Electronic transitions of Ar,Xe, N2, CO physisorbed on Ag(111) and Al(111)

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N₂, CO) on Ag(111) and Al(111) surfaces at ～20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes.In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases.The FWHM of the electronic excitations of CO and N₂ adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ～1eV.The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates.Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).     

Nonwetting and sticking of the heavy rare gases on potassium surfaces

We have investigated the adsorption, growth, and desorption of Ar, Kr, and Xe on K mono- and multilayers on Ru(001) at 6.5 K. In all cases, three-dimensional growth from very low rare gas (RG) coverages on, clearly indicates nonwetting, and the desorption energies for low RG coverages are much lower than the RG sublimation energies, and lower on K multi- than on K monolayers. Nevertheless, initial sticking coefficients of the RG on both types of K layers are well defined and not dominated by nucleation or by impurities or defects. Their values and dependence on type of substrate can be understood qualitatively, and quantitatively for Ar in terms of a recently developed quantum mechanical model, extending adsorption dynamics studies to nonwetting systems.     

Influence of Ar,Kr, and Xe layers on the energies and lifetimes of image-potential states on Cu(100)

The influence of well-ordered adlayers of Ar, Kr, and Xe on the energetic and dynamical properties of image-potential states on Cu(100) has been investigated in a comprehensive study using time-resolved two-photon photoemission (2PPE). The effect of these insulating films varies systematically with the electron affinity EA of the condensed rare gases and with the film thickness. For the electron-repulsive Ar layers (EA=-0.25 eV), a strong lifetime increase of the n=1 state from 40 fs on clean Cu(100) to as much as 10 ps at a coverage of 5 monolayers is observed. For Kr and Xe layers (EA=+0.3 and +0.5 eV, respectively), decoupling from the metal is less efficient. These layers exhibit quantum-well-like resonances of the n=2 state as a function of layer thickness. The energies of the series of states depend characteristically on the affinity level and the dielectric constant of the films. A microscopic model is developed that includes the discrete atomic structure of the adsorbate layers. It is capable of describing the experimental results to a high degree of quantitative agreement. PACS 78.47.+p; 73.20.At; 77.55.+f     

Formation and migration properties of the rare gases He,Ne, Ar,Kr, and Xe in nickel

The energies of formation and migration of various rare gas-point defect complexes in an f.c.c. nickel lattice have been calculated for He, Ne, Ar, Kr, and Xe. Formation energies of rare gas atoms at interstitial sites are compared with those in substitutional sites. Binding energies are presented for self-interstitials and vacancies trapped to the various rare gas substitutionals. We also present migration energies and migration paths for various rare gas interstitials and substitutionals with and without trapped vacancies and self-interstitials. The migration energies are compared with the breakup energies for the corresponding complexes. We find that divacancy-rare gas complexes are rather stable and will migrate at relatively low energies compared to other substitutional rare gas migration processes.     

Coupled plasmon and phonon dynamics in embedded Na clusters

We investigate, from a theoretical perspective, the coupled electronic and ionic/atomic dynamics of Na clusters embedded in Ar matrices. The system is described by time-dependent density-functional theory for cluster electrons and classical motion for Na⁺ ions as well as for Ar atoms. The interaction with the surrounding Ar atoms is modelled by polarization potentials plus core repulsion. We use this model to study coupled electronic and ionic/atomic motion in embedded clusters following a very short laser pulse. For excitations in the non-linear regime, we find clear signs for the coherent coupling of the Mie plasmon resonance with ionic vibrations (phonons). In addition, an incoherent line stretching is observed which can be traced back to the turning point of ionic vibrations. The coupling to the atomic motion of the surroundings leads to a slow and far reaching rearrangement of the matrix. PACS 36.40.Gk; 36.40.Vz; 31.15.EW     

Electronic and geometric structure of doped rare-gas clusters: surface, site and size effects studied with luminescence spectroscopy

