Direct observation of collective modes coupled to molecular orbital–driven charge transfer

Abstract

Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Tunable dynamics of a flake on graphene: Libration frequency

“…Beyond NEMs and nanomechanical devices, the dynamics of a molecule on surfaces of substrates, in particular the coupling of their vibrational and libration modes with the electronic states via charging, the thermal and photoexcitation brought about new insights to fundamental features [29][30][31][32]. Also extraordinary properties of graphene flakes of diameter ∼10-15Å attained due to quantum confinement of their electrons in small size are attracting interest for applications in sensors, catalysis, supercapacitors, bioimaging, luminescence, and spintronic devices [33][34][35][36][37].…”

Optical fiber-based photocathode

“…Finally, we embedded our fiber-based photocathode inside a bulk cathode structure which is optimized for use in high extraction fields as typically employed in compact DC guns (typically in the keV to hundreds of keV regimes [7][8][9]22,28 ) or radio-frequency (RF) guns ranging into MeV energies. 27 The feasibility of this arrangement was tested in a DC electron gun at an acceleration field of 7 MV/m, and beam energy of 70 kV (see the supplementary material for details).…”

“…Ultrafast electron diffraction (UED) is a highly successful technique for structural dynamics investigation of phase transitions, electron-phonon coupling, and chemical reactions. [1][2][3][4][5][6][7][8][9] Metal photocathodes are currently the most commonly employed electron sources due to their robustness, ultrafast temporal response, and compatibility with high electric fields. They provide sub-picosecond electron pulses with coherence lengths up to a few nanometers.…”

Electron energy analysis by phase-space shaping with THz field cycles

“…Diffraction patterns of crystalline materials or real-space images of complex morphologies obtained in a pump-probe way have already elucidated numerous ultrafast phenomena in gaseous, solid, and liquid environments, 3–8 and researchers are continuing to explore even more fundamental processes 9 or more complex materials 10 …”