Wechselwirkung von Elektronen mit Gitterschwingungen in Ammoniumchlorid und Ammoniumbromid

Energy loss spectra of polycrystalline NH₄Cl and NH₄Br films have been studied between 10 and 400 meV using 30-keV electrons. The resolution of the spectrometer was between 4 and 10 meV, the scattering angle smaller than 1.2×10^?4 radian. Strong energy losses and gains were found near 20 meV corresponding to excitation of translational lattice vibrations wellknown from reststrahl measurements. Weaker energy losses above 100 meV are due to excitation of ammonium ion fundamentals and their interaction with torsional vibrations and, presumably, with translational lattice vibrations. In this energy loss range agreement is stated concerning the general behaviour between energy loss and infra-red absorption spectra. In the finer details, however, striking deviations occur, which must be ascribed to the different interaction mechanisms of lattice vibrations with electrons and photons, respectively.     

Electron energy losses by excitation of transition radiation

Inelastic scattering of KeV electrons in thin films with energy losses corresponding to the infrared spectral region is discussed taking retardation into account, that is allowing for radiative modes. For sufficiently thin target films Cerenkov radiation does not play any role. Rather it turned out even for nonrelativistic electrons, that in the spectral region of negligible absorption, where for example excitation of phonons or surface phonons does not occur, the energy loss spectrum is exclusively due to the creation of transition radiation. By means of high resolution spectroscopy of electrons inelastically scattered from TiO₂ films it is shown experimentally that this conclusion is indeed correct.     

Vibrational spectroscopy of oxygen on the (100) and (111) surfaces of lanthanum hexaboride

Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O₂ produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ～ 734 cm^?1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm^?1 with full widths at half maxima of only 12 cm^?1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB₆. On the boron-terminated LaB₆(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O₂.     

Spectroscopy of vibrational modes in metal nanoshells

In this work we study the spectrum of vibrational modes in metal nanoparticles with a dielectric core. Vibrational modes are excited by the rapid heating of the particle lattice that takes place after laser excitation, and can be monitored by means of pump-probe spectroscopy as coherent oscillations of transient optical spectra. In nanoshells, the presence of two metal surfaces results in a substantially different energy spectrum of acoustic vibrations than for solid particles. We calculated the energy spectrum as well as the damping of nanoshell vibrational modes. The oscillator strength of the fundamental breathing mode is larger than that in solid nanoparticles. At the same time, in very thin nanoshells, the fundamental mode is overdamped due to instantaneous energy transfer to the surrounding medium. PACS 78.67.-n; 78.67.Bf; 63.22.+m     

N23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N₂₃-excitation and the low energy N₂₃VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N₂₃VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.     

Phonon-assisted stimulated emission in 2H type PbI2

The stimulated emission in 2H type PbI₂ crystals has been observed under N₂ laser beam excitation at 4.2 K. The analysis of its gain spectrum shows that the emission is induced by the recombination of free exciton assisted by the emission of one longitudinal optical phonon. At a higher level of excitation, the gain spectrum peak shifts to lower energy side. Possible mechanisms of this spectral change are discussed.     

Phononenspektroskopie von Ammoniumchlorid und Ammoniumbromid mit keV-Elektronen

The high resolution energy loss spectrum of NH₄Cl in the spectral region of internal lattice vibration has been reinvestigated. In order to assure the assignement made, the spectra of the deuterated ammonium halides ND₄Cl and ND₄Br has been studied too. The structure of the films, which were investigated, was checked by electron-diffraction and by electron microscopy. The deuterated films show the shift of energy losses towards lower energy losses as expected. Unshifted losses are assigned to partially deuterated material. Two types of spectra are observed for NH₄Cl depending on the size of the crystals and presumably on the perfectnes of the lattice. Unlike the external vibrations, which are strongly influenced by surface effects, at least for the strongest bands good agreement is found with the absorption measurement concerning the positions of the peaks.     

Electron energy loss study of titanium dioxide,barium titanate and silica in the range between 0.02 and 2eV

This work discusses the energy loss spectra of TiO₂, BaTiO₃, silica, and graphite in the range between 0.02 and 2eV. The observed energy losses of crystalline TiO₂ and BaTiO₃ are located energetically between the excitation energies of the transversal and longitudinal optical phonons. This means that the energy losses are due to three (TiO₂) or two (BaTiO₃) different types of surface phonons. The energy loss spectra of the amorphous TiO₂, BaTiO₃, and silica are broadened and show certain similarities to the one-phonon density of states like Raman and infra-red absorption spectra. Rather sharp energy losses in the spectrum of the amorphous TiO₂ are partially interpreted as vibrational excitation of TiO or TiO₂ molecular states.In the energy loss range between 0.2 and 2eV the spectra of the amorphous and crystalline polar material are due to the coupling of the beam electrons to the electromagnetic field of the transition radiation. In the spectrum of graphite, however, which is referred to as comparison, the excitation of the almost free conduction electrons predominates.Part of this work has been presented at the Third International Conference on Thin Films Basic Problems, Applications and Trends, Budapest, Hungary, 1975 [1] and at the International Conference on Lattice Dynamics, Paris 1977 [2]     

