Sapphire (0 0 0 1) surface modifications induced by long-pulse 1054 nm laser irradiation

We have investigated modifications of sapphire (0 0 0 1) surface with and without coating, induced by a single laser pulse with a 1054 nm wavelength, 2.2 s duration, 7.75 mm spot and energy of 20-110 J. A holographic optical element was used for smoothing the drive beam spatially, but it induced small hotspots which initiated damage on the uncoated and coated surfaces. The individual damage effects of hotspots became less pronounced at high fluences. Due to high temperature and elevated non-hydrostatic stresses upon laser irradiation, damage occurred as fracture, spallation, basal and rhombohedral twinning, melting, vitrification, the formation of nanocrystalline phases, and solid-solid phase transition. The extent of damage increased with laser fluences. The formation of regular linear patterns with three-fold symmetry ( directions) upon fracture was due to rhombohedral twinning. Nanocrystalline -Al₂O₃ formed possibly from vapor deposition on the coated surface and manifested linear, triangular and spiral growth patterns. Glass and minor amounts of -Al₂O₃ also formed from rapid quenching of the melt on this side. The - to -Al₂O₃ transition was observed on the uncoated surface in some partially spalled alumina, presumably caused by shearing. The nominal threshold for laser-induced damage is about 47 J cm⁻² for these laser pulses, and it is about 94 J cm⁻² at the hotspots.     

Dynamics of NF3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF₃ on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF₃ adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF₃ is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F⁺ central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF₃ (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F⁺, N⁺, NF⁺, and ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the and ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF₃ molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF₃ crystal. In this process, some NF₃ molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.     

Aromatic interactions in the close packing of phenyl-imides at Cu(1 1 0) surfaces

The oxidation of aniline at Cu(1 1 0) surfaces at 290 K has been studied by XPS and STM. A single chemisorbed product, assigned to a phenyl imide (C₆H₅N(a)), is formed together with water which desorbs. Reaction with preadsorbed oxygen results in a maximum surface concentration of phenyl imide of 2.8 × 10¹⁴ mol cm⁻² and a surface dominated by domains of three structures described by , and unit meshes. However, concentrations of phenyl imide of up to 3.3 × 10¹⁴ mol cm⁻² were obtained from the coadsorption of aniline and dioxygen (300:1 mixture) resulting in a highly ordered biphasic structure with and domains. Comparison of the STM and XPS data shows that only half the phenyl imides at the surface are imaged. Pi-stacking of the phenyl rings is proposed to account for this observation.     

The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

Spectroscopic characterization of the potential energy functions of Ne2 Rydberg states in the vicinity of the Ne(S0) + Ne(4p′) dissociation limits

The gerade autoionizing Rydberg states of Ne₂ have been studied in the range 162 000-172 000 cm⁻¹ by 1 + 1′ resonant two-photon excitation from the Ne₂ X ground state via different vibrational levels of the Ne₂ C state. A rotationally resolved part of the spectrum allowed the determination of the potential energy functions of two states of 1_g and characters in the vicinity of the Ne(2p⁶¹S₀) + Ne (2p⁵4p′) dissociation limit. The presence of maxima in these potential energy functions is interpreted as originating from a repulsive interaction between the Rydberg electron and the neutral atom.     

Shell model calculations of the adsorption and diffusion of K, Cl, and KCl on KCl(0 0 1)

Equilibrium adsorption positions and diffusion pathways of the ions K⁺ and Cl⁻ as well as of the molecule KCl on the terrace of the (0 0 1) surface of KCl were determined by shell model calculations allowing relaxations of the crystal lattice in the vicinity of the adsorbed species. For the ions each one adsorption position was found, in which the ions are located above the hollow site at the center of a slightly distorted square formed by two cations and two anions of the uppermost surface layer of the KCl crystal. Adsorption energies of −1.52 eV for K⁺ and −1.61 eV for Cl⁻ were calculated. Jumps of the ions occur from these positions to adjacent hollow positions in the ±[1 0 0] and ±[0 1 0] directions with a jump distance of a₀/2. The activation energies for the jumps result as 0.142 for K⁺ and 0.152 eV for Cl⁻ and the mean diffusion lengths as and . For the KCl molecule four distinct adsorption minima with energies between −0.932 and −0.825 eV were found. Because of the smaller lattice relaxation caused by the molecule the adsorption energies are considerably lower than for the single ions. In the position with the largest adsorption energy the ions of the admolecule are again placed above adjacent hollow sites. In two more adsorption positions only one ion is at the hollow site and the other one in a top position above an oppositely charged ion of the surface. In the fourth position with the smallest adsorption energy both ions are in top positions. Jumps between the different adsorption positions proceed by rotations of the molecule, in which one of its ions remains essentially attached to a local minimum position. The diffusion and desorption of a KCl molecule was studied by a Monte Carlo method, resulting in a mean diffusion length x_s (nm) = 0.39 exp[0.84 (eV)/2kT], which agrees rather well with an experimental value of . Values for the mean stay time as well as for the surface diffusion coefficient are derived.     