The electronic and geometric structure of rare gas clusters doped with rare-gas atoms Rg = Xe, Kr or Ar is investigated with fluorescence excitation spectroscopy in the VUV spectral range. Several absorption bands are observed in the region of the first electronic excitations of the impurity atoms, which are related to the lowest spin-orbit split atomic ³P₁ and ¹P₁ states. Due to influence of surrounding atoms of the cluster, the atomic lines are shifted to the blue and broadened (“electronical cage effect”). From the known interaction potentials and the measured spectral shifts the coordination of the impurity atom in Ar_N, Kr_N, Ne_N and He_N could be studied in great detail. In the interior of Kr_N and Ar_N the Xe atoms are located in substitutional sites with 12 nearest neighbours and internuclear distances comparable to that of the host matrix. In Ne_N and He_N the cluster atoms (18 and 22, respectively) arrange themselves around the Xe impurity with a bondlength comparable to that of the heteronuclear dimer. The results confirm that He clusters are liquid while Ne clusters are solid for N≥ 300. Smaller Ne clusters exhibit a liquid like behaviour. When doping is strong, small Rg_m-clusters (Rg = Xe, Kr, Ar, m≤10 ²) are formed in the interior sites of the host cluster made of Ne or He. Specific electronically excited states, assigned to interface excitons are observed. Their absorption bands appear and shift towards lower energy when the cluster size m increases, according to the Frenkel exciton model. The characteristic bulk excitons appear in the spectra, only when the cluster radius exceeds the penetration depth of the interface exciton, which can be considerably larger than that in free Rg_m clusters. This effect is sensitive to electron affinities of the guest and the host cluster.     

X@C20F20(X=He,Ne,Ar,Kr)几何结构和 电子结构的理论研究

采用密度泛函理论中的广义梯度近似, 对X@C₂₀F₂₀(X=He, Ne, Ar, Kr)几何结构和电子结构进行了计算研究. 几何结构优化发现: 惰性气体原子X内掺到C₂₀F₂₀笼后, 均稳定于碳笼中心, 随着内掺X原子序数的增大, X原子对C₂₀F₂₀笼的影响越来越大. 能隙、内掺能和振动频率计算表明: 内掺X原子使得C₂₀F₂₀的稳定性得到了显著提升, X@C₂₀F₂₀(X=He, Ne, Ar, Kr)都具有良好的稳定性, 并且随着X原子序数的增大, 其稳定性也基本呈现逐渐增强的趋势. 电子结构研究发现: X原子对X@C₂₀F₂₀费米能级附近的占据轨道基本没有贡献, 而对其未占据轨道贡献较大. 计算还发现: 在X@C₂₀F₂₀中, He 和Kr分别从C₂₀F₂₀的C 笼上获得了0.126和0.271个电子, 而Ne和Ar却分别向C笼转移了0.060和0.012个电子. 由此可见: X原子与C原子之间都发生了电荷转移, C笼上的C原子与惰性气体原子X间形成了一定的离子键.     

Surface excitations in magnetic systems with a singlet ground state

We have investigated surface excitations in a system wherein the ionic ground state is a magnetic singlet. The pseudospin formalism is employed to account for the crystal-field and exchange interactions between the ions in a Heisenberg ferromagnet with the singlet-triplet crystal-field-only level scheme. The Hamiltonian was studied in the molecular field approximation to find the possible ground states. Surface excitations for the simple cubic structure were investigated for the (001) surface in the random phase approximation. Analytic expressions have been obtained for the thermodynamic Green functions. For a fully magnetized molecular field ground state, there are in general two bulk bands, the spin-wave and the excitonic. Surface modes were found to exist below the spin-wave band, above the excitonic band and between the bands. The dispersion curves can exist only over one or two limited regions of the two-dimensional wave-vector parallel to the crystal surface.     

利用重正化激子方法计算离域离子态或激发态的一阶分子性质

近年来,重正化激子方法已快速发展为一种新兴的低标度从头算激发态量子化学方法. 该方法假定大体系的整体激发态是其若干子体系局域激发态的线性组合,并利用有效哈密顿理论来高精度计算这些局域态之间的耦合. 在本文中,通过将有效哈密顿理论拓展到普适的有效算符方案,实现了重正化激子方法对于大体系离域电子态的一阶分子性质(如电荷布居、跃迁偶极矩)的高效计算. 对于四种不同的分子聚集体(氨、甲醛、乙烯、吡咯)的离域离子态和激发态的重正化激子计算测试证实：新方法可以同时实现大体系低能离域电子态的能量和波函数性质的高效高精度计算.     