Raman scattering from soft to-phonon in IV–VI compound semiconductors

The soft TO-phonon modes were investigated below the cubic-rhombohedral transition temperature in Pb_1?xGe_xTe (x=0.05) and SnTe. We observed resolved Raman spectra from ordinary and extraordinary modes. The temperature dependence of the spectra was analyzed in terms of phonon-phonon interactions. We also discussed origins of extra Raman spectra which appear even in the high temperature phase. The energy gain of valence bonding electrons due to lattice distortions was estimated to be ≈10⁹erg/cm³ by analyzing anomalous temperature behavior of the optical dielectric constant. This is about two order of magnitude larger than the depth of the well of free energy.     

Strongly coupled optical phonons in the ultrafast dynamics of the electronic energy and current relaxation in graphite

Ultrafast charge carrier dynamics in graphite has been investigated by time-resolved terahertz spectroscopy. Analysis of the transient dielectric function and model calculations show that more than 90% of the initially deposited excitation energy is transferred to a few strongly coupled lattice vibrations within 500 fs. These hot optical phonons also substantially contribute to the striking increase of the Drude relaxation rate observed during the first picosecond after photoexcitation. The subsequent cooling of the hot phonons yields a lifetime estimate of 7 ps for these modes.     

Resonance Raman scattering by localized and resonant modes of Ag2 molecules in rare gas matrices

We demonstrate for the first time that excitation of silver containing rare gas matrices at 406.74 nm results in resonance Raman spectra which show low energy localized and resonant modes. As they combine with the Ag₂ stretching vibration and its overtones, they can unambiguously be attributed to the Ag₂ molecule. In Xe-matrices the coupling of lattice phonons and impurity vibrations is documented by side bands which resemble the phonon density of states of the host lattice. Two trapping sites are observed in Kr-matrices.     

Fluorescence excitation spectroscopic characterisation of F2 - colour centers in a LiF crystal

LiF crystals with F₂ ^- centers contain a whole series of F-based colour centers with spectrally overlapping absorption spectra. On the short-wavelength side the absorption spectrum of the F₂ ^- centers is overlapping with the absorption spectra of other colour centers and is therefore not directly accessible by transmission measurements. Fluorescence excitation spectroscopy is employed here to gain information on the absorption spectrum of F₂ ^- centers in spectral regions where the absorption by other centers is present. The fluorescence signal emitted beyond 1200 nm is detected as a function of the excitation wavelength in the range from 350 nm to 1100 nm. The F₂ ^- center absorption cross-section spectrum in the wavelength range from 690 nm to 1100 nm and from 350 nm to 410 nm is determined from the fluorescence excitation quantum distribution and the transmission spectrum. Influences of fluorescence re-absorption and Förster-type energy transfer on the fluorescence excitation quantum distribution and the subsequent F₂ ^- center absorption cross-section determination are discussed. PACS 61.72.Ji; 78.40.Ha; 78.55.Fv     

Resonance Raman spectroscopic study for radial vibrational modes in ultra‐thin walled TiO2 nanotubes

The study reports the observation of radial vibrational modes in ultra‐thin walled anatase TiO₂ nanotube powders grown by rapid breakdown anodization technique using resonant Raman spectroscopic study. The as‐grown tubes in the anatase phase are around 2–5 nm in wall thickness, 15–18 nm in diameter and few microns in length. The E_{g(ν1,ν5,ν6)} phonon modes with molecular vibrations in the radial direction are predominant in the resonance Raman spectroscopy using 325 nm He–Cd excitation. Multi‐phonons including overtones and combinational modes of E_{g(ν1,ν5,ν6)} are abundantly observed. Fröhlich interaction owing to electron–phonon coupling in the resonance Raman spectroscopy of ultra‐thin wall nanotubes is responsible for the observation of radial vibrational modes. Finite size with large surface energy in these nanotubes energetically favor only one mode, B_1g(ν4) with unidirectional molecular vibrations in the parallel configuration out of the three Raman modes with molecular vibration normal to the radial modes. Enhanced specific heat with increasing temperatures in these nanotubes as compared to that reported for nanoparticles of similar diameter may possibly be related to the presence of the prominent radial mode along with other energetic phonon mode. The findings elucidate the understanding of total energy landscape for TiO₂ nanotubes. Copyright © 2015 John Wiley & Sons, Ltd.     

