Ein neuer Photoionisationsdetektor für die Gas-Chromatographie

Zusammenfassung In einem Gas-Chromatographen wird der FID durch einen neuen Photoionisationsdetektor (PID) ersetzt. Der Photoionisationsdetektor wird kurz beschrieben und auf seine operativen Vorteile (einfache Anwendung, Fehlen von Betriebsmitteln wie Brenngase usw.) hingewiesen. Es werden einige Messungen mit dem modifizierten Gerät vorgenommen und die Ergebnisse mit denen des FID verglichen. Bedingt durch systemspezifische Vorteile des Photoionisationsdetektors—verglichen mit dem FID ist der Rauschpegel stark vermindert und wegen des größeren Wirkungsgrades der Photoionisation steht ein höheres Signal zur Verfügung—sowie durch das Fehlen von O₂ und seinen Problemen ist eine erhebliche Empfindlichkeitssteigerung des Gesamtsystems festzustellen. Außerdem ergibt sich, wiederum verglichen mit dem FID, ein weiterer Dynamikbereich. Weiter werden mit Hilfe des Photoionisationsdetektors gewonnene Meßergebnisse mit publizierten Ergebnissen anderer Detektoren verglichen und die Vorteile des PID aufgezeigt.

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A new photoionization detector for gas chromatography

Summary In a commercially available GC the FID was replaced by a newly designed photoionization detector (PID). The PID and its operational advantages (ease of use, no neccessity for combustion gas and the related facilities) are described briefly. Some measurements are made with the such modified GC and the results are compared with those obtained with the FID. Specific advantages of the PID — compared to a FID — the noise level caused by the system is lower and (due to the higher ionization efficiency of the PID) the signal is larger — and due to the lack of oxygen and its altendant problems the overall system sensitivity is strikingly better. In addition, the dynamic range — again compared with the FID — is wider. Further, performance data obtained with the PID are compared with published data of other detectors and the advantages of the PID are shown clearly.

     

Coherent nonlinear transport in quantum rings

While equilibrium properties of mesoscopic systems are well understood, many questions are still debated concerning the non-equilibrium properties, which govern nonlinear transport. Nonlinear transport measurements have been performed on two-terminal semiconductor quantum rings in the open regime, where the rings are used as electron interferometers and show the Aharonov–Bohm effect. We observe a magnetic field asymmetry of the nonlinear conductance, compatible with the non-validity of the Onsager–Casimir relations out-of-equilibrium. In particular, the voltage-antisymmetric part of the nonlinear conductance of these two-terminal devices is not phase rigid, as it is the case for the linear conductance. We show that this asymmetry is directly related to the electronic phase accumulated by the electrons along the arms of the ring and can be tuned using an electrostatic gate.     

Ti:sapphire second-harmonic-pumped visible range femtosecond optical parametric oscillator

A β-barium borate (BBO) femtosecond optical parametric oscillator (OPO) pumped by the second-harmonic of a modelocked 82 MHz Ti:sapphire laser is described. With intracavity dispersion compensation, pulse durations down to 30 fs are obtained with a total average power up to 100 mW. The current wavelength range, with a 400 nm pump, is from 566 nm to 676 nm. Unusual tuning characteristics, including bichromatic emission, are presented in detail and explained theoretically.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.     

TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p_x and p_y) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p_z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA=0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p_z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p_z components on and off easily.Experimental evidence of these features has been achieved by exploiting the selection rules for e–hh and e–lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.