Individual Molecules on Surfaces: From Chemical Reactions to Ultra-Short Timescales
Department of Physics, University of Regensburg, 93040 Regensburg, Germany
We investigated different kind of π-conjugated molecules in a combined scanning tunneling (STM) and atomic force microscope (AFM). Whereas both measurement channels show features with sub-molecular resolution, the information they can provide is truly complementary. For example, STM allows the direct imaging of the unperturbed molecular orbitals , whereas the AFM channel directly reveals the molecular geometry [2, 3]. Further, we used the AFM-derived technique Kelvin probe force spectroscopy (KPFS) with sub-molecular resolution to image the polarity of individual bonds inside a single molecule .
The possibility of tailoring optical waveforms has allowed scientists to steer ultrafast electronic motion directly via the oscillating carrier wave of light - a principle dubbed "lightwave electronics" . Despite tremendous interdisciplinary efforts to combine ultrafast temporal resolution with atomic spatial resolution, the ultrafast dynamics of individual molecular orbitals has remained out of reach.
Terahertz (THz) scanning tunnelling microscopy  (THz-STM) has introduced a new paradigm by combining STM with lightwave electronics. In THz-STM, the electric field of a phase-stable single-cycle THz waveform acts as a transient bias voltage across an STM junction. In analogy to the all-electronic pump-probe scheme introduced recently in STM  these voltage transients may result in a net current that can be detected by time-integrating electronics.
By means of a low-noise low-temperature lightwave-STM we entered an unprecedented tunnelling regime, where the peak of a terahertz electric-field waveform opens an otherwise forbidden tunnelling channel through a single molecular orbital. In this way, the terahertz peak removes a single electron from an individual pentacene molecule's highest occupied molecular orbital within a time window of ∼ 100 fs - faster than an oscillation cycle of the terahertz wave. This quantum process allowed us to capture a microscopic real-space snapshot of the molecular orbital on a sub-cycle time scale. By correlating two successive state-selective tunnelling events, we directly tracked coherent THz vibrations of a single molecule in the time domain .
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 F. Albrecht et al., JACS 137, 7424 (2015).