Quantum beats at the metal/organic interface

Abstract

Time resolved two-photon photoemission (TPPE) is used to probe the unoccupied electronic structure of monolayer films of dicarbonitrile-quaterphenyl (NC-Ph 4-CN) on Ag(111) and cobalt phthalocyanine (CoPc) on Ag(100). For both samples, photoelectron spectra show a well-formed series of electronic states near the vacuum level. These are assigned as the 1 ≤ n ≤ 4 image-potential states (IPS's) based on the scaling of their binding energies and lifetimes. Time domain measurements at energies near the vacuum level exhibit intensity oscillations (quantum beats) which are due to excitation of an electronic wave packet of the n ≥ 4 IPS's. The wave packets remain coherent until population decay renders them unobservable. These measurements clearly demonstrate that the classical image-potential state structure is retained to high order (n ∼ 6) in the presence of aromatic organic adlayers. This represents the first definitive observation via TPPE of quantum beats of electronic origin at the metal/organic interface.

Plasmonic Photoemission from Single-Crystalline Silver

“…In a previous 2PP experiment on Ag (111) surface where femtosecond laser excitation was tuned through ENZ, we observed the same feature, and concluded that it could not be attributed to an excited-state population-dependent Auger decay, because it appeared with comparable intensities relative to other features with nanosecond and femtosecond laser pulse excitations. 75 Instead, because its onset coincided with the longitudinal plasmon frequency, we assigned it to a collective process where light interaction induces an internal plasmon field, locking the 2PP signal to 2ℏω p .…”

“…52,77 The "2ℏω p " feature could only be attributed to the collective plasmon response, and was supported by the occurrence of a similar signal for the Ag(100) surface. 75 In fact, this 2ℏω p peak has been observed in the 2PP spectra of Ag (111) and (100) surfaces for excitation with ℏω L ≥ ℏω p , but its nonband structure and collective origin led to its misassignment, or neglect. 82,84−86 In the following, by reasoning that the 2ℏω p feature represents a nearly isotropic bulk plasmonic response, we reexamine the plasmonic photoemission of Ag (111) and Ag(100) surfaces, and explore it for Ag (110) surface for ℏω L ≥ ℏω p .…”

“…75 In fact, this 2ℏω p peak has been observed in the 2PP spectra of Ag (111) and (100) surfaces for excitation with ℏω L ≥ ℏω p , but its nonband structure and collective origin led to its misassignment, or neglect. 82,84−86 In the following, by reasoning that the 2ℏω p feature represents a nearly isotropic bulk plasmonic response, we reexamine the plasmonic photoemission of Ag (111) and Ag(100) surfaces, and explore it for Ag (110) surface for ℏω L ≥ ℏω p . We report (1) dramatic changes in the metallic response through ENZ resulting from whether the collective plasmonic response screens the optical field, or allows it to penetrate as longitudinal plasmons; (2) non-Einsteinian, collective plasmon-induced photoemission for ℏω L ≥ ℏω p ; (3) the preferential plasmon decay exciting hot electrons from the E F ; and (4) evidence that the atomic scale surface corrugation couples transverse optical fields to the longitudinal plasmon response.…”

“…81 Upon developing the nanosecond laser-based nonlinear two-photon photoemission (2PP) spectroscopy, Steinman and co-workers could also explore photoemission mediated by laser excitation of single particle transitions in crystalline Ag samples. 82,83 They interpreted their 2PP spectra of the Ag (111) surface primarily in terms of single particle excitations. In addition, they found a signal component, which appeared abruptly for ℏω L ≥ 3.84 eV, and did not tune in proportion to ℏω L , as expected for Einsteinian photoemission.…”

“…To define how light couples with the electronic system of plasmonic metal, we characterize by 2PP spectroscopy the nonlinear response of atomically defined single-crystal Ag (111), (100), and (110) surfaces changes when the excitation light is tuned through ENZ, under ultra-high-vacuum conditions where the electronic response involves either single-particle excitations of the surface and bulk electronic bands or the collective bulk plasmon response. 2PP spectroscopy has been used to investigate the unoccupied electronic structure and ultrafast electron phase and population dynamics in metals, 89,90,93,94 semiconductors, 95−98 superconductors, 92 topological materials, 99,100 and atomic and molecular films on solid surfaces.…”