C. Jäger

Spectral properties of carbon black

Abstract The internal structure of carbon black particles considerably influences the optical behavior of the material, apart from the shape and agglomeration state of the primary particles. In this paper the correlation between internal structure and spectral behavior of carbon black is investigated experimentally. The carbon blacks were produced by resistive heating of graphite electrodes and condensation in a cooling gas atmosphere. The internal structure of the primary carbon black particles was investigated by high-resolution transmission electron microscopy, electron energy loss spectroscopy, 13 C NMR spectroscopy, and Raman spectroscopy. The primary particles were found to consist of bent or plane structural subunits. The UV π−π* absorption feature of the produced carbon blacks varies in position between 196 and 265 nm depending on the state of bending of the graphene layers in the subunits of particles and/or the dimensions of the plane graphitic microcrystallites and the incorporation of hydrogen. The different curvature radii of the graphene layers or the sizes of microcrystallites can be summarized by an integral dimension like the ratio of sp2/sp3 hybridized carbon atoms. In the mid-infrared spectral region, the absolute value of the absorption coefficient κ is dominated by a continuous absorption due to free charge carriers which are also influenced by the ratio of sp2/sp3 hybridized carbon in the primary particles. The appearance of prominent bands is related to the existence of functional groups, like C–Hn, CO and/or C–O–C.

Steps toward interstellar silicate mineralogy - VII. Spectral properties and crystallization behaviour of magnesium silicates produced by the sol-gel method

Amorphous silicate particles are generally assumed to be the main dust component in the envelopes of oxygen-rich evolved stars and may be considered the precursors of the pure crystalline enstatite and forsterite particles detected by ISO. We present optical constants in the broad wavelength range 0.2-500 µm for a unique series of pure amorphous Mg-silicates (Mg/Si in the range 0.7-2.4). They have been prepared by the sol-gel process, a chemical technique based on the condensation of Mg- and Si-hydroxides in a liquid phase. The salient feature of these Mg-silicates is the very small content of Si-OH bonds in the silicate network, which considerably reduces the activation energy of crystallization and, thus, decreases the temperature threshold for crystallization as well as crystallization time. The astrophysical relevance of our sol-gel silicates is shown by a comparison of optically thin model spectra based on dust emissivities with ISO-SWS spectra of AGB stars and with 10 µm emission profiles of such stars obtained by ground-based spectroscopy. As paradigmatic cases of AGB spectra with respect to the appearance of the silicate bands, TY Dra (slender bands and deep trough between them) and R Cas (broad bands and widely filled-up trough) were used, for which ISO-SWS spectra are available. The dust emissivity derived from TY Dra can be excel- lently reproduced by the models, suggesting that the dust grains consist indeed of pure amorphous Mg-silicates. Satisfactory agreement was also found with the mean 10 µm profiles of some groups of AGB stars and supergiants. Spectra with strong dust emission in the silicate trough like R Cas require additional contributions by other dust components, probably oxides. A rough orientation on the spectral properties of such potential trough opacity contributors has been obtained by subtracting a pure silicate spectrum (TY Dra) from a spectrum with a nearly filled trough and a less pronounced 20 µm band (R Cas). In agreement with other amorphous silicates, the spectral index of the new silicate analogues amounts to −2.

TEMPERATURE DEPENDENCE OF THE SUBMILLIMETER ABSORPTION COEFFICIENT OF AMORPHOUS SILICATE GRAINS

We have measured mass absorption coefficients of amorphous silicate materials for wavelengths between 100 μm and 2 mm (5-100 cm-1) and at temperatures between 300 and 10 K. For both interstellar analog MgSiO3 and simple silica SiO2, we find evidence for a strong temperature and frequency dependence. We define two distinct wavelength regimes, 500 μm-1 mm and 100-250 μm, for which the absorption coefficient presents different trends with frequency. To evaluate this frequency dependence, we fit our absorption coefficient using two power laws with spectral index β that varies with temperature. We do not find a significant variation of β with temperature between 100 and 250 μm, whereas between 500 μm and 1 mm a pronounced anticorrelation between T and β exists. Globally, β-values decrease from 2.5 to 1.5 between 10 and 300 K. This anticorrelation for interstellar analog grains has the same trend as the one observed using the balloon-borne experiment PRONAOS. We show that physisorbed water is not responsible for the observed temperature and frequency dependence and that OH groups could be at the origin of the submillimeter properties of the materials. As discussed in the literature, OH groups are often related to tunneling processes in two-level systems (TLS). In the case of the more complex MgSiO3 silicates, TLS could also be produced by the Mg+2 ions, which act as network modifiers, similar to how they act with OH groups.

Connectivities of Coordination Polyhedra in Phosphate Glasses from 31P Double-Quantum NMR Spectroscopy

Abstract High-resolution double-quantum nuclear magnetic resonance (NMR) spectroscopy in solids at a magnetic field of 11.75 T is applied to determine the structure of phosphate glasses. The connectivities between Q(n) coordination polyhedra, identified by their 31 P chemical shifts are established. Moreover, more extended structural features, such as rings or linear chains composed of Q(2) groups are resolved, since the chemical shift contains information about the local geometry, i.e. bond lengths and angles, which are in turn influenced by the surrounding Q(n) groups. From the study of two sodium phosphate glasses, containing 35 and 58 mol% of Na2O a motif involving sequences of at least three Q(2) groups could be identified in both compositions. Extensions of this new approach are discussed.

