Scanning tunneling microscopy on self-assembled calix[4]resorcinarene monolayer adsorbates on Au(111)

Self-assembled Monolayers of calix[4]resorcinarene receptor molecules on Au(111) were studied by UHV scanning tunneling microscopy and X-ray photoelectron spectroscopy. Highly ordered monolayers were observed with domains oriented at an angle of 60° relative to each other. Molecularly resolved images were investigated and lattice constants found which depended on the preparation solvent. The STM images of two samples, one prepared in 1 mM chloroform/ethyl alcohol adsorbate solution and one in 1 mM hexane adsorbate solution are consistent with having a ×2 and 4×2 lattice, respectively. Received: 22 September 1999 / Accepted: 28 March 2000 / Published online: 11 May 2000     

Growth of extra-thin ordered aluminum films on Si(110) surfaces

The formation of surface structures upon Al deposition onto a Si(110) surface was studied by LEED and AES. The “4 × 6”, “1 × 9”, 2 × 1, 1 × 1 ordered Si(110)-Al surface phases and epitaxial Al(110) domains were observed depending on Al coverage and substrate temperature. The formation phase diagram was drawn for the Al/Si(110) system.     

Electronic structure and local interactions on a S(100) 2×1 surface with submonolayer Ba overlayers

The electronic structure and ionization energy for the system Ba/Si(100)2×1 have been studied as functions of the submonolayer coverage. It is found that there is an energy gap in the surface states spectrum and that the Ba/Si(100)2×1 interface is semiconducting up to 1.5 monolayers of Ba. Two surface bands induced by Ba adsorption have been detected. The evolution of the spectrum with increasing degree of Ba coverage points to the existence of two nonequivalent “adsorption sites,” which differ in binding energy by 0.11 eV. The development of the Ba-induced bands is found to terminate at a coverage corresponding to the minimum ionization energy and close to one monolayer. The adsorption bond is shown to have a primarily covalent character. Zh. éksp. Teor. Fiz. 114, 2145–2152 (December 1998)     

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (×) R30^° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures. Received: 3 August 2000 / Accepted: 4 August 2000 / Published online: 7 March 2001     

Carbon monoxide adsorption kinetics on molybdenum (100)

In an ultrahigh vacuum system (10^?10 torr) LEED, Auger spectroscopy, flash desorption and work-function measurements have been used to follow the kinetics of adsorption of carbon monoxide on the (100) face of a molybdenum single crystal. At room tempeiature CO adsorbs into three β states β₁, β₂, β₃, which are associated with different work-function changes: the phase β₃ is found to correspond to a decrease of work-function whereas the phases β₁ and β₂ correspond to an increase. The total change of surface potential is about 0.50 eV at saturation. An ordered C (2×2) structure is found for the β₂ + β₃ phases, and is associated at their maximum coverage (about half a monolayer) with a work-function change of + 0.070 eV with respect to the clean surface. At completion of the monolayer a P (1×1) structure is observed.     

Intense femtosecond laser-induced second-harmonic generation in atmospheric-pressure air

Second-harmonic generation (SHG) in atmospheric-pressure air is experimentally studied using single focused linear-polarized Ti:sapphire intense femtosecond laser pulses at 810 nm. The efficiency of SHG is found to reach a maximum at the optical breakdown threshold of ≈2.9×10¹⁴ W/cm². The spectral distribution and polarization property of the second harmonic are investigated. The contribution to SHG from electric-field-induced third-order mixing plays the main role even after the optical breakdown had occurred. Received: 23 May 2000 / Published online: 16 August 2000     

Photoemission studies of ultrathin Cs,Ba/InN interfaces

Cs/InN and Ba/InN interfaces were studied by UV photoelectron spectroscopy in the submonolayer coverage range for the first time. Normal photoemission spectra from the valence band and spectra from In 4d, Ba 5p, Ba 4d, and Cs 5p core levels were investigated in the excitation energy range of 60–800 eV. It was found that metallization of the interface and narrowing of the valence band is observed upon increasing coverage.     