Modelling of phase transitions and reaction at CO adsorption on oxygen precovered Pd(1 1 1)

Using the interaction parameters up to the third neighbors and activated form of O and CO diffusion and their reaction, the model has been proposed for Monte-Carlo simulations describing the catalytic O + CO → CO₂ reaction and occurring phase transitions on Pd(1 1 1) surface. Upon adsorption of CO the pre-adsorbed oxygen transforms from p(2 × 2)_O phase into and phases in the limit of room and moderate temperatures, respectively. We demonstrate that the kinetic effects determine both the occurrence of the p(2 × 1)_O and disappearance of the phases at moderate and low temperatures, respectively. Using reaction rate as a fit parameter, we show that at room temperature the start of the reaction can be synchronized with the occurrence of phase.     

Laser excitation spectrum of C3 in the region 26 000-30 700 cm

The vibrational structure of the electronic state of C₃ in the region 26 000-30 775 cm⁻¹ has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born-Oppenheimer components of the state, and by the effects of the double minimum potential in the Q₃ coordinate, recognized by Izuha and Yamanouchi [16]. The present work shows that there is also strong anharmonic resonance between the overtones of the ν₁ and ν₃ vibrations. For instance, the levels 2 1⁺ 1 and 0 1 ⁺ 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm⁻¹ apart, centered about the expected position of the 2 1⁺ 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm⁻¹ (a vibrational energy of over 5000 cm⁻¹) can now be assigned. A vibronic level at 30181.4 cm⁻¹, which has a much lower B′ rotational constant than nearby levels of the state, possibly represents the onset of vibronic perturbations by the electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36].     

Electron-phonon interaction in magnesium: From the monolayer to the Mg(0 0 0 1) surface

We report ab initio study of the electron-phonon coupling in a free standing magnesium monolayer and at the Mg(0 0 0 1) surface. The calculations were carried out using a linear-response approach in the plane-wave pseudopotential representation. Eliashberg spectral function α²F(ω) averaged over electron states at the Fermi surface is presented for the monolayer while for the Mg(0 0 0 1) surface, we compute the electron-phonon spectral function α²F_k,i(ω) for surface states at the and points.     

Rotational and vibrational structure in the 288 nm band system of the FeCl2 radical

The laser excitation spectrum of the 288 nm band system of FeCl₂, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. Vibronic transitions are observed from the ground state to two close-lying excited electronic states that differ in inversion (g, u) parity. Two extensive progressions in the symmetric stretching vibration have been identified, referred to as Progressions A and B. The main features of Progression A, which is based on the band, are allowed transitions to the excited electronic state of ungerade symmetry. Progression B is built on the band and consists of vibronically induced transitions to the gerade excited state. A substantial decrease in the symmetric stretching vibrational wavenumber is observed on excitation . Local perturbations are found to cause relative shifts between the different isotopomers. Several vibronic bands have been recorded and analysed at rotational resolution for the three isotopomers Fe³⁵Cl₂, Fe³⁵Cl³⁷Cl, and Fe³⁷Cl₂ in natural abundance. All bands show perpendicular rotational structure of a linear molecule, and have been unambiguously assigned to a Ω = 5-4 transition, consistent with the inverted ⁵Δ_g ground state predicted by ab initio and DFT calculations. The zero-point averaged FeCl bond length is determined to be in the upper and lower electronic states. The results show that the molecule is linear in both states.     

Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S₁ ← S₀ transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the band and 3047 rotational lines of the band have been assigned. The band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the ΔK_a = ± 1 transitions were observed. The excited state is identified to be the S₁¹A₁(ππ^∗) state. In contrast with this, the band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S₂¹B₂(ππ^∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.     

Coadsorption of CO and C6H6 on Co(0 0 0 1)

We have studied the influence of CO on the adsorption of benzene on the Co(0 0 0 1) surface using LEED, XPS, TDS and work function measurements. CO was found to reduce the benzene adsorption, but even at saturation CO exposure no complete blocking was observed. Thermal desorption of the coadsorbed layer featured CO and H₂ peaks indicating partial dehydrogenation of benzene and retaining of the CO bond. Ordered LEED structures were found with all coverages: Pre-adsorption of CO led to patterns already seen for pure carbon monoxide adsorption. Pre-adsorption of benzene showed the known structure of pure benzene also with small CO exposures, but higher CO exposures yielded a mixture of and patterns.     

