Design and application of a femtosecond optical parametric oscillator for time-resolved spectroscopy of semiconductor heterostructures

Femtosecond pulses of a collinearly pumped Optical Parametric Oscillator (OPO) are applied for investigations of the carrier dynamics in ternary and quaternary semiconductor quantum wells. The design and the specifications of the OPO are given in detail. We show that no measurable jitter exists between the pump pulses and the output pulses of the OPO. Therefore, it is possible to use the OPO and its pump laser for two-color experiments with a time resolution limited by the pulse lengths. We present and discuss results of transient four-wave mixing experiments on (InGa) As/InP quantum wells, and find a new kind of polarization-dependent quantum beat phenomenon. In addition, non-degenerate experiments on quantum wells from the quaternary (InGaAl) As material system, using two pulses at different wavelengths (one from the OPO and one from the pump laser), are discussed as a novel experimental technique to study carrier trapping into quantum wells.     

3,3-Dichlorotetrahydrofurans by reductive addition of 3-bromo 1,1,1-trichloroalkanes to carbonyl compounds

3,3-Dichlorotetrahydrofurans 5 are prepared by cathodic reduction of 3-bromo-1,1,1-trichloroalkanes 1 in the presence of aldehydes aund ketones; 5 can be partially or fully dechlorinated.     

Additives atomares punktdipol-(APUDI)-modell zur berechnung der 1h-NMR-werte von benzoiden kohlenwasserstoffen

Our empirical atomic point dipole (APUDI) model simply allows the calculation of ring-current produced chemical shifts of the ring protons of 17 planar benzenoid hydrocarbons with high accuracy. It simulates the effect of the anisotropy of diamagnetic susceptibility of the π-systems by assumption of two effective atomic magnetic point dipoles, located perpendicular above and below each carbon atom of the π-system at 70 pm, the distance of the maximum of π-electron density. Application of the McConnell equation on each of these point dipoles leads after summation for each ring proton j to a corresponding geometry factor GF_j which was correlated by linear least squares method with 160 literature values of experimental chemical shifts leading to a value of (- 12.5± 0.7) × 10^-36m³, the effective anisotropy of diamagnetic susceptibility per atomic point dipole with a correlation coefficient of 0.943. The chemical shift values may be obtained in the APUDI-model by the least squares equation δ^calc_j p me>= - 12.5 × 10^-36 GF_j - 5.16. This empirically determined slope may be interpreted as the average value of the z-component of the experimental diamagnetic susceptibility per atomic point dipole of these benzenoid aromatic compounds. Besides the simple applicability the APUDI-model yields superior results for the chemical shifts of sterically crowded protons in comparison to many known ring current models.     

Messung der lichtelektrischen Leitung in Anthrazenkristallen

The drift mobility of holes in pure Anthracene has been measured by the transient photoconductivity technique used byKepler andLeblanc. The mobility is 0.8 cm²/Vsec and varies asT ^?2. In Tetracene-doped crystals the drift-mobility of holes decreases with growing Tetracene concentration and it varies exponentially as-T ^?1, which suggests that Tetracene acts as a trap. The trap depth is 0.45 eV. By pre-irradiation with α-particles or X-rays the photocurrent is reduced and it is supposed that α-irradiation produces deep traps, whereas X-rays produce as well shallow as deep traps.     

A ligand field approach to orthoaxial complexes

First order perturbation considerations are applied to general orthoaxial chromophores. These contain monatomic or linear ligands which are so positioned around the central ion that Cartesian coordinate axes can be placed through the ligands. The zero order functions used are the orbitals of the partially filled shell which are proper basis functions of the regular octahedron and can be thought of as general molecular orbitals of a particular regularly octahedral chromophore. Certain degeneracies not demanded by the proper chromophoric symmetry are rationalized in terms of the holohedrized symmetry, which also determines the number of parameters required for the full parametrization. All the results obtained are, apart from a minor correction given explicitly, formally equivalent to those obtained earlier using the angular overlap model and the electrostatic model. The extra correction required in the present more general treatment is probably unimportant in view of the approximations involved, in particular that of using a basis restricted to five orbitals.