where the energy
goes, we can get a much better picture of what happens – at the most basic
level – when molecules interact."
As will be reported in the June 21 issue of the journal Science, Dlott and postdoctoral
research associates Zhaohui Wang and Andrei Pakoulev used pulses from a mid-infrared
laser to excite the hydroxyl stretching vibrations in different alcohols. Then
they probed the laser-pumped molecules with pulses of visible light to monitor
the energy flow through intervening methylene groups and at the terminal methyl
groups.
The researchers studied vibrational energy flow in ethanol, 1-propanol, 1-butanol
and 2-propanol. "For each additional methylene group in the path between
the hydroxyl and the terminal methyl group, the time for vibrational energy
transfer was increased by about 400 femtoseconds," Dlott said.
The corresponding speed is a little faster than Mach 1, which is the speed of
sound in air at sea level, but it is only about one-third the speed of sound
in ethanol.
"The efficiency is low – only about 1 percent of the energy is going
to the methyl groups," Dlott said. "This isn’t like hitting the
end of a metal bar with a hammer and having nearly all the vibrational energy
move to the other end. In a molecule, there are many paths for the energy to
follow. With our laser, we are tuned to only one location at a time, and we
only measure the energy being transferred to that one location."
Although vibrational energy transfer through a molecule is reminiscent of electronic
energy transfer, it is fundamentally quite different, Dlott said. "Electronic
energy transfer typically involves through-space interactions. As our observations
show, vibrational energy transfer is mechanical, and occurs via through-bond
interactions."
To prove the energy is flowing through the alcohol molecules, rather than around
them, Dlott and his colleagues took a look at another molecule, tert-butanol.
They saw no energy transfer between the hydroxyl and the terminal methyl groups.
"In tert-butanol, the central carbon atom has no carbon-hydrogen stretching
modes along the major axis of the molecule," Dlott said. "As a result,
the through-bond energy transfer is choked off. Absolutely no vibrational energy
gets through."
The researchers' findings provide an important new perspective on the mechanics
of molecules and on through-bond energy transfer, and could lead to a better
understanding of chemical processes in general.
"It’s like seeing people leave a room, but you don’t know whether
they are going home or going someplace else," Dlott said. "With our
advanced form of vibrational spectroscopy, we can see where the energy is going."
The National Science Foundation, the Air Force Office of Scientific Research
and the Army Research Office supported this work.