Fundamentals and advantages of ultrafast micro-structuring of transparent materials

Experimental investigations using femtosecond and picosecond laser pulses at 800 nm illuminate the distinctions between the dynamics and nature of ultrafast processing of dielectrics compared with semiconductors and metals. Dielectric materials are strongly charged at the surface on the sub-ps time scale and undergo an impulsive Coulomb explosion prior to thermal ablation. Provided the laser pulse width remains in the ps or sub-ps time domain, this effect can be exploited for processing. In the case of thermal ablation alone, the high localization of energy accompanied by ultrafast laser micro-structuring is of great advantage also for high quality processing of thin metallic or semiconducting layers, in which the surface charge is effectively quenched. Received: 17 January 2003 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/670-53-500, E-mail: d.ashkenasi@lmtb.dt RID="**" ID="**"Present address: LMTB GmbH, Berlin, Fabeckstr. 60–62, D-14195 Berlin, Germany     

Dc plasma-enhanced chemical vapour deposition growth of carbon nanotubes and nanofibres: <Emphasis Type="Italic">in situ</Emphasis> spectroscopy and plasma current dependence

Dc plasma-enhanced CVD growth of nanotubes and nanofibres is studied as a function of plasma power (3–40 W). The dependence of the nanotube/nanofibre morphology for growth on thin iron films and lithographically prepared individual nickel dots is investigated. In both cases, large differences in the morphology of the carbon nanostructures are observed as the plasma power is changed. In situ optical emission spectroscopy is used to obtain insight into the important parameters affecting the growth. The best growth results are found for intermediate plasma powers (15 W). PACS 81.07.-b; 52.70.Kz     

Molecular fusion within fullerene clusters induced by femtosecond laser excitation

Molecular fusion is induced in clusters of fullerene molecules on excitation with fs laser pulses. The dependence of the mass distributions of the fused products on the initial cluster distribution are studied and results for (C₆₀)_N and (C₇₀)_N clusters are compared. The fused products decay by emitting C₂ molecules and the fragmentation spectrum is used to determine the initial excitation energy of the fused species. The threshold excitation energy needed to induce fusion is consistent with the energetic thresholds for molecular fusion of fullerenes determined previously in single collision experiments.     

The intramolecular Diels-Alder reaction: Stereocontrol through nonsynchronous bond formation?

Nonsynchronous bond formation in the transition state of the intramolecular Diels-Alder reaction is suggested to be an important determinant of the stereochemical outcome of the cyclization.     

Dissociation of NH3 to electronic ground-state fragments by high-intensity CO2 laser radiation

The dissociation of NH₃ into electronic ground-state fragments NH₂(²B₁) + H by high-intensity CO₂ laser radiation has been observed under essentially collision free conditions and as a function of NH₃ pressure, using laser fluorescence excitation for the NH₂ detection. Luminescence from excited NH₂ (²A₁) produced directly by the CO₂ laser has also been observed. The relative yield of NH₂ (²A₁) is estimated to be smaller than that of NH₂ (²B₁) by several orders of magnitude.     

Temperaturabh?ngigkeit des Null-Ladungspunktes von Kobalt- und Nickeloxiden und Hydroxyden

Ohne Zusammenfassung     

Internal conversion of the 6.2 keV transition in181Ta

The total conversion coefficient of the 6.2 keV Mössbauer transition in¹⁸¹Ta was measured by X- and gamma-ray spectroscopy. The result of 86 ± 20 disagrees with the literature value of 46, but agrees with theory if a nuclear penetration parameter deduced from relative conversion electron intensities is taken into account. The weak 6.2 keV gamma ray has intensity per decay that is less than the previous value by a factor of 8.5.