Actuating and probing a single-molecule switch at femtosecond timescales

Accessing ultra-fast non-equilibrium phenomena is enabled  by terahertz  (THz)  scanning tunneling  microscopy [1] (THz-STM) through 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. These  voltage transients may result  in a net current that  can be detected  by time-integrating  electronics.  The recent  development of this lightwave STM has enabled the combined femtosecond and  sub-angstrom resolution in observing matter [2]. We now demonstrate the first combined  femtosecond and  sub- angstrom access in the control of matter. Ultrafast localized electric fields in lightwave STM enable  exerting  atom-scale femtosecond forces to selected  atoms. By shaping atomic forces on the intrinsic timescale  of molecules, coherent  atomic motion can now be excited. Utilizing  this coherent structural dynamics, we can modulate the quantum  transitions of a single-molecule switch by up to 39 %.  We directly visualize  the coherent excitation of the switch in the first femtosecond single-molecule movie [3]. To resolve the impact of coherent control  of the  single-molecule switch, alongside, we introduce single-shot action spectroscopy in lightwave STM as  the  first  concept  resolving   individual   path-selective reaction  events of a single molecule in space and time. With this novel concept, we detect the outcome of every single  laser shot and further separate the statistics of the two inverse reaction paths. Our results open a new chapter in the control and observation  of reactions of individual molecules directly on the relevant ultrafast and ultrasmall scales.

[1) T. L. Cocker et al., Nature Photon. 7, 620 (2013).

[2) T. L. Cocker et al., Nature 539, 263 (2016).

(3) D.Peller et al.,in preparation.