Showing papers by "Till Jahnke published in 2002"

Characterization of a high-pressure microdischarge using diode laser atomic absorption spectroscopy

Abstract: A high-pressure dc glow discharge based on micro-structured-electrode (MSE) arrays was investigated by diode laser atomic absorption spectroscopy. The microdischarge was studied at constant current in pure Ar for pressures ranging from 50 to 400 mbar. The measurements of the absolute population density of the excited 1s5 ,1 s 4 ,1 s 3, and 1s2 levels of Ar in the discharge revealed a population density of excited atoms in metastable and resonance states in the range 10 12 –10 15 cm −3 . The gas temperature and the electron number density were evaluated from the analysis of the absorption line profiles, taking into account significant broadening mechanisms. The gas temperature, derived from the Doppler broadening, was found to increase with pressure from 380 K at 50 mbar to 1100 K at 400 mbar. The electron number density was calculated from the Stark broadening and shift, and it ranges from 9 × 10 14 to 5 × 10 15 cm −3 . The MSE-sustained discharges are combining the non-equilibrium character with the advantage of high-pressure, which recommends them for non-thermal plasma processing e.g. surface treatment, plasma chemistry and generation of UV and VUV radiation.

Circular dichroism in K-shell ionization from fixed-in-space CO and N2 molecules.

Abstract: We have measured the angular distributions of 1s photoelectrons excited by circularly and linearly polarized light from fixed-in-space CO and N2 molecules, in the vicinity of their shape resonances. A strong circular dichroism, i.e., a strong dependence on the sense of rotation of the polarization vector of the photons, is found for both molecules. State-of-the-art one-electron multiple scattering and partially correlated random phase approximation calculations are in good agreement with many, but not all, aspects of the experimental data.

Mechanisms of photo double ionization of helium by 530 eV photons.

Abstract: We have measured fully differential cross sections for photo double ionization of helium 450 eV above the threshold using the COLTRIMS technique. We have found an extremely asymmetric energy sharing between the two electrons and an angular asymmetry parameter β ≃ 2 for the fast electrons and β ≃ 0 for the slow electrons. The asymmetric energy sharing together with the asymmetry parameter β ≃ 2 for the fast electron indicate that the fast electron absorbs not only most of the photon energy but also its angular momentum. The electron angular distributions show that the very slow electrons (about 2 eV) are emitted isotropically with a slight backward emission to the momentum of the fast electron which is expected from the shake-off mechanism. At the same time, the slow electrons at higher energies are mostly emitted at 90 degree to the fast electron indicating an (e,2e) like collision between the two electrons. All our data are in good agreement to CCC calculations.

Energy sharing and asymmetry parameters for photo double ionization of helium 100 eV above threshold in single-particle and Jacobi coordinates

Abstract: We present a joint experimental and theoretical study of the double differential cross section for the double ionization of helium by absorption of one 179 eV linear polarized photon (100 eV above threshold) We show the energy sharing and the asymmetry parameters in both single-particle coordinates and in Jacobi coordinates An asymmetric sharing of the electron excess energy and a breakup of the electron pair preferentially parallel to the polarization axis is found This marks a deviation from the propensity rule favouring perpendicular relative electron motion which has been observed to hold at energies closer to the double-ionization threshold

Microstructured electrode arrays: a source of high-pressure nonthermal plasma

Abstract: High-pressure discharges are generated using Micro-Structured Electrode (MSE) arrays with electrode distances between 50 and 250 µm. These arrays consist either of a planar electrode system bonded to a dielectric substrate or of a matrix of holes perforated in a composite sheet made out of two metallic foils separated by an insulator. Stable discharges were produced for pressures ranging from 10 to 1000 mbar using DC and radio frequency excitation. The optical appearance is that of a homogeneous glow discharge. Spectral investigations have revealed the presence of a large number density of metastable atoms and highly energetic electrons, while the discharge has a non-thermal character. The non-equilibrium high -pressure plasma provided by the MSE arrays is suitable for surface activation, thin film deposition and reduction of pollutants.