The curved feature of ultrasound-treated graphitic sheets

When examined under a high-resolution transmission microscope (HRTEM), highly oriented pyrolitic graphite (HOPG), after ultrasound treatment, is found to contain some bent graphitic sheets. These bent structures are ordered graphitic sheets, which have specific bend angles that are a multiple of 30° (from 30° to 180°). We speculate that the creation and variation of bend angles is a result of interplay between the conformation of sp³-like defects and the ultrasound wave impact. Received 8 September 2000 / Accepted: 6 November 2000 / Published online: 23 May 2001     

Pressure-induced dimerization kinetics of fullerene C60

Dimerization kinetics was studied for fullerene C₆₀ by IR spectroscopy at a pressure of 1.5 GPa in the temperature range 373–473 K. The kinetic curves for the formation of a dimer (C₆₀)₂ were obtained using its analytical IR band at 796 cm^?1. Under the assumption that pressure-induced C₆₀ dimerization is a second-order irreversible reaction, the reaction rate constants were determined at different temperatures. The corresponding activation energy and preexponential factor were found to be 134±6 kJ/mol and (1.74± 0.24)×10¹⁴ s^?1, respectively. The specific features of the solid-phase C₆₀ dimerization in simple cubic and face-centered cubic fullerite phases are discussed.     

Electronic structure of ultrathin Cs coatings on a Si(100)2×1 surface

The electronic structure and ionization energy of submonolayer Cs coatings on a Si(100)2×1 surface is investigated by threshold photo-emission spectroscopy. Two surface bands induced by Cs adsorption are observed, and their evolution is studied as a function of coverage. It is found that there are two “adsorption locations” for Cs atoms, where they interact with active dangling bonds at the surface. It has been determined that the Cs/Si(100)2×1 interface is semiconducting all the way down to monolayer coverage. The results show that Cs adsorption is predominantly of a covalent character. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 699–702 (10 May 1997)     

The chemisorption of O2, CO,D2 and C2H4 over epitaxially grown rhenium crystalline films

The adsorption and desorption of oxygen, carbon monoxide, deuterium and ethylene has been studied over rhenium films using thermal desorption spectroscopy, low energy electron diffraction and Auger electron spectroscopy. The films, obtained by evaporating rhenium onto a platinum (111) single crystal, grow over the substrate forming (0001) basal plane rhenium surfaces. Oxygen chemisorbs on this film, forming an ordered structure, consisting of three (2 × 1) overlayer domains and giving a saturation coverage of half a monolayer of atomic oxygen. CO chemisorption is mainly molecular, although some dissociation occurs at temperatures above about 700 K. A complicated LEED pattern is obtained when saturating the surface at 150 K with CO, but it changes to a (2 × 2) or (2 × 1) structure upon heating. Also, CO chemisorption can be modified by predissociated CO or preadsorbed oxygen on the rhenium surface. Deuterium desorbs in three peaks, starting at temperatures as low as 150 K. Ethylene desorbs partially intact at around 250 K, the rest decomposing and yielding hydrogen, that appears as two main peaks at 357 K and 460 K during thermal desorption. We conclude that epitaxially grown films may be an alternative to single crystals for studying chemisorption over well ordered surfaces.     

Wavelength-modulation spectroscopy using a frequency-doubled current-modulated diode laser

We have studied atomic absorption in an argon discharge by wavelength-modulation spectroscopy with a frequency-doubled diode laser. The tunable wavelength-modulated radiation at 430 nm was generated by frequency doubling a current-modulated 860-nm diode laser in a KNbO₃ crystal. 2f-, 4f- and 6f-harmonic spectra as a function of diode laser modulation depth were measured on a Doppler-broadened sample of excited argon atoms produced in a capacitively coupled plasma chamber. Characterisation of the harmonic signals was accomplished. Minimum detectable absorbances of 7.7×10^-5 and 1.9×10^-4 based on a 3σ criterion (σ being the standard deviation of the noise) were estimated for 2f- and 4f-harmonic detection of the frequency-doubled radiation with a time constant of 0.1 s. The concentrations of argon in the 1s₄ state were found to be in the range of 3×10⁸ to 1.2×10¹¹ cm^-3 for the experimental conditions studied. Received: 25 February 2002 / Revised version: 4 April 2002 / Published online: 14 May 2002     

First Z-scan n2 measurements on crystal hosts for ultraviolet laser systems

We report the results of the first measurements of the non-linear Kerr refractive index, n₂, for LiBaF₃ and LiLuF₄ crystal hosts, known as prospective UV-emitting tunable laser media when doped with Ce³⁺ ions. These n₂ values (2.7×10^-16 and 1.5×10^-16 cm²/W at 532 nm, respectively), obtained using the well-established Z-scan technique, are important for the characterization of new optical materials particularly in relation to their potential ultrafast applications. Received: 6 June 2000 / Published online: 5 October 2000     

Quantitative analysis of dilute mixtures of SO<Subscript>2</Subscript> in N<Subscript>2</Subscript> at 7.4 μm by difference frequency spectroscopy