Freezing and melting of binary mixtures confined in a nanopore

This paper reports on a Grand Canonical Monte Carlo study of the freezing and melting of Lennard–Jones Ar/Kr mixtures confined in a slit pore composed of two strongly attractive structureless walls. For all molar compositions and temperatures, the pore, which has a width of 1.44?nm, accommodates two contact layers and one inner layer. Different wall/fluid interactions are considered, corresponding to pore walls that have a larger affinity for either Ar or Kr. The solid/liquid phase diagram of the confined mixture is determined and results compared with data for the bulk mixture. The structure of the confined mixture is studied using 2D order parameters and both positional g(r) and bond orientational G₆(r) pair correlation functions. It is found that in the confined solid phase, both the contact and inner layers have a hexagonal crystal structure. It is shown that the freezing temperature of the Ar/Kr confined mixture is higher than the bulk freezing point for all molar compositions. Also, it is found that the freezing temperature becomes larger as the ratio α of the wall/fluid to the fluid/fluid interactions increases, in agreement with previous simulation studies on pure substances confined in nanopores. In the case of pore walls having a stronger affinity for Kr atoms (ε _Ar/W<ε _Kr/W), it is observed that both the contact and inner layers of the confined mixture undergo, at the same temperature, a transition from the liquid phase to the crystal phase. The freezing of Ar/Kr mixtures confined between the walls having a stronger affinity for Ar (ε _Ar/W?>?ε _Kr/W) is more complex: for Kr molar concentration lower than 0.35, we observe the presence of an intermediate state between all layers being 2D hexagonal crystals and all the layers being liquid. This intermediate state consists of a crystalline contact layer and a liquid-like inner layer. It is also shown that the qualitative variations of the increase of freezing temperature with the molar composition depend on the affinity of the pore wall for the different components. These results confirm that, in addition to the parameter α the ratio of the wall/fluid interactions for the two species, η=?_Ar/W/?_Kr/W, is a key variable in determining the freezing and melting behaviour of the confined mixture.     

Conductivity of irradiated Kapton in relation to energy loss of ions and electrons

Abstract

We have studied the effects of 2.5 MeV electron irradiation and ion (C, N, F, Si and Kr) bombardment on the electrical conductivity of a polyimide (Kapton-H) with ion energies ranging between 320 keV (N) and 1.25 GeV (Kr). In this wide range of situations we have tried to sort out the respective effects of nuclear and electronic excitation energy losses.

For all ion irradiation the conductivity is found to scale with the electronic excitation absorbed dose: i.e. a power law of conductivity versus absorbed dose with an exponent around 9 is observed. At a given absorbed dose (in Gray units) the efficiency of each ion to enhance conductivity is found to be proportional to the electronic energy loss; electrons are much less efficient than ions and thus collective excitations are required to achieve this process.

The nuclear energy loss can perhaps play some role at conductivities higher than 100 Ω^?1 m^?1, but its effects are negligible in the range explored here.     

Complete two-electron spectra in double photoionization: the rare gases Ar,Kr, and Xe

A new spectroscopic technique, giving complete two-dimensional e(-)-e(-) coincidence spectra in single photon double photoionization, is presented. The technique resolves the states of doubly charged ions and provides spectra of the individual electrons emitted in formation of each final dication state. Complete spectra for double photoionization of Xe, Kr, and Ar at photon energies up to 51 eV have been recorded. Overall and surprisingly, the np(4) 3P, 1D, and 1S states are populated according to their statistical weights. When the evident autoionization is excluded, the supposedly favored 3P states are in fact disfavored. Detailed information on the autoionization processes is also made available.     

UV photoemission from physisorbed atoms and molecules: Electronic binding energies of valence levels in mono- and multilayers

Angle-resolved UV photoemission spectra were measured for Ar, Kr, Xe, CO, O₂ and N₂ adsorbed on a Ni(110) surface at 20 K. The different gases were adsorbed also on the Ni(110) surface which had been precovered by mono- and multilayers of the same gases. Upon physisorbing one of these species onto the bare and precovered Ni surface, binding energy shifts up to 3 eV were found. These shifts will be explained by work function changes of the substrate onto which the gas is physisorbed. It will be shown that for the investigated gases the binding energy referred to the vacuum level is an atomic or molecular property which is independent of the substrate, to a first approximation. By physisorption of a known gas the work function of any substrate can be evaluated by UPS. The density of valence states for bulk Ar, Kr and Xe will be discussed. There is evidence that the conduction band can be seen in the secondary electron background of the UP spectra.     

Plasma line emission during single-bubble cavitation

Emission lines from transitions between high-energy states of noble-gas atoms (Ne, Ar, Kr, and Xe) and ions (Ar(+), Kr(+), and Xe(+)) formed and excited during single-bubble cavitation in sulfuric acid are reported. The excited states responsible for these emission lines range 8.3 eV (for Xe) to 37.1 eV (for Ar(+)) above the respective ground states. Observation of emission lines allows for identification of intracavity species responsible for light emission; the populated energy levels indicate the plasma generated during cavitation is comprised of highly energetic particles.     

Low-temperature time-resolved spectroscopy of APb2X5 crystals (A ≡ K, Rb; X ≡ Cl, Br)

The paper reports on a low-temperature (8 K) time-resolved spectroscopic study of excitonic states and radiative relaxation of electronic excitations in undoped APb₂X₅ crystals (A ≡ K, Rb; X ≡ Cl, Br) performed under selective photoexcitation by synchrotron radiation. The study has revealed a variety of channels of radiative relaxation of intrinsic electronic excitations, which should be primarily assigned to specific features of the electronic structure of the crystal.     

