Klaus-Werner Brzezinka

Femtosecond laser irradiation of indium phosphide in air: Raman spectroscopic and atomic force microscopic investigations

Surface modification and ablation of crystalline indium phosphide was performed with single and double 130 fs pulses from a Ti:sapphire laser. The morphological features resulting from laser processing, have been investigated by means of micro Raman spectroscopy as well as by optical, atomic force and scanning electron microscopy. The studies indicate amorphous, ablated and recrystallized zones on the processed surface. In the single-pulse irradiation experiments, several different threshold fluences could be assigned to the processes of melting, ablation and polycrystalline resolidification. Residual stress has been detected within the irradiated areas. Double-pulse exposure experiments have been analyzed in order to clarify the effect of cumulative damage in the ablation process of indium phosphide.

Vibrational excitation and energy redistribution after ultrafast internal conversion in 4-nitroaniline

Nonequilibrium vibrational excitations of para-nitroaniline (PNA, 4-nitroaniline) occurring after internal conversion from the photoexcited charge transfer state are studied by picosecond anti-Stokes Raman scattering. Vibrational excess populations with distinctly different picosecond rise and decay times are found for a number of modes with frequencies between 860 and 1510 cm−1, including the overtone of a non-Raman active mode. A nonthermal distribution of vibrational populations exists up to about 6 ps after photoexcitation. The time-resolved experiments are complemented by steady-state infrared and Raman measurements as well as calculations based on density functional theory, providing a detailed analysis of the steady-state vibrational spectra of PNA and two of its isotopomers. A weakly Raman active vibration at about 1510 cm−1 displays the fastest rise time and a pronounced excess population and—thus—represents the main accepting mode. We suggest that an out-of-plane mode giving rise to the overtone...

Studies of amorphous carbon using X-ray photoelectron spectroscopy, near-edge X-ray-absorption fine structure and Raman spectroscopy

Abstract We report core level and valence band X-ray photoelectron spectroscopy (XPS), carbon K and oxygen K near-edge X-ray-absorption fine structure spectroscopy (NEXAFS), and Raman spectroscopy results of plasma-deposited amorphous carbon generated from fullerene C 60 . In comparison with evaporated C 60 , the C 1 s peak is broader and asymmetric for the amorphous carbon film and its shake-up satellites disappear. The valence band shows three fairly broad peaks. Only one prominent π * resonance occurs in the NEXAFS spectrum. Recognizable structures appear in the σ * region indicating the formation of new bonds. In the Raman spectrum the typical D and G bands were observed. The amorphization of C 60 and post-plasma functionalization of the surface after exposure to atmosphere cause changes in the electronic structure.

Pulsed-laser deposition and boron-blending of diamond-like carbon (DLC) thin films

Abstract Diamond-like carbon (DLC) films, both pure and doped with boron, were prepared by pulsed laser deposition (PLD) with a XeCl excimer laser employing polycrystalline graphite and boron carbide targets. As substrates served silicon(111) wafers. The deposition parameters such as the laser intensity, vacuum, supporting gas conditions, substrate temperature, target-substrate distance, substrate combination and composition could be controlled independently, and thus, were used to modify the film properties and composition. Optical emission diagnostics of the laser plasma was performed at various locations between the target and the substrate. In the high power regime (> 108 W cm−2), pulsed laser evaporation resulted in the emission of excited C2 molecule radicals. High incident energies were necessary for surmounting potential barriers to the formation of sp3 bonds. Films with the highest sp3 content were formed with small distance between target and substrate, high laser intensities (I ≈ 109 W cm−2), and low base pressures (

Is the indigo molecule perturbed in planarity by matrices

Abstract Raman spectra of pure synthetic indigo and of Maya blue of a Mexican clay sculpture are compared. The Raman spectrum of Maya blue shows extra bands assigned to vibrational modes of Bu symmetry as well as an increasing intensity of some other bands. The partial removal of the mutual exclusion rule for the centrosymmetric indigo molecule is supposed to indicate a loss of its planarity due to strong adsorption at the palygorscite matrix.