We describe a 7.4-μm source based on difference frequency generation with 6.5 mW of 1278-nm radiation from an extended cavity laser and 66 mW of 1544-nm radiation from another extended cavity laser, amplified in an erbium-doped fibre amplifier. Optimum focusing of the input beams in the 5×5×10-mm³ AgGaSe₂ crystal, and the spatial and temporal characteristics of the output beam, are determined. The source is used for accurate determination of line parameters for selected lines of the ν₃ band of SO₂, centred at 1361 cm^-1. Subsequently, these lines are used for performing quantitative analysis of gas mixtures containing SO₂ at concentration levels down to 4 ppm without relying on any calibration with certified gas mixtures. This demonstrates the potential of infrared spectroscopy as a primary method for low-concentration gas analysis. Received: 16 January 2003 / Revised version: 19 February 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +45-4593/1137, E-mail: jh@dfm.dtu.dk     

Oxygen on Ni(110): Surface phases and related absolute coverages

The adsorption of oxygen on Ni(110) was investigated by nuclear reaction analysis (NRA), XPS, Δφ, temperature programmed reaction spectroscopy (TPRS) and LEED. At 423 K, (3 × 1), (2 ×1) and (3 ×1) phases are formed in sequence with increasing O₂ exposure. The coverage in the (2 ×1) phase was determined by NRA, the coverages in the other phases being determined via this calibration by XPS, TPRS and Δφ. Contrary to previous reports, the maximum in the intensity of half-order beams from the (2 × 1) phase is associated with a coverage of (5.6 ± 0.5) × 10¹⁴ O atoms cm⁻² or 0.49 ± 0.05 monolayers, and not 0.25 monolayers. The two (3 ×1) phases are associated with θ = 0.33 ± 0.03 and 0.64 ± 0.06 monolayers respectively. Oxygen adsorbed at 295 K is not at thermodynamic equilibrium. Annealing to T > 400 K causes significant decreases in Δφ and the formation of the (2 ×1) phase for θ > 0.3.     

Metastable structures of Dy layers adsorbed on Mo(112) and their transformations

Ordering of dysprosium on Mo(112) up to 1.5 monolayers has been investigated by LEED and work function analysis after adsorption at 100 K and annealing between 200 and 1000 K. At low annealing temperatures (< 350-600 K) ordered structures are found, which are changed or even destroyed irreversibly by annealing steps to higher temperatures. At coverages, θ, up to 0.3 monolayer a (6×1) not strictly commensurate chain structure is seen, which coexists up to θ = 0.58 with a one-dimensionally incommensurate c( 1.56×2) structure. At higher coverages up to the physical monolayer at θ≈ 0.77, incommensurate ( n×2) followed by oblique ( n×1) structures are seen with n continuously variable with coverage. The second layer forms a p (1.33×1) structure. Annealing to higher temperatures causes irreversible structural transitions with strongly coverage dependent properties. Up to θ = 0.58, only a glass-like disordered phase is formed, which cannot be ordered again. In contrast, the rectangular incommensurate structures between 0.58 < θ < 0.68 remain unchanged upon annealing, whereas the structures at higher coverages and those of the second layer are transformed into commensurate (s×1) structures with integer s. Geometrical models are presented for the non-annealed structures and possible origins for the two-dimensional concentration dependent vitrification of the Dy layers are discussed. Received 15 June 2001     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001     

Coulomb excitation of 74Ge beam

⁷⁴Ge beam was Coulomb-excited on a ^natPb target. Ten E2 matrix elements including diagonal matrix elements for 5 low-lying states have been determined using the least-squares search code GOSIA. The expectation values of the rotational invariants 〈Q ²〉 and 〈cos3δ〉 show the small and triaxial deformation of the two lowest members of the ground-state band , while the 0₂ ⁺ and 2₂ ⁺ states are found to be almost spherical. Received: 31 August 2000 / Accepted: 4 December 2000     

A leed-aes study of the structure of sulfur monolayers on the Mo(100) crystal face

The formation of ordered phases of sulfur on the molybdenum (100) crystal face has been studied by Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Thermal Desorption Spectroscopies (TDS). Sulfur was deposited from a S₂ molecular flux streaming out of an Ag₂S containing electrochemical cell inside the UHV chamber. The use of a controlled flux of S₂ allowed the careful determination of saturation values for the monolayer, as well as the formation of multilayers of sulfur. This allowed the calibration of Auger intensities in terms of sulfur coverage. Various ordered structures, c(2 × 2), (1 × 2), $\text{2}\text{1}\text{?}\text{1}\text{1}$ and c(2 × 4), were observed by LEED for different values of the S coverage. Real space models for these structures are proposed that satisfy the coverage values observed and place sulfur atoms only on high symmetry four-fold sites on the (100) molybdenum surface.