Scientists have found a fascinating way to push an atom with controlled forces so quickly that they can control the movement of an individual molecule in less than a trillionth of a second. The extremely sharp needle of their unique ultra-fast microscope serves as a technical foundation: it scans molecules carefully, like a turntable. A study by physicists from the University of Regensburg, published in the journal Nature, showed that pulses of light falling on this needle could turn it into an ultra-fast “atomic hand.” This allows you to manipulate molecules and inspire new technologies.
Atoms and molecules are constituent parts of almost all matter that surrounds us. Interacting with each other according to the rules of quantum mechanics, they form complex systems with an infinite variety of functions. To study chemical reactions, biological processes in a cell, or new ways of collecting solar energy, scientists would like to not only observe individual molecules but even control them.
Most intuitively, people learn through tactile exploration, such as pushing, pulling, or tapping. Naturally, we are accustomed to macroscopic objects that we can directly touch, squeeze or push with force. Likewise, atoms and molecules interact through forces, but these forces are extreme in many ways. First, the forces between atoms and molecules occur at very small distances. In fact, these objects are so small that a special length scale was introduced to measure them: 1 Angstrom (1Å = 0,000,000,000.1 m). Second, at the same time, atoms and molecules move and wiggle extremely fast. In fact, their movement is faster than a picosecond (1 ps = 0,000,000,000,001 s). Therefore, to directly control a molecule as it moves requires a tool to create ultra-fast forces on an atomic scale.
More than 30 years ago, Eigler and Schweitzer showed that using a scanning tunneling microscope, static forces can be applied to individual atoms. In such a microscope, a very sharp needle is used to detect atoms and molecules by scanning over them, like in a record player. A team of scientists from Regensburg and Zurich has now solved the problem of making such forces fast enough to directly control the molecule as it moves, and thereby control its reactions and transitions. A team from Regensburg led by Rupert Huber and Jasha Repp has created a world-unique ultrafast microscope that combines femtosecond laser pulses, giving access to ultrafast time scales, with scanning tunneling microscopy, which allows the visualization of individual molecules.
Since light is an electromagnetic wave, its oscillating carrier wave can act as an ultra-fast force. Even faster than one light field oscillation cycle. When they applied ultrafast light waves to the atomic tip of the microscope, they could indeed exert this effect locally, on individual parts of the molecule. “So we can use a light-illuminated needle as an atomic-scale ultra-fast ‘hand’ to push the individual atoms in a molecule,” explains Dominic Peller, lead author of the new study.
The team noticed that the ultra-fast atomic forces were strong enough to cause the molecule to vibrate. This movement was so strong that it changed the probability of switching the molecule to 39%. “We can control the amplitude and direction of vibrations at will and thereby modulate the probability of a molecule’s reaction on a femtosecond scale,” the scientist emphasizes.
Moreover, it turned out that only when the “atomic hand” applies ultrafast forces to very specific parts of the molecule, it causes vibrational motion. As the scientists found out from a comparison with quantum mechanical calculations performed by Nikolai Molle in Zurich, this is due to the fact that the molecule clings to the surface through key atoms. Only by applying ultra-fast forces to these particular atoms could scientists selectively control the vibrations of the molecule.
This development finally provides the most direct control over molecular reactions. Ultrafast atomic forces are expected to help understand and manipulate key processes in chemistry and biology to inspire future technologies based on single-molecular devices. Thus, the ubiquitous ultrafast movement of the elementary constituent of matter can not only be observed but also controlled and used with unprecedented precision.