Emission from impurity molecular centres in binary solutions of inert crystals

The structure of impurity excitations in solid binary solutions XeKr, XeAr, KrAr has been studied using the luminescence vacuum u.v. spectroscopy. Emission from biatomic heteronuclear molecules of inert elements was separated. The existence of two excited states of heteronuclear molecules of different atomic configurations is predicted. The pair potential of inert element interaction in the excited and ground states are analysed and force constants found. The peculiarities of the heteronuclear molecule formation as a function of concentration and temperature for solid binary solutions of inert gases have been discussed.     

Excitation and emission of metal electrons in atom-surface collisions

Electron-hole pair excitation and ionization probabilities are calculated for atomic collisions with metal surfaces at high incident energies. The method adopted is based on a Sudden Collision Approximation, and a realistic model is employed for the bound and continuum electronic states involved. The parameters used in the calculations are for Ar, He, H atoms impinging on a Li surface at 300 eV. The main results are: (1) Only single electron-hole pair excitations are important; multiple pair contributions are small. (2) The transitions are dominated by the behavior of the electronic wavefunctions in the tunneling region and may serve as a probe of this regime. (3) The excitation efficiency is in the order H ? Ar ? He, the effectiveness of hydrogen being due to its stronger, longer-range coupling. (4) The maximum excitation probabilities are for electrons ejected with relatively low excess energies. (5) Total transition probabilities are about 0.5 per collision for H, and about 0.1 for Ar, indicating that these are important, easily detectable processes. Experiments in this field should provide important information on electronic wavefunctions at the metal-gas interface, and on gas-metal interactions at high energies.     

Quantum-well resonances and image states in the Ar/Cu(1 0 0) system

A theoretical study of the effect of an atomically thin rare gas layer on the dynamics of excited electronic states at metal surfaces is presented for the case of a few mono-layers of Ar on a Cu(1 0 0) surface. We develop a 3D-microscopic model with predictive capabilities of the interaction of an electron with an Ar layer physisorbed on a metal surface. It takes into account the 3D structure of the Ar layer as well as its dielectric character. The dynamics of the excited electron on the surface is treated within a wave-packet propagation approach. The calculations show that two different types of excited states are present at the Ar/Cu(1 0 0) surface. (i) Image states that are repelled into vacuum as compared to their position on clean Cu(1 0 0) surfaces, leading to a decrease of their binding energies and to an increase of their lifetimes. (ii) Quantum-well resonances, corresponding to quasi-stationary states localised inside the Ar layer; they are associated with the quantisation of the conduction band in the finite size Ar layer. The present results on image states nicely agree with very recent time-resolved two-photon-photo-emission experiments by Berthold, Feulner and Höfer.     

Optical spectroscopy study of modifications induced in cerium dioxide by electron and ion irradiations

UV-visible absorption spectroscopy and Raman spectroscopy were used to study damage production in cerium dioxide epitaxial films and polycrystalline sintered samples after irradiation with electrons for three energies to span the threshold displacement energies of cerium and oxygen atoms, and 2.4-MeV Cu ions. Neither amorphization nor specific colour-centre bands were detected. Evolutions of the refractive index were derived from the interference fringes in the optical transmission spectra of epilayers after irradiation. No significant change of the refractive index occurred for the 1.0-MeV electron irradiation, whereas a maximum decrease by 28?±?8% was deduced for the 1.4-MeV and 2.5-MeV energies. These modifications are consistent with ballistic damage on the cerium sublattice for high electron energies producing Ce³⁺ ions. However, no significant change of refractive index was found for the Cu ion irradiation. This likely stems from the high rate of Frenkel pair recombination in the collision cascades induced by more energetic recoils than for the electron irradiations, combined with electronic excitations and hole capture on Ce³⁺ ions. This study reveals modifications of the electronic structure upon irradiation that could take place in other non-amorphizable oxide systems.     

Resonant enhancement of inelastic light scattering in the fractional quantum Hall regime at ν=1/3

Strong resonant enhancements of inelastic light scattering from the long wavelength inter-Landau level magnetoplasmon and the intra-Landau level spin wave excitations are seen for the fractional quantum Hall state at ν=1/3. The energies of the sharp peaks (FWHM 0.2 meV) in the profiles of resonant enhancement of inelastic light scattering intensities coincide with the energies of photoluminescence bands assigned to negatively charged exciton recombination. To interpret the observed enhancement profiles, we propose three-step light scattering mechanisms in which the intermediate resonant transitions are to states with charged excitonic excitations.