Th. Henning

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, Cue605O and/or C–O–C.

Matrix-isolated Nano-sized Carbon Grains as an Analog for the 217.5 Nanometer Feature Carrier

The effect of particle shape or particle clustering on the extinction behavior of nano-sized hydrogenated carbon grains is investigated experimentally. The particles were extracted by a molecular beam technique at different condensation and clustering states and isolated in an argon matrix for UV spectroscopy. The state of clustering in the samples was controlled by transmission electron microscopy analysis. The simple spherical morphology of the matrix-isolated nonagglomerated particles permitted the derivation of reliable optical constants. A clear correlation was found between the measured UV feature width and the degree of particle clustering, in agreement with theoretical investigations based on the derived optical data. Therefore, the results prove unambiguously the expectation that the optical properties of carbonaceous grain material are strongly influenced by the particle shape and the clustering degree. For particles produced in hydrogen-containing atmospheres, the UV extinction peak was shifted blueward to a position close to the 217.5 nm hump. This shift was found to be nearly independent of the amount of hydrogen in the condensation zone. We also discuss the infrared spectra of the hydrogenated carbon materials. The astrophysical implications of the results are discussed with regard to the observational as well as the elemental abundance constraints.

Structural processing of enstatite by ion bombardment

During their lifetime, cosmic dust silicates suffer from a continuous processing by annealing, cosmic ray and UV irradiation, destruction and possibly also interstellar recondensation. Since the discovery that a significant proportion of star- dust silicates leaves their star in crystalline form, the question arose as to why the interstellar silicate dust component does not show any indication of crystallinity. Amorphization due to ion irradiation is one possible explanation for the effect. In this paper, the results of irradiation experiments of submicrometre-sized clinoenstatite (MgSiO3) particles with 400 keV Ar + and 50 keV He + ions are presented. The irradiation doses have been varied between 1×10 16 and 1×10 18 ions/cm 2 for He + ions and 1×10 14 up to 5×10 14 ions/cm 2 for Ar + ions. These doses are comparable to those values that an interstellar silicate grain should be exposed to during its average life-time of 4×10 8 years. Threshold values for amorphization have been amounted to 1×10 17 and 3×10 14 ions/cm 2 for 50 keV He + and 400 keV Ar + ions. Besides the structural changes in the microcrystallites morphological modifications in the grains, but no change of the chemical composition are found. Conclusions of potential astrophysical relevance have been drawn.

On the Azimuthal Structure of Thermal Convection in Circumstellar Disks

We study the three-dimensional (3D) global structure of thermal convection in a protoplanetary accretion disk. For that purpose, we have developed a new, versatile 3D hydrocode that incorporates radiation transport and tensor viscosity in different coordinate systems. The code was extensively tested internally by recalculating the most recent models of circumstellar disk convection. With our new code we were able, for the first time, to simulate the large-scale (azimuthal) structure of circumstellar disk convection as it will develop under the assumption of an α viscosity. We find that axisymmetry is broken and convective motions with an azimuthal wavelength of about 4 pressure scale heights form.

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.

Facts and Artifacts in Interstellar Diamond Spectra

Absorption spectra of presolar diamonds extracted from the Murchison meteorite have been measured in the extended wavelength range 0.2-500 μm in order to make available optical properties of this supposed component of interstellar carbon dust. In contrast to terrestrial natural and synthetic diamonds, spectra of the meteoritic diamonds show prominent bands in the middle-IR. In this Letter, experimental evidence is presented that the OH band at 3200 cm-1 and the CH bands in the 2800-3000 cm-1 range are not intrinsic features of the diamonds and that the band at 1100 cm-1 contains an artificial component due to the extraction procedure. In addition, in our spectra a conspicuous band at 120 cm-1 was found. If the intrinsic character of this band, which, up to now, is unidentified, is confirmed, it would offer a chance to observe interstellar diamonds, e.g., by the ISO satellite. We encourage laboratory astrophysicists and observers to study this promising possibility.

Formation and spectroscopy of carbides

We review the evidence for carbides in space both from infrared spectroscopy and direct measurements on presolar grains extracted from primitive meteorites. The paper includes a discussion of the structural properties of silicon carbide and metal carbides and their formation routes from the gas phase. In addition, we present spectroscopic data in the infrared, which are required for a better understanding of astronomical spectra.

Carbon - From Space to Laboratory

In this review, the nature of carbon-containing molecules and carbonaceous solids present in meteorites, comets, and the interstellar medium is discussed. Carbon plays an active role in the lifecycle of stars and the interstellar medium. It is the basis of a rich interstellar chemistry and the main component of pre-biotic organic material in space. The aim of the review is to build a bridge between astronomical spectroscopy and laboratory studies relevant to the investigation of cosmic carbon. Special emphasis is given to the structural variety of carbon-containing species and their characterization by experimental techniques.

S1 ← S0 transition of 2,3-benzofluorene at low temperatures in the gas phase

The S(1)((1)A())<--S(0)((1)A()) absorption spectrum of jet-cooled 2,3-benzofluorene (Bzf) has been measured by cavity ring-down spectroscopy. The potential energy surfaces of the S(n=0,1,2) states of Bzf have been investigated with calculations based on the time-dependent density functional theory (TD-DFT). At the B3LYP/TZ level of theory, TD-DFT does not deliver a realistic difference between the excited S(1) and S(2) potential energy surfaces, a problem which can be avoided by introducing a reference geometry where this difference coincides with the observation. In this geometry, an expression for the Herzberg-Teller corrected intensities of the vibronic bands is proposed, allowing a straightforward assignment of the observed a() modes below 900 cm(-1), including realistic calculated intensities. For vibronic bands at higher energies, the agreement between calculated and observed modes is deteriorated by substantial Dushinsky rotations and nonparabolicities of the potential energy surface S(1).

The sticking efficiency of quartz crystals for cosmic sub-micron grain collection

Abstract Mass flux monitoring of cosmic dust grains is possible by means of microbalance sensors, such as those used in the GIADA-MBS system, onboard the Rosetta mission. An important parameter, which determines the efficient collection of grains with time, is the sticking efficiency of the electrode-coated quartz crystals forming the sensor of the microbalance. In principle, an improving of performances should be obtained by applying a thin coating of adhesive material onto the sensor surface. In order to evaluate the sticking properties for coated and uncoated microbalance crystals, we bombarded them with 1.2 μm silica spheres and 0.64 μm irregular SiC grains, having velocities of up to 55 m s −1 . We studied individual impacts obtaining the sticking efficiency as a function of the grain velocity. For silica sphere projectiles, the presence of the adhesive coating slightly increases the sticking, whereas for irregular SiC particles it is noticeably high even for the uncoated crystal (0.7–0.8). We conclude that, for micrometer irregular grains, in the studied velocity range, the microbalance sensor without any sticking coating guarantees rather high collection performances.