31P NMR investigations of binary alkaline earth phosphate glasses of ultra phosphate composition

31P NMR CW and FT MAS measurements have been used to investigate the structural composition of x(MeO)·(1−x)(P2O5) glasses (Me = Ca, Sr, Ba, x = 0.25−0.50). The influence of the structural composition on components of the chemical shift tensors is determined and discussed by means of a correlation between isotropic chemical shifts in crystalline phosphates and molecular structure published recently by Sternberg et al. [Z. Phys. Chem. Neue Folge 168 (1990) 115].

Temperature effects on the mid-and far-infrared spectra of olivine particles

The absorption spectra of the olivine particles of different Mg/Fe content were measured in the infrared spectral region between 5 and 100 µm, while the particles were continuously cooled down to 10 K. Measurements independently carried out on different samples of synthetic forsterite, natural olivine, and synthetic fayalite at laboratories in Kyoto and Jena. The positions of the olivine infrared bands were measured for these samples in detail at up to seven individual temperatures in the interval between 300 K and 10 K. According to the different widths of the olivine bands in different wavelength regions, spectral resolutions of 2, 1, 0.5, 0.25, 0.2, and 0.125 cm −1 were used in order to measure the band positions with high accuracy. While in general the band positions and their temperature-dependent shift agree very well for the Kyoto and Jena samples, the positions of some very strong bands differ, which is probably a consequence of different particle shapes. For the two long-wavelength forsterite bands at 49 and 69 µm, the sharpening and strengthening of the bands were quantified. The widths of these bands differ for the Kyoto and Jena samples, which is discussed in terms of different crystal quality and particle coagulation of the samples. Our new data can be used to derive dust temperatures from the observed peak positions for crystalline silicate dust in circumstellar regions.

DIRECT-DETECTION OF CONNECTIVITIES IN GLASSES BY 2D NMR

Abstract 31 P two-dimensional (2D) nuclear magnetic resonance (NMR) is proposed a powerful tool for direct investigation of the connectivities between like or different Q n units in glasses or the medium-range order. The entire experiment was carried out under conditions of fast magic angle sample spinning (MAS). The magnetization transfer was obtained by recoupling of the dipole-dipole interaction despite the fast MAS. The mixing time dependence of the 2D spectra for a binary Na 2 OP 2 O 5 glass suggests an almost regular alternation of Q 2 and Q 3 units for that glass.

Spectral Properties of Gas-phase Condensed Fullerene-like Carbon Nanoparticles from Far-ultraviolet to Infrared Wavelengths

Carbon solids are ubiquitous material in interstellar space. However, the formation pathway of carbonaceous matter in astrophysical environments, as well as in terrestrial gas-phase condensation reactions, is not yet understood. Laser ablation of graphite in different quenching gas atmospheres, such as pure He, He/H2, and He/H2O at varying pressures, is used to synthesize very small, fullerene-like carbon nanoparticles. The particles are characterized by very small diameters between 1 and 4 nm and a disturbed onion-like structure. The soot particles extracted from the condensation zone obviously represent a very early stage of particle condensation. The spectral properties have been measured from the far-ultraviolet (FUV; λ = 120 nm) to the mid-infrared (MIR; -->λ = 15 μm). The seedlike soot particles show strong absorption bands in the 3.4 μm range. The profile and the intensity pattern of the 3.4 μm band of the diffuse interstellar medium can be well reproduced by the measured 3.4 μm profile of the condensed particles; however, all the carbon which is left to form solids is needed to fit the intensity of the interstellar bands. In contrast to the assumption that onion-like soot particles could be the carriers of the interstellar ultraviolet (UV) bump, our very small onion-like carbon nanoparticles do not show distinct UV bands due to (π-π*) transitions.

Identification and Spectral Properties of Polycyclic Aromatic Hydrocarbons in Carbonaceous Soot Produced by Laser Pyrolysis

Carbon soot has been prepared by laser-induced pyrolysis of a mixture of ethylene (C2H4) and benzene (C6H6) vapor. The soluble part of the carbonaceous powder has been separated from its insoluble counterpart by soxhlet extraction in toluene. Several techniques were applied to obtain information on the composition of the extract. These included UV/visible and IR spectroscopy in solid and liquid phase, gas chromatography combined with mass spectrometry, gas-phase laser spectroscopy in a supersonic jet, and matrix spectroscopy in helium droplets, the latter being also combined with mass spectrometry. The analysis revealed that the carbonaceous powder contained various polycyclic aromatic hydrocarbons (PAHs). The highest concentration was found for the three-membered catacondensed PAHs, phenanthrene and anthracene. The results are discussed in view of the possible role of these molecules as interstellar dust components.

Optical extinction by spherical carbonaceous particles

Abstract We set up a simple model for the optical properties of carbon particles in which the particles are considered to be a mixture of two materials-a graphitic and an amorphous material. Several models for the geometrical arrangement of these materials are discussed. Based on these models, the optical extinction cross-sections of small spherical carbon particles of different internal structures are calculated in the framework of classical electromagnetic theory. For ordered structures such as onion-shaped particles, analytical formulas are used, while for disordered structures effective-medium formalisms are applied which take into account the anisotropy of graphite and which have become available only recently. We find that the shape and position of the UV feature in the optical extinction at ca. 220 nm depends on the internal structure of the particle.