Динаміка збудження автоіонізаційних станів в атомі рубідію

З використанням методу електронної спектроскопії, нами досліджені функції збудження (ФЗ) деяких дублетних та квартетних автоіонізаційних станів (АІС), які представлені на рис.1. Похибка при визначенні ефективних перерізів збудження не перевищувала 30%

Infrared Dielectric Function of Gold Films in Relation to Their Morphology

Though data concerning gold’s optical properties differ substantially in the literature, the causes for these discrepancies are poorly understood. Surface quality affects the optical response considerably, not only through crystal defects but also through the existing morphology. If optical data analysis is done under the assumption of ideally flat surfaces, the obtained dielectric function or dynamic conductivity and any model parameters represent an effective description that may differ from bulk values according to various preparation conditions. To show this finding in detail, we performed spectroscopic ellipsometry measurements of evaporated gold films in the mid-infrared range, below the onset of the interband transitions, and investigated the sample morphology by means of atomic force microscopy. This study yields effective Drude-model parameters that vary with film morphology over a range that includes most of the published values. Introducing a Bruggeman effective medium to model rough films as a mixture of bulk metal and empty volume makes it possible to find a relation between metal volume fraction and effective plasma frequency. In such a model, the plasma frequency and also the dielectric background resulting from interband transitions decrease as the fraction of empty volume inclusions increases. In contrast, while metal volume fraction is much less influential to relaxation time, the density of the gold crystallites’ grain boundaries yields a strong effect. We thus found a plasma frequency, relaxation rate, and dielectric background for the most ideal gold films at room temperature of 7.37(40) × 10⁴ cm^–1, 221(1) cm^–1, and 9.6(3), respectively

Anisotropic Resistance of the Clean and Oxygen-Covered Cu(110) Surface in the Infrared

With mid-infrared spectroscopy at room temperature, we investigated the adsorption of oxygen on the clean, single-crystalline Cu(110) surface resulting in the well-known (2 × 1)­O–Cu(110) <i>added-row</i> reconstruction. We observed an anisotropic change of broadband reflectance which corresponds to an anisotropic surface resistance change. The resistance change is more pronounced by a factor of 7 for the plane of light incidence parallel to the [11̅0] direction. However, even perpendicular to this direction, a small but significant change is observed. A qualitative explanation of the anisotropic baseline shift can be given within existing theory, but for a quantitative description anisotropic electronic scattering of the bulk is crucial. Our results may be relevant for the optical behavior of nanocrystallites

Angstrom-Scale Distance Dependence of Antenna-Enhanced Vibrational Signals

The resonantly enhanced near-field of micrometer-sized gold antennas has been probed with Angstrom-scale resolution. <i>In situ</i> surface-enhanced infrared spectroscopic vibrational signals of carbon monoxide (CO) layers cold-condensed on the antennas in ultrahigh-vacuum conditions are compared to the signals of CO layers with corresponding thicknesses on a flat gold surface. Vibrational signals of CO as well as the shift of the plasmonic resonance frequency were used to analyze the distance dependence of the near-field. The signal enhancement induced by the antennas not only decays monotonically from the surface but, in contrast to classical near-field models, shows a maximum between 10 and 15 Å and decays also toward the surface of the antenna. This effect is attributed to the spill-out of the electron wave function, as expected from quantum mechanical calculations

Acoustic Surface Plasmon on Cu(111) as an Excitation in the Mid-Infrared Range

We report on an investigation of the acoustic surface plasmon on Cu(111), an electronic excitation in the infrared range related to the Shockley surface state. As shown here by high-resolution electron energy loss spectroscopy, it contributes together with other low-energetic electronic transitions to a broad excitation feature. Our analysis is similar to that recently reported for the Au(111) surface and clarifies that the group velocity of the acoustic surface plasmon is <i>slower</i> than the Fermi velocity of the surface state. The acoustic surface plasmon thus overlaps with the electron–hole pair continuum and may therefore influence adsorption processes as well

Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light

We present in situ infrared spectroscopy as a powerful tool for the qualitative and quantitative analysis of the charge transfer through the prototypical interface between the organic semiconductor 4,4′-bis­(<i>N</i>-carbazolyl)-1,1′-biphenyl (CBP) and MoO₃ that in organic electronic devices is often used to improve their performance. Due to the different infrared vibrational spectra, charged and neutral species of CBP molecules can be well distinguished, which allows the measurement of the amount of charged species in the vicinity of the interface. The quantitative analysis of CBP thickness-dependent infrared transmission spectra delivered the extension of the space charge region from the interface into the CBP on a nanometer scale. The clear influence of the deposition sequence on these interface properties was clarified by further studies of the inverted layer structures

