STM tracks ultrafast molecule motion


Rebecca Pool

Friday, November 11, 2016 - 14:45
Image: Ultrafast vibration of single pentacene molecules on a gold surface [Dominik Peller]
Germany-based researchers have recorded the ultrafast motion of a single molecule directly in time and space by combining a femtosecond laser with a scanning tunnelling microscope.
The physicists from the University of Regensburg built an ultrafast terahertz scanning tunelling microscope, in which the electron tunnelling process is controlled via lightwave electronics.
Here, femtosecond light pulses manipulate electron motion on fast timescales, so the researchers could watch single molecules of pentacene move with ultrafast temporal and atomic spatial resolution.
To track the ultrafast single molecule motion, the researchers built a Besocke-style STM that operates under ultrahigh vacuum and at temperatures down to 11K; the tip-sample spacing of this compact type of STM is typically insensitive to thermal drift
For molecular analysis, terahertz radiation enters the vacuum chamber of the STM as a collimated beam and is focused onto the tip by a parabolic mirror fixed to the STM scan unit.
Terahertz pulses are then modulated with an optical chopper to transiently open an otherwise 'forbidden' tunelling channel and remove a single electron from the highest occupied orbital of the pentacene molecule.
Using this method, the researchers could record some 100 femtosecond snapshot images of the molecule's orbital structure with sub-ångström spatial resolution.
What's more, through pump-probe measurements they could track molecular vibrations at terahertz frequencies.
"In the near-future we envisage watching single-electron lightwave electronics in molecular circuits, single-molecule movies, and chemical reactions in four dimensions," highlights Professor Rupert Huber from the Department of Physics at Regensburg.
"[Here we will] visualise the initial reaction steps of key elementary processes in chemistry and biology," he adds.
Research is published in Nature.
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