Fig. 5: Tracking coherent interference of valence electronic states in molecules.
From: Real-space subfemtosecond imaging of quantum electronic coherences in molecules
a, Schematic illustration of the photon-induced transitions in the four-layer experiment. At a bias where the HOMO of a PTCDA molecule rises close to EF,t, photons of the laser pulse can dipole couple the HOMO with the LUMO of a PTCDA molecule either by a sequential two-photon transition (black arrows) or by a single-photon transition from the blue tail of the laser pulse (blue arrow). b,c, Spatial variation of the ultrafast laser-induced tunnelling current on PTCDA molecules in the four-monoloayer coverage for the HOMO (−1.8 V; b) and LUMO (+1.4 V; c). . d, Real-space and real-time resolved maps of the laser-induced tunnelling current as a function of the delay between pulses 1 and 2. Scale bars, 1 nm. Dashed black and white ellipses in all the panels are drawn to highlight the contrast in the variation of the laser-induced tunnelling current upon change of the delay between pulses 1 and 2 in two individual PTCDA molecules undergoing a HOMO to LUMO oscillation. The d.c. bias and set current for constant-current operation mode of the STM are –1.8 V and 50 pA, respectively. e, Temporal oscillation of the population in the HOMO and LUMO states as a function of the delay between pulses 1 and 2. Solid lines are experimental results, whereas dotted lines are the simulated temporal evolution of the population in HOMO and LUMO using the two-state analytical model. The reproducibility of the data in b–d was confirmed in five individual measurements.
