Selected fluorescence and dissociation lifetimes on N2O+ in its Ã(2Σ+),000 and 100 states

Radiative and radiationless lifetimes together with the quantum yield towards fluorescence of individual vibronic states of molecular ions can be determined by threshold electron—fluorescence photon coincidence measurements. This method is here applied to the X?²Π_{$\text{3}\text{2,1,2}$} and òΣ⁺ states of N₂O⁺ which were also studied by high resolution photoionization resonance spectroscopy.     

Infrared ellipsometric view on monolayers: towards resolving structural details

The optical constants in the infrared spectral range and the thickness of a surface layer are simultaneously determined by reflection based spectroscopic infrared ellipsometry. In the past experimental progress has been used to increase sensitivity with the aim to detect ever thinner layers. Reaching the monolayer limit by now, methodic efforts focus on revealing structural details such as anisotropy and lateral heterogeneity caused primarily by molecular orientational order. The basis of the method and present methodical approaches are outlined. Aspects of using synchrotron radiation for infrared ellipsometry and of setting up an infrared beamline are discussed.     

Quasi-Norm interpolation error estimates for the piecewise linear finite element approximation of p-Laplacian problems

Summary. In this work, new interpolation error estimates have been derived for some well-known interpolators in the quasi-norms. The estimates are found to be essential to obtain the optimal a priori error bounds under the weakened regularity conditions for the piecewise linear finite element approximation of a class of degenerate equations. In particular, by using these estimates, we can close the existing gap between the regularity required for deriving the optimal error bounds and the regularity achievable for the smooth data for the 2-d and 3-d p-Laplacian.Mathematics Subject Classification (1991): 65N30     

Design and fabrication of 0.5 micron GaAs Schottky barrier diodes for low-noise terahertz receiver applications

Recent technological advances have made possible the development of heterodyne receivers with high sensitivity and high spectral resolution for frequencies in the range 1,000–3,000 GHz (1–3 THz). These receivers rely on GaAs Schottky barrier mixer diodes to translate the high-frequency signal to a lower frequency where amplification and signal processing are possible. At these frequencies, the diode quality is a major limitation to the performance of the receiver. The design, fabrication and DC evaluation of a diode for this frequency range is presented. A figure-of-merit cut-off frequency of over 10 THz is achieved with a record low zero biased capacitance of 0.5 fF. Results from RF tests are also given.This work has been supported in part by the National Science Foundation under contract ECS-8720850 and the US Army.     

The origin and elimination of spurious peaks in threshold electron photoionization spectra

Autoionization electrons in threshold photoionization spectra are known to occur. The effect of such electrons on the interpretation of threshold electron spectra is discussed and a new method is described which enables one to discriminate between zero kinetic energy electrons and energetic electrons in photoionization experiments with little ambiguity. The method largely overcomes the main problem present in some previously described threshold electron studies; namely, the line-of-sight transmission of energetic electrons which gives rise to non-threshold electron peaks. A rejection ratio of 25 is readily obtained for 0.038 eV electrons compared with threshold (zero kinetic energy) electrons and the (small) transition probability for energetic electrons falls off more rapidly than for analysers previously described. Design parameters based on electron trajectory calculations are given for the analyser used, and photoionization results on argon are presented.     

Direct observation of rotational states of molecular ions in photoionization processes

Optical transitions from H₂ (X ¹Σ_g⁺) to specific vibration-rotation states of the hydrogen molecular ion (X ²Σ_g⁺) have been directly observed and the spectrum is herein reported. A new method of zero kinetic energy electron analysis has been employed for the detection of the specific states and non-threshold electron peaks are not observed. It is shown that auto-ionization processes that produce threshold electrons will reveal ionic states that otherwise are not detectable.     

Investigation of GaAs Schottky barrier diodes in the THz range

The properties of GaAs Schottky barrier diodes as video detectors and mixing elements were investigated in the frequency range from 0.8–2.5 THz. For the most sensitive diode, the video responsivity and system noise temperature were measured as a function of incident laser power. The highest video responsivity was 2,000 V/W at 214m and 60 V/W at 118m. For five diodes differing in doping, capacitance, series resistance and anode diameter, the system noise temperature was measured at 214m and 118m. The best single sideband (SSB) values are 12,300 K and 24,200 K at 214m and 118m, respectively. The system noise temperature versus frequency is given over the range from 0.5–3 THz for two specific diodes demonstrating that the sharpness of the I–V characteristics is only of secondary importance for mixer perfomance at such high frequencies.     

Brilliant,coherent far-infrared (THz) synchrotron radiation

A new technology for generating steady state, brilliant, broadband, coherent, far-infrared (FIR) radiation in electron storage rings is presented, suitable for FIR spectroscopy. An FIR power increase of up to 100 000 compared to the normal, incoherent synchrotron radiation in the range of approximately 5 to approximately 40 cm(-1) could be achieved. The source is up to 1000 times more brillant compared to a standard Hg arc lamp. The coherent synchrotron radiation is produced in a "low alpha" optics mode of the synchrotron light source BESSY, by bunch shortening and non-Gaussian bunch deformation.