Main Content

2019

  • M. Reutzel, A. Li, and H. Petek, Coherent two-dimensional multiphoton photoelectron spectroscopy of metal surfaces, Physical Review X 9, 011044 (2019).

    Light interacting with high-electron-density materials elicits an ultrafast coherent many-body screening response on sub- to few-femtosecond timescales, which makes its experimental observation challenging. Here, we describe the coherent two-dimensional (2D) multiphoton photoemission (mPP; 𝑚=2–5) study of the Shockley surface (SS) state of Ag(111) as a benchmark for spectroscopy of the coherent nonlinear responses of metals to intense optical fields in the perturbative regime; similar 2D signatures can be expected for coherent responses in other materials like low-dimensional semiconductors and strongly correlated materials. Employing interferometric time-resolved multiphoton photoemission spectroscopy (ITR-mPP), we correlate the coherent polarizations excited in the sample with photoelectron energy distributions where the interaction terminates and photoelectrons carry away the information on their excitation. By measuring the nonresonant three- and four-photon photoemission of the SS state, as well as its replicas in above-threshold photoemission (ATP), we record the coherent response of the Ag(111) surface by 2D photoemission spectroscopy and relate it to its band structure. A 2D analysis of the SS state and its ATP replicas shows similar behavior, indicating that they are 𝑚th- and 𝑚⁢th+1-order coherent processes in a contradiction of the common attribution of ATP as a sequential process where a photoelectron excited above the vacuum level absorbs one or more additional photons. We interpret the mPP process by an optical Bloch equation model, which reproduces the main features of the surface state coherent polarization dynamics in ITR-mPP experiments: The distributions of spectroscopic components in 2D photoelectron spectra of coherent mPP are shown to follow systematically the 𝑛/𝑚 ratio, where 𝑛 and 𝑚 are orders of the induced coherence and the photoemission process contributing to the signal.

    DOI: 10.1103/PhysRevX.9.011044

  • M. Reutzel, A. Li, B. Gumhalter, and H. Petek, Non-linear plasmonic photoelectron response of Ag(111), Physical Review Letters 123, 017404 (2019).

    Photons can excite collective and single-particle excitations in metals; the collective plasmonic excitations are of keen interest in physics, chemistry, optics, and nanotechnology because they enhance coupling of electromagnetic energy and can drive nonlinear processes in electronic materials, particularly where their dielectric function ϵ(ω) approaches zero. We investigate the nonlinear angle-resolved two-photon photoemission (2PP) spectroscopy of the Ag(111) surface through the ϵ(ω) near-zero region. In addition to the Einsteinian single-particle photoemission, the 2PP spectra report unequivocal signatures of nonlocal dielectric, plasmonically enhanced, excitation processes.

    DOI: 10.1103/PhysRevLett.123.017404

  • T. Bohamud, M. Reutzel, M. Dürr, and U. Höfer, Dynamics of proton transfer reactions on silicon surfaces: OH-dissociation of methanol and water on Si(001), The Journal of Chemical Physics 22, 224703 (2019).

    The reaction dynamics of methanol and water on Si(001) were investigated by means of molecular beam techniques. The initial sticking probability s0 was determined as a function of the kinetic energy of the incoming molecules, Ekin, and surface temperature, Ts. For both, methanol and water, a nonactivated reactional channel was observed; the dynamics were found to be determined by the reaction into the datively bonded intermediate state. A low conversion barrier was deduced for the conversion from this intermediate into the final state. It is attributed to the reaction mechanism, which proceeds via proton transfer from the OH-group of the datively bonded molecules to a Si surface atom. Despite this low conversion barrier, adsorption into the intermediate and further reaction via proton transfer were found to be largely decoupled.

    DOI: 10.1063/1.5092804

  • G. Mette, A. Adamkiewicz M. Reutzel, U. Koert, M. Dürr, and U. Höfer, Controlling an SN2 reaction by electronic and vibrational excitation: tip-induced ether cleavage on Si(001), Angewandte Chemie 131, 3455-3458 (2019).

    Controlling chemical reactions beyond thermally activated reaction schemes can open alternative reaction channels, and thus lead to new final products. Herein, we show for tetrahydrofuran (THF) cleavage on Si(001), the surface analogue of an SN2 reaction, that excitation by electrons from the tip of a scanning tunneling microscope (STM) not only opens new reaction channels, but that different final products can be selectively addressed by the type of excitation: Above a threshold voltage of 2.5 V, direct excitation by electron transfer into the antibonding C−O orbital of the THF molecules induces ether cleavage of the datively bonded intermediate of THF on Si(001). Below the threshold, ether cleavage is induced by multiple excitation of vibrational modes. In both modes of excitation, additional final configurations were observed when compared to the thermally activated reaction. The branching ratios of the final configurations are different for the two different excitation mechanisms, which in turn can be controlled by the applied sample bias.

    DOI: 10.1002/anie.201806777