Bulk and surface properties of highly dispersed VOx/ZrO2, VOx/SiO2 and VOx/TiO2/SiO2 systems and their relevance for propane oxidation

Highly dispersed vanadium-doped metal oxides such as VOx/ZrO2, VOx/SiO2 and VOx/TiO2/SiO2 with vanadium contents between 0 and 25 mole% were prepared by special bulk preparation methods (coprecipitation and sol–gel, followed by freeze-drying). Bulk and surface properties of the obtained mixed oxide solid solutions were thoroughly investigated by different analytical methods (Raman and FTIR spectroscopy, TPD, H2-TPR, oxygen isotope measurements etc.). Moreover, the catalytic behaviour of the oxides was studied for the example of the oxidative dehydrogenation (ODH) of propane to propylene. Independent of the preparation method, the catalytic behaviour of vanadium-doped ZrO2 and TiO2 phases is very similar. Both metal oxide solid solutions are very active in propane ODH whereas the catalytic activity of VOx/SiO2 is relatively low. On the other hand, the reduction of the catalytic activity is accompanied by an improved selectivity for the formation of propylene. The correlation between the catalytic activity and the acidity of the oxide systems is discussed. Oxidation experiments with 18O2 clearly show that the ODH reaction occurs according to the Mars–van Krevelen mechanism.

Vibrational analysis and excited-state geometric changes of betaine-30 derived from Raman and infrared spectra combined with ab initio calculations

We present a combined experimental and theoretical study of vibrational modes and excited-state geometry changes of betaine-30 (B-30). Infrared and Raman spectra were recorded under electronic off-resonant excitation conditions and also in resonance with the charge transfer transition of B-30. Comparing these spectra with the corresponding vibrational patterns calculated by Hartree–Fock methods we obtained the assignments of the vibrational modes. Excited-state geometry changes were calculated for a smaller model system of B-30 using configuration interaction with single excitations. The calculations predict that the central phenoxide and pyridinium rings move from a twisted conformation in the electronic ground state into a perpendicular position in the first excited state. In addition, the pyridinium ring tilts and the nitrogen atom becomes pyramidalized. In correspondence, we measured two strong low-wavenumber Raman bands with large origin shifts at 291 and 133 cm−1. They were assigned to N-inversion and torsional vibrational modes and are expected to mediate the excited state and back-electron transfer reaction of B-30. Copyright

Transition metal oxide/carbon composite catalysts for n-alkane aromatization: structure and catalytic properties

Nanocrystalline particles of high temperature pretreated titania, zirconia or hafnium oxide, embedded in a carbon matrix, have been found to catalyze the aromatization of n-octane into ethylbenzene (EB) and o-xylene (OX) with high selectivity. The carbon matrix itself is catalytically not active, but seems to co-operate with the transition metal oxides in such a way that the various metal oxide/carbon composite materials exhibit equal selectivity patterns. In detail, the carbon component stabilizes a high dispersion of the oxides during the high temperature pretreatment procedure. This thermal treatment results in a destruction of surface acidity of the oxides, which would otherwise be responsible for undesirable consecutive and parallel reactions. Moreover, the carbon component is involved in the deep dehydrogenation of alkanes to multiple unsaturated alkenes. This is explained by the ability of surface carbon atoms to interact with hydrogen. The bulk and surface structure of the catalysts have been characterized by XRD, specific surface area measurements, XPS, UPS, Raman spectroscopy, in situ ESR and DRIFT spectroscopy.

Physical chemistry of the femtosecond and nanosecond laser–material interaction with SiC and a SiC–TiC–TiB2 composite ceramic compound

The interaction of nanosecond laser pulses in the ultraviolet wavelength range and femtosecond laser pulses in the near-infrared region with the semiconductor SiC and the composite compound SiC–TiC–TiB2 was investigated. Surface analytical techniques, such as XPS, depth profile (DP), and micro-Raman spectroscopy (μ-RS) were used to identify the chemical changes between untreated and laser-treated areas. Single-pulse irradiation led to material modifications in the condensed state in most instances. Multi-pulse results differed depending on the pulse duration. Crystal structure changes were observed as a consequence of laser-induced melting and resolidification. In air contact all components underwent oxidation reactions according to thermodynamic expectations. Exceptions were observed under exclusion of oxygen, SiC was reduced to elemental Si.

Synthesis and characterization of nanoscaled and nanostructured carbon containing materials produced by thermal plasma technology

Single-walled carbon nanotubes (SWNT) and carbon nitride films are synthesized by a direct current (dc) arc discharge at slightly reduced pressure and by an inductively coupled r.f. plasma (ICP) at atmospheric pressure, respectively. By treatment with nitric acid SWNT are purified from by-products and characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. ICP allows the deposition of carbon nitride films on steel substrate. The deposition of the films creates a relatively strong radial symmetric profile. The structure of the films are rather independent of the distance between substrate and plasma. Raman, Fourier transform infrared (FTIR), and energy dispersive X-ray (EDX) spectroscopy are used for analyzing the carbon nitride materials.