An ordered monolayer of phthalocyanine on a silver(111) surface reacts with the stoichiometric amount of coadsorbed Fe atoms to form iron(II)-phthalocyanine. This surface-confined redox reaction was studied with X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The formation of iron(II)-phthalocyanine was confirmed by comparison with the directly deposited complex. No side reactions such as the coordination of Fe atoms on the peripheral nitrogen atoms of the phthalocyanine molecules were observed.

Achieving control over light−matter interaction in custom-tailored nanostructures is at the core of modern quantum electrodynamics. In strongly and ultrastrongly coupled systems, the excitation is repeatedly exchanged between a resonator and an electronic transition at a rate known as the vacuum Rabi frequency Ω R . For Ω R approaching the resonance frequency ω c , novel quantum phenomena including squeezed states, Dicke superradiant phase transitions, the collapse of the Purcell effect, and a population of the ground state with virtual photon pairs are predicted. Yet, the experimental realization of optical systems with Ω R /ω c ≥ 1 has remained elusive. Here, we introduce a paradigm change in the design of light−matter coupling by treating the electronic and the photonic components of the system as an entity instead of optimizing them separately. Using the electronic excitation to not only boost the electronic polarization but furthermore tailor the shape of the vacuum mode, we push Ω R /ω c of cyclotron resonances ultrastrongly coupled to metamaterials far beyond unity. As one prominent illustration of the unfolding possibilities, we calculate a ground state population of 0.37 virtual photons for our best structure with Ω R /ω c = 1.43 and suggest a realistic experimental scenario for measuring vacuum radiation by cutting-edge terahertz quantum detection. KEYWORDS: Quantum electrodynamics, ultrastrong coupling, terahertz, metamaterials I n the strong coupling regime of quantum electrodynamics (QED), where the vacuum Rabi frequency Ω R exceeds the dissipation rates of the electronic excitation and the resonator, new eigenmodes called cavity polaritons emerge. This universal principle is found in a large variety of systems, ranging from atoms 1 to excitons in semiconductors, 2,3 molecules, 4 mid-IR plasmonic structures, 5−9 circuit QED systems at GHz frequencies, 10−13 and structures in the THz spectral range. 14−16 In ultrastrongly coupled structures, Ω R becomes comparable to the resonance frequency ω c itself; the rotating-wave approximation of light−matter interaction falters, and antiresonant coupling terms describing the simultaneous creation of correlated light and matter excitations become relevant. 17−19 Most prominently, the ground state is theorized to be a modified squeezed quantum vacuum with a finite population of correlated virtual photon pairs. 17,19 For sufficiently large values of the relative coupling strength Ω R /ω c ≳1, subcycle switching of Ω R 6,9 may release these photons 17,19,20 in analogy to Unruh− Hawking radiation emerging at the event horizon of black holes. 21 These spectacular perspectives have fuelled the quest of the QED community for ever greater relative coupling strengths, ultimately aiming for Ω R /ω c beyond unity.The key strategy for boosting Ω R /ω c , also referred to as g/ω c , comprises increasing the dipole moment of the electronic transition, decreasing the resonator mode volume and ω c , or enhancing the overlap of the photonic mode and the electroni...

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We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.

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