Methane diode laser overtone spectroscopy at 840 nm

Overtone absorption lines of ¹²CH₄ have been examined by using a tunable diode laser (TDL) spectrometer in the region around (840 nm) where the combination overtone band ν₁+3ν₃ lies. The spectrometer sources are commercially available heterostructure GaAlAs TDLs operating in the “free-running” mode, which allowed the detection of the line positions within . The wavelength modulation spectroscopy (WMS) and the second harmonic detection technique permitted the measurements of minimum absorbances of the order of ?5×10^-6. This allowed to observe the weakest lines of the band with absorption cross-sections of the order of ?2×10^-25 cm²/molecule or /amagat. For some of them self-, air-, He- and H₂-broadening coefficients have been obtained at room temperature.     

A novel approach to measure the step line tension and the step dipole moment on vicinal Au(0 0 1) electrodes

In this paper we introduce a new experimental approach to determine the potential dependence of the step line tension on metal electrodes in contact with an electrolyte: (0 0 1) and (1 1 n) surfaces of single crystal gold electrodes were investigated by impedance spectroscopy in solutions containing weakly adsorbing anions, such as , F⁻ and . Within the limits of error the shift in the potential of zero charge is proportional to the step density of the vicinal surfaces indicative of a well-defined dipole moment per step length. The dipole moments per step atom are 6.8 ± 0.8, 5.2 ± 0.4, 5.8 ± 0.5 × 10⁻³ eÅ for , , and F⁻ containing electrolytes, respectively. Using the values for the pzc and the potential dependence of the capacitance curves, the potential dependence of the surface tension of the vicinal surfaces is determined. The line tension of the steps is then calculated from the difference between the surface tension of the stepped and the step free surface. Our results represent the first experimental confirmation of a recent theoretical model proposing that in absence of specifically adsorbed ions the step line tension should decrease (roughly linear) with potential.     

High-resolution infrared and subterahertz spectroscopy of the v2 = 1, v5 = 1, and v3 = 2 levels of CH3Cl

High-resolution Fourier-transform infrared spectra between 1235 and 1680 cm⁻¹ and subterahertz spectra between 250 and 630 GHz of monoisotopic ¹³CH₃³⁵Cl have been recorded and analyzed simultaneously, with all Coriolis, α-resonance, and l-type interactions in the polyad of the v₂ = 1, v₅ = 1, and v₃ = 2 levels taken into account. Several α-resonances (Δk = ±2, Δl = ?1) generating perturbation-allowed transitions have been assigned in the rovibrational spectra. These resonances enabled us to determine accurately and independently the ground state rotational and centrifugal distortion parameters A₀ = 5.205 746 9 (55) cm⁻¹ and . Even , which is, however, correlated to higher-order α-resonance terms, was determined. With 51 upper state parameters varied, about 5800 rovibrational wavenumbers and more than 550 rotational frequencies pertaining to the excited vibrational states were fitted within their experimental accuracy.     

Fourier transform emission spectroscopy of CoCl in the 500 nm region

Fourier transform emission spectra of CoCl were obtained with a tube furnace-DC discharge source in the 450-500 nm spectral region. In addition to observing two different band systems, which were assigned as [20.7] and [21.3] by Adam et al. [J. Mol. Spectrosc. 212 (2002) 111], we obtained data for additional sub-bands and vibrational levels. In contrast to the previous work, intense Q branches are seen in the [21.3]- bands which indicate that these bands are likely ΔΛ≠0 transitions. The equilibrium rotational constant B_e for the ground state is 0.1801104(28) cm⁻¹, the equilibrium bond length is 2.065122(16) Å and ΔG_1/2 is 430.418(5) cm⁻¹.     

Electronic structure of the Sb-induced Si(1 1 3)2 × 5 surface

The surface electronic structure of Sb/Si(1 1 3)2 × 5 was investigated by angle-resolved photoemission spectroscopy experiments. This reveals Sb/Si(1 1 3)2 × 5 to have three surface bands with anisotropic two-dimensional characteristics. The band widths of the surface bands along is larger than along . The number of surface bands of Sb/Si(1 1 3)2 × 5 and their band dispersions along and are quite analogous with those of Sb/Si(1 1 3)2 × 2 composed of Sb adatom and Si tetramer chains. The electronic structure analogy suggests that Sb/Si(1 1 3)2 × 5 and Sb/Si(1 1 3)2 × 2 have common building blocks such as Sb adatom and Si tetramer chains.