Wintersemester 2018/2019URL zum ICS-Kalender dieses Seminars
Kirchhoff-Institut für Physik, Otto-Haxel-Hörsaal
We investigated different kind of π–conjugated molecules in a combined scanning tunnelling (STM) and atomic force microscope (AFM). Whereas both measurement channel s 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]. When applied to STM-based single-molecule synthesis and on-surface chemistry, the combination of these techniques enables a direct quantification of the interplay of geometry and electronic coupling in real space [3, 4]. In particular, in many cases only the AFM channel enables discriminating different binding sites inside a single molecule, which is a prerequisite to obtain a full atomistic description of regioselectivity in on-surface chemistry . Similarly, in the case of hydrogen-bonded molecular assembly the AFM provides direct insight into the bond rearrangement upon crystallization in two dimensions , which is elusive for STM.
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” . 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 win dow 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 .
 J. Repp
et al., Phys. Rev. Lett.
94, 026803 (2005)
 L. Gross et al., Science 325, 1110 (2009)
 F. Albrecht et al., JACS 137, 7424 (2015)
 N. Kocić et al., JACS 138, 5585 (2016).
 L. Patera et al., Angew. Chem. Int. Ed. 56, 10786 (2017)
 E. Goulielmakis et al., Science 317, 769 (2007)
 T. L. Cocker et al., Nature Photon. 7, 620 (2013)
 S. Loth et al., Science 329, 1628 (2010)
 T. L. Cocker et al., Nature 539, 263 (2016)