Ultrafast Laser Spectroscopy Tracks Energy Flow through Molecules

CHAMPAIGN, Ill., June 20 -- 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."