Using THz waves to control electrons in a scanning tunneling microscope (STM) has opened the door to ultrafast atomic-scale microscopy. Recently, we combined low-temperature STM with ultrafast single-electron lightwave control to push microscopy to the ultimate spatio-temporal quantum limit via a novel state-selective tunneling regime. The peak of a THz electric-field waveform transiently opens an otherwise forbidden tunneling channel through a single molecular orbital. This process allowed us to record the first-ever ~100 fs sub-Å snapshot images of a single molecule’s orbitals and the first femtosecond movie of a single vibrating molecule.<br/>So far, stroboscopic data acquisition has averaged single-molecule dynamics over multiple laser shots. To resolve a single-molecule reaction, however, its statistics need to be analyzed separately for different, possibly irreversible reaction paths. Here, we introduce single-shot detection in lightwave STM as the first approach resolving individual unidirectional quantum events on simultaneous atomic and femtosecond scales. We study a magnesium phthalocyanine molecule, which adsorbs on sodium chloride in a bistable geometry. When we inject a single electron into an unoccupied orbital, the molecule starts to rearrange on the surface, statistically causing unidirectional switching events back and forth between the adsorption geometries. By monitoring a non-resonant tunneling current, the outcome of every single-electron injection is detected and the statistics of the different reaction paths are analyzed separately – electron by electron.<br/>In single-shot pump-probe experiments, we locally apply an atomic-scale field stimulus to the molecule and time-resolve its subsequent transient reaction statistics. We find that we can controllably introduce a coherence in the molecule that strongly modulates the switching rate by &gt;40% on the femtosecond scale. Analyzing ultrafast movies of the spatio-temporal reaction statistics shows that this coherence is excited locally: only when the pump stimulus directly exerts an atomic-scale force on the hot spots of electrostatic interaction that shape the energetic landscape, the switching rate is modulated. Introducing single-shot detection of unidirectional reaction events together with ultrafast atomic-scale force stimuli in single-molecule microscopy, this novel scheme provides a route towards coherent control of chemistry at the single-molecule level.

Single switching events of one molecule observed by femtosecond STM

2019 Fall National Meeting

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