Real-space subfemtosecond imaging of quantum electronic coherences in molecules

Understanding and controlling chemical transformations hinges on tracking electron motion within molecules. While attosecond science allows for the generation and tracking of the temporal evolution of electron motion, it lacks real-space resolution. Conversely, STM excels at spatially resolving valence electron density but lacks the temporal resolution for ultrafast dynamics. This study bridges this gap by combining STM with attosecond technology to directly visualize quantum electronic coherences induced in molecules by short laser pulses. The researchers aim to achieve ångström-scale spatial and subfemtosecond temporal resolution, overcoming previous limitations and eliminating the need for reconstruction from indirect measurements. This approach promises to provide unprecedented insights into electron dynamics in complex systems, moving beyond previous methods that only provide non-local temporal information or static spatial information.

Existing techniques in attosecond science can track the temporal evolution of coherent superpositions of quantum states of valence electrons, but only non-locally. The local time-evolution of electron density is typically inferred from features in spectra, a process often challenging and indirect. Light-matter interactions at the molecular scale have been utilized to probe electron motion at relevant length (ångström) and timescales (100 attoseconds to femtoseconds). Lightwave-driven STM has advanced to the point of generating and tracking fast tunnelling currents, and even coherently controlling electron tunnelling. However, simultaneous real-space and real-time imaging of electron dynamics remained elusive until this study.

The experiment utilizes orthogonally polarized near-infrared laser pulses with slightly different carrier frequencies, focused at the apex of a tungsten nanotip in tunnel contact with PTCDA molecules on a Au(111) surface. The non-collinear polarizations overlap, creating homodyne beating. The researchers measured the correlation of the laser-induced tunnelling current as a function of the delay between the two laser pulses. They observed that only photon-driven tunnelling, not field-driven tunnelling, occurred at the laser intensities used. The variation of the laser-induced tunnelling current with different polarizations and DC bias was also studied. The laser-induced tunnelling current showed features at voltages corresponding to the HOMO and LUMO resonances of PTCDA and the Au(111) surface state, indicating that the current arises from transitions involving these states. The dependence of the laser-induced current on the tunnel gap width mirrored that of the DC tunnelling current. For single-pulse experiments, the researchers tuned the bias voltage to control which initial state (HOMO or surface state) participated in the laser-induced transition to the LUMO or surface state. They then performed spatially resolved topographic scans of the single-pulse-induced tunnelling current at various biases, imaging the spatial profile of the HOMO and LUMO. For the time-resolved experiments, the delay between the two pulses was varied to induce and track coherent interference between the HOMO and surface state. Spatially resolved topographic scans were performed at different delays to generate space-time maps of the electron density, revealing coherent oscillations between the states. Multilayer PTCDA films were also studied to investigate coherences involving only molecular states, revealing oscillations between the HOMO and LUMO with a different frequency and amplitude.

The study successfully demonstrated real-space and real-time imaging of quantum electronic coherences in molecules. In the single-pulse experiments, spatial imaging of the HOMO and LUMO of PTCDA molecules was achieved by tuning the bias voltage to align the molecular orbitals with the Fermi level of the tungsten tip. In the two-pulse experiments, coherent oscillations between the HOMO and the Au(111) surface state were observed in a monolayer of PTCDA molecules, with an oscillation period of ~2.7 fs. This coherent interference was mapped in space and time, revealing the dynamics of electron density between the two states. The researchers developed a two-level model that accurately predicted the observed population dynamics. Using multilayers of PTCDA molecules to decouple the molecular states from the substrate, they further observed coherent oscillations between the HOMO and LUMO of the PTCDA molecules with a period of ~1.4 fs. Ångström-scale resolution was achieved, mapping the spatial distribution of the HOMO and LUMO orbitals. The observed behavior in the multilayer system is attributed to the specific vertical stacking of PTCDA molecules, leading to variations in the local density of states (LDOS). DFT calculations supported the experimental observations and provided values for the transition dipole moments. The results show that the method achieves spatial resolution well below the diffraction limit of the near-infrared laser pulses used. The amplitude of oscillations in the multilayer system did not decrease monotonically with delay, suggesting the involvement of one- and two-photon transitions and/or more than two electronic levels.

The successful real-space and real-time imaging of electronic coherences in molecules addresses the long-standing challenge of directly observing ultrafast electron dynamics in real space. This approach significantly advances our ability to understand chemical transformations driven by electron transfer processes. The demonstrated ångström-scale resolution and subfemtosecond temporal resolution provide a powerful tool for investigating a wide range of complex molecular systems, including photosynthetic molecules and other light-harvesting systems, two-dimensional materials, and superconductors. The ability to control the population of the involved orbitals also opens up exciting possibilities for manipulating electron dynamics and controlling chemical reactions.

This work presents a groundbreaking method for real-space and real-time imaging of quantum electronic coherences in molecules. The combination of STM and attosecond laser pulses enables ångström-scale spatial and subfemtosecond temporal resolutions. The study provides insights into electron dynamics in both monolayer and multilayer systems, highlighting the potential for controlling and manipulating electron motion. Future research could focus on applying this technique to more complex molecular systems and exploring its applications in areas such as photocatalysis and quantum information science.

The study primarily focused on PTCDA molecules on a Au(111) surface. The generalizability of the method to other molecular systems and substrates needs further investigation. The two-level model used to interpret the data is a simplification; the involvement of multiple electronic levels could influence the observed dynamics. The experimental setup is complex and requires sophisticated laser technology, potentially limiting its accessibility.