(OTTAWA - September 2, 1999) -- Scientists at the National Research Council of Canada have developed a technique that allows them to follow the ultrafast internal processes that lead to electronic-structural rearrangements in molecules. This discovery, which may eventually provide insights into the new field of "molecular electronics" and biologicial processes such as vision and photosynthesis, was made by a multidisciplinary team of physicists and chemists at NRC's Steacie Institute for Molecular Sciences (SIMS). Their work was reported in the September 2 issue of Nature.
This SIMS group specializes in femtosecond laser technology, molecular dynamics and intense field physics. A femtosecond (10-15 s) is an extremely short duration: one femtosecond is to one minute as one minute is to the age of the universe. Modern lasers can now routinely produce pulses in this range.
The Nature article describes a new femtosecond technique for following and distinguishing the electronic rearrangements and atomic motions in a molecular process, even though they occur on the same time scale. The idea is to use ionization (removal of an electron) to get a picture of what the molecule was doing at the moment of ionization. By measuring the removed electron with a technique called photoelectron spectroscopy, Dr. Albert Stolow and co-workers were able to follow both the atomic motions and the electronic rearrangements that accompany them.
Femtoseconds are the time scale for ultrafast chemical reactions and internal motions in molecules. When a molecule is excited, both its atoms and electrons can begin to move. The electrons, being very light and fast, might easily adjust to the motions of the much heavier atoms. In many important cases, however, the electronic rearrangements and atomic motions occur on the same time scale, making it difficult to discern what is happening. Femtosecond lasers are used as an ultrafast stroboscope to watch the molecular motions as they occur. One laser pulse starts the process and a second takes a 'snapshot' at a later time. This is repeated until the whole time behaviour is known. If a femtosecond 'snapshot' were recorded of the atomic motion only, we would be missing the critical element of how the electrons are rearranging at the same time.
According to Dr. Albert Stolow, the project leader, there is a world wide effort in the development of increasingly smaller devices for information and telecommunications technology. "Such devices will soon be so small that they begin to approach the size of molecules. As such, the molecular way of thinking becomes increasingly profitable," said Dr. Stolow. The burgeoning field of 'molecular electronics' considers molecules themselves as electronic devices and, using fundamental physical principles, tries to lay the foundation for the next generation of devices.
Femtosecond time scale processes underlie many of the phenomena we see around us in the natural world. Vision and photosynthesis are interesting cases because they are both biological systems and paradigms for molecular electronics. In both these cases, a complex mixing of atomic and electronic motions is critical to the function.
"We will seldom be interested only in the static properties of molecular systems. We want to know how they change as a function of time, in other words, their dynamics," said Dr. Stolow. The design of active molecular scale devices must include consideration of these dynamics process. Another critical issue in molecular systems is long term stability. Excited molecules have many undesired paths they can follow. The design of stable, efficient devices requires the rates of decay (e.g. breaking) to be much slower than the rate of the desired process (e.g. switching). We expect that fundamental studies will shed new light onto complex chemical and biological processes as well as open new avenues for the rational design of molecular devices."
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