Seminars Archive

Floquet Engineering of coherent charge transfer through molecule-dressed vacuum

Abstract
We explore ultrafast coherent processes at metal surfaces by interferometric multiphoton photoemission spectroscopy and microscopy. Intense laser illumination of metal surfaces induces coherent polarization creating a ladder of quasi-energy states, which undergo photoemission when excited to above the vacuum level. Interferometric measurements with identical pump-probe 20 fs laser pulses scanned with 50 as pulse delay resolution interrogate the coherent response of metals. We apply coherent photoelectron spectroscopy to investigate photoemission from Ag(111) surface through a monolayer of upright standing trans-1,2-bis(4-bipyridyl)ethylene (BPE) molecules. Four photon photoemission measurements interrogate charge transfer from the Shockley surface state of BPE/Ag(111) interface, to image potential states of the BPE/vacuum interface. Photoelectrons emerge coherently through the molecular overlayer spanning 1.5-2 nm length, raising questions how the molecular overlayer acts as a coherent charge transfer medium. We find that instead of flowing through the intramolecular conjugated bond framework, BPE molecules dress the intermolecular vacuum to support quantum well waveguides states that mediate the electron transfer. The frequency-time analysis of photoelectron spectra inform that the coherent Floquet-to-image potential state charge transfer occurs on <2 fs time scale through non-Markovian screening response of the substrate. The measurements suggest a role of dressed vacuum in photoinduced charge transfer in molecular media.