Spectroscopic identification of the reaction intermediates in the thermal decomposition of trimethylindium at GaAs(100) surfaces

High resolution electron energy loss spectroscopy (HREELS) has been used to study the adsorption and thermal decomposition of trimethylindium (TMIn) on Ga-terminated GaAs(100) surfaces. HREEL spectra recorded for adsorption at room temperature are dominated by strong CH₃ deformation and stretching modes and indicate that the surface species is based on methyl groups. The intensities of these bands decrease with increasing temperature consistent with a primary decomposition route involving the loss of CH₃ groups from the surface. A small upward shift in the frequency of the symmetric and asymmetric CH₃ deformation modes is also observed with increasing temperature and indicates that decomposition takes place via an exchange reaction in which CH₃ groups switch from In to Ga due to the stronger Ga-C bond. At temperatures greater than 350°C, the spectra are dominated by CH₂ rocking, deformation and stretching vibrations. The presence of a surface methylene species at elevated temperatures suggests a second, minority decomposition pathway which involves dehydrogenation of surface CH₃ groups to CH₂.     

BaBPO5晶体晶格振动光谱研究与第一性原理计算

通过偏振拉曼光谱和第一性原理计算对非线性光学晶体BaBPO₅的 晶格振动模式进行了研究. 实验得到了不同几何配置下、在100–1600 cm^-1范围内的晶体偏振拉曼光 谱与傅里叶变换红外吸收谱, 结合因子群分析方法研究了晶体的外振动与内振动模式特征. 分析表明拉曼振动主要来自于PO₄四面体和BO₄四面体的振动, 且PO₄基团振动具有较强的拉曼与红外活性. 此外,根据第一性原理对晶体拉曼振动进行了数值模拟, 进一步明确了拉曼峰与晶体中原子振动的对应关系, 计算表明拉曼光谱中位于672 cm^-1峰位来自晶体中B–O–P键的伸缩振动, 这是晶体中PO₄四面体和BO₄四面体共顶点连接的特征结构在光谱中的体现.     

Interaction of Electrons with Indole,Tryptophan, and their Derivatives in the Gas Phase

The electron energy loss spectra (EELS) of indole, 3-indolyl propionic acid, 3-indolealdehyde, 3-dimethylaminomethylindole, tryptophan, and N-acetyl-L-tryptophan in the gas phase upon excitation by monokinetic electrons with an energy of E₀ = 11–50 eV are obtained. The structure of EELS is determined in the main by the indole chromophore; the side groups, except for the C=O group of 3-indolealdehyde, exert an insignificant influence. The energy of the lower triplet level ³L_a is 3.3 eV for indole and its derivatives and 3.2 eV for tryptophan and N-acetyl-L-tryptophan. Four singlet transitions in the region of 4.4–7.2 eV have been identified. The molecules studied, except for tryptophan, fluoresce in the gas phase on excitation by electrons. At low values of E₀ (10–25 eV), the fluorescence spectra are similar and are due to the indole fluorophore. Just as in the case of optical excitation, fluorescence on excitation by electrons is associated with the ¹L_b-S₀ transitions. An increase in the energy E₀ up to 60–80 eV leads to dissociation of a portion of the indole molecules and to the appearance of additional bands in the fluorescence spectrum. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 468–472, July–August, 2005.     

Subsurface-channeling-like energy loss structure of the skipping motion on an ionic crystal

The skipping motion of Ne+ ions in grazing scattering from the LiF(001) surface is studied for velocity below 0.1 a.u. with a time-of-flight technique. It is demonstrated that suppression of electronic excitation and dominance of optical phonon excitation in the projectile stopping results in an odd 1,3,5,... progression of the energy loss peaks, a feature usually ascribed to subsurface channeling. The experimental findings are well reproduced by parameter-free model calculations where thermal vibrations are the dominant cause for the ion trapping and detrapping.     

Raman spectroscopy of gold nanoparticles in polycrystalline LiF film

Results of the Raman spectroscopy analysis of a new composite material based on a thin polycrystalline LiF film containing gold nanoparticles are presented. The formation of spherical gold nanoparticles in the film has been confirmed by the X-ray structural analysis and observation of the optical plasmon resonance absorption spectrum with a maximum at 534 nm. The obtained composite layers have been subjected to annealing by ruby laser (λ = 694 nm) in the spectral region on a descending long-wavelength wing of the plasmon absorption band of gold nanoparticles. Raman spectroscopy has been applied for the first time to the investigation of the modification of the shape of gold nanoparticles in LiF during laser annealing. The experimental Raman spectra are compared with calculated modes of in-phase bending vibrations generated in gold nanoparticles.     

Wirkungsquerschnitte für die Anregung von Molekülschwingungen durch schnelle Elektronen

The differential cross section for the excitation of infra-red inactive and — in dipole approximation — infra-red active fundamental vibrations by fast electrons is calculated. The energy loss spectra of CO₂, N₂O, C₂H₄ are measured with 33 keV electrons in the small energy loss range up to 0.5 eV. The experimentally determined “particular” cross sections

$$\sigma = \int\limits_{\vartheta = 0}^{1.1 \cdot 10^{ - 4} } {d\sigma }$$     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.