Hauptinhalt

2021

  • A. Adamkiewicz, T. Bohamud, M. Reutzel, U. Höfer, and M. Dürr, Tip-induced β-hydrogen dissociation of an alkyl group bound on Si(001), Journal of Physics: Condensed Matter 33, 344004 (2021).

    Atomic-scale chemical modification of surface-adsorbed ethyl groups on Si(001) was induced and studied by means of scanning tunneling microscopy. Tunneling at sample bias > +1.5V leads to tip-induced C-H cleavage of a \beta-hydrogen of the covalently bound ethyl configuration. The reaction is characterized by the formation of an additional Si-H and a Si-C bond. The reaction probability shows a linear dependence on the tunneling current at 300 K; the reaction is largely suppressed at 50 K. The observed tip-induced surface reaction at room temperature is thus attributed to a one-electron excitation in combination with thermal activation. arXiv:2103.16641

  • D. Novko, V. Despoja, M. Reutzel, A. Li, H. Petek, and B. Gumhalter, Plasmonically assisted channels of photoemission from metals, Physical Review B 103, 205401 (2021).

    We analyze recently measured nonlinear photoemission spectra from Ag surfaces that reveal resonances whose energies do not scale with the applied photon energy but stay pinned to multiples of bulk plasmon energy ℏ⁢𝜔𝑝 above the Fermi level. To elucidate these unexpected and peculiar features we investigate the spectra of plasmons generated in a solid by the optically pumped electronic polarization and their effect on photoemission. By combining quadratic response formalism for calculations of photoemission yield, a nonperturbative approach to inelastic electron scattering, and first-principles calculations for the electronic structure, we demonstrate the dependence of probability amplitude for single- and multiplasmon excitations on the basic parameters characterizing the photon pulse and the system. The resulting multiexcitation spectrum evolves towards a truncated plasmonic coherent state. Analogous concept is extrapolated to interpret plasmon generation by multiphoton excited electronic polarization. Based on this we elaborate a scenario that the thus created real plasmons act as supplementary frequency-locked pump field for non-Einsteinian plasmonically assisted channels of photoemission from metals. The established paradigm enables assignment and assessment of the observed linear ℏ⁢𝜔𝑝 and nonlinear 2⁢ℏ⁢𝜔𝑝 electron yields from Ag. Such effects may be exploited for selective filtering of optical energy conversion in electronic systems.

    DOI: 10.1103/PhysRevB.103.205401

  • A. Li, M. Reutzel, Z. Wang, D. Novko, B. Gumhalter, and H. Petek,  Plasmonic photoemission from single-crystalline silver,  ACS Photonics 8, 247-258 (2021). 

    Optical fields interacting with solids excite single particle quantum transitions and elicit collective screening responses that define their penetration, absorption, and reflection. The interplay of these interactions on the attosecond time scale defines how optical energy transforms to electronic, setting the limits of efficiency for processes such as solar energy harvesting or photocatalysis. Our understanding of light–matter interactions is primarily based on specifying the electronic structure of solids and initial particles or fields occupying well-defined states, and the outcome of their interaction culminating in photoelectron or photon emission and analysis, with scant ability to follow the transitional, ultrafast many-body interactions that define it. The optical properties of metals transubstantiate from metallic to dielectric when the real part of their dielectric response function, Re[ε(ω)], passes through zero: at low frequencies, Re[ε(ω)] < 0, and the collective free electron plasmonic response confers high reflectivity; at high frequencies, Re[ε(ω)] > 0, and the fields penetrate as charge-density or longitudinal plasmon waves. How such collective plasmonic responses decay on the femtosecond time scale into single particle excitations is cardinal to plasmonics, but not sufficiently well described by experiment or theory. We examine the spectroscopic signatures of the nonlinear single particle and collective excitations of the low index crystals of silver by nonlinear two-photon photoemission spectroscopy, at frequencies where the bulk dielectric response passes through zero. We find that the transition through zero dielectric region is reflected in the nonlinear photoemission spectra, and in particular, the bulk plasmons decay by giving rise to a non-Einsteinian plasmonic photoemission component. This response, where the energy of photoelectrons is not defined by the incoming photons, occurs when photons excite the longitudinal plasmons, which then decay by exciting photoelectrons selectively from the Fermi level. Such mode of plasmon decay into hot electrons is contrary to the general agreement, but confirms a theoretical prediction by J. J. Hopfield from 1965. Our experiment illuminates a more energy efficient optical-to-electronic energy flow in metals that so far has escaped scrutiny.

    DOI: 10.1021/acsphotonics.0c01412

  • H. Petek, Y. Dai, A. Ghosh, A. Li, Z. Zhou, M. Reutzel, S. Yang, and C. B. Huang, Light Matter, Emerging Trends in Chemical Applications of Lasesrs, ACS Symposium Series (book chapter), pages 153-171 (2021).

    At the dawn of the age of quantum computation, we consider light as a reagent that affects and can control the properties of matter. Light is a probe of the quantum nature of matter, capturing the electronic and molecular structure, vibrations, rotations, etc. through single particle and collective excitations. Light also probes electron and nuclear spin interactions of atomic, molecular, and solid-state matter, and most profoundly their time dependent interactions. But light can have a more active role. Optical fields also create quantum superposition states, dress electronic states through Stark and Floquet processes, and weave spin textures in space and time. This enables light as it penetrates matter to transiently transpose in energy and modify electronic bands creating new transient electronic structures defined by its field strength, its vectorial polarization, as well as its consequent spin properties. We perform multiphoton photoemission electron spectroscopy and microscopy to probe how optical fields modify the electronic properties of matter, such as the energy-momentum dispersions of electronic bands. We further perform ultrafast photoemission electron microscopy to observe how optical fields, interacting with photonic nanostructures with a defined geometrical charge, create matter-wave surface plasmon polariton fields that focus the optical spin-orbit interaction into topological plasmonic vortices, with meron or skyrmion topological spin textures. Such fields represent new topological quasiparticles, that break the time-inversion symmetry, and enable light-matter interactions on deep subwavelength spatial and femtosecond temporal scales. We describe our recent ultrafast optical field and spin, Floquet and Poincaré engineering, of matter.

    DOI: 10.1021/bk-2021-1398.ch008