One-Dimensional Plasmonic Excitations in Gold-Induced Superstructures on Si(553): Impact of Gold Coverage and Silicon Step Edge Polarization

Free charge carriers confined to atomic chains such as the gold-induced superstructures on the stepped Si(553) surface enable experimental insight into one-dimensional physics. Embedding into the higher dimensional substrate allows for additional couplings between the free charge carriers and their surroundings, which might modify the one-dimensional characteristics. The gold atom superstructures on Si(553) consist of a parallel arrangement of metallic chains from Au and Si atoms on the terraces and of parallel Si step edges with some of the Si atoms having dangling bonds with one unpaired electron. The metallic chains give rise to localized plasmonic excitations. We have studied these plasmonic resonances with infrared spectroscopy that enables the detection of resonance shifts as small as 1 meV or even less. The plasmonic behavior of the conductive chains of the high- and the low-coverage gold superstructures on Si(553) is investigated at various temperatures and additionally after filling electrons into certain electronic states by placing gold adatoms onto the high-coverage structure. When cooling to 20 K, the strong plasmonic signals of the undoped superstructures become even stronger but shift to lower frequencies, which is attributed to the temperature dependent change of the orientational polarization of the Si dangling bonds. Regarding their plasmonic resonance shifts, the conductive atom chains work just like refractive index sensors

Impact of Metal-Optical Properties on Surface-Enhanced Infrared Absorption

Surface-enhanced infrared absorption (SEIRA) spectroscopy using resonant metallic nanostructures is increasingly attracting interest during the last decade. Nevertheless, the impact of the metals’ intrinsic properties on SEIRA is still little studied. We present an experimental work on this topic, examining the infrared-optical resonance spectra of linear nanoantennas made of five of the most common metals (gold, silver, copper, aluminum, and iron) with respect to the intrinsic and radiation damping. Highly material- and size-dependent ratios of the two damping contributions were found and discussed. Using layers of organic probe molecules, we obtained SEIRA enhancement factors for the different nanoantennas and experimentally verified the predicted relationship between the plasmonic damping mechanisms and the SEIRA enhancement. The multitude of our experimental data for the ratio between the intrinsic electronic damping and the radiation damping is compared with the measured SEIRA enhancement of the various nanoantennas and therefore deliver the proof that the best SEIRA enhancement is achieved when both damping mechanisms equally contribute. Furthermore, it is shown that for a given nanoantenna geometry, the red-shift away from the plasmonic extinction maximum is strongly dependent on material parameters

How Intrinsic Phonons Manifest in Infrared Plasmonic Resonances of Crystalline Lead Nanowires

Single-crystalline lead nanowires with length of about one micrometer and with effective diameters of a few tens of nanometers have been grown on vicinal silicon by a self-assembly process. They show strong plasmonic resonances in the infrared with a remarkable enhancement of the extinction upon the cooling below room temperature. The increase of the plasmonic extinction at resonance is linear with decreasing temperature but saturates before the Debye temperature is approached. This observation is in full accordance with the quasi-static description of plasmonic extinction with the intrinsic damping dominated by phonons and thus linearly temperature dependent well above the Debye temperature. The different slopes of this linear function for different wire thickness indicate the importance of surface and near surface phonon properties that can be described by a Debye temperature that is lower than the bulk value. The careful spectral analysis also yields temperature independent contributions to the electronic scattering rates for various wire thicknesses and, furthermore, a resonance frequency decreasing with temperature, which corresponds to the predicted trend in renormalization theory for electron–phonon interaction in a metal like lead

Spatial Extent of Plasmonic Enhancement of Vibrational Signals in the Infrared

Infrared vibrations of molecular species can be enhanced by several orders of magnitude with plasmonic nanoantennas. Based on the confined electromagnetic near-fields of resonantly excited metal nanoparticles, this antenna-assisted surface-enhanced infrared spectroscopy enables the detection of minute amounts of analytes localized in the nanometer-scale vicinity of the structure. Among other important parameters, the distance of the vibrational oscillator of the analyte to the nanoantenna surface determines the signal enhancement. For sensing applications, this is a particularly important issue since the vibrating dipoles of interest may be located far away from the antenna surface because of functional layers and the large size of biomolecules, proteins, or bacteria. The relation between distance and signal enhancement is thus of paramount importance and measured here with <i>in situ</i> infrared spectroscopy during the growth of a probe layer. Our results indicate a diminishing signal enhancement and the effective saturation of the plasmonic resonance shift beyond 100 nm. The experiments carried out under ultra-high-vacuum conditions are supported by numerical calculations