University of Bremen

About: University of Bremen is a education organization based out in Bremen, Germany. It is known for research contribution in the topics: Population & Context (language use). The organization has 14563 authors who have published 37279 publications receiving 970381 citations. The organization is also known as: Universität Bremen.

The temperature dependence (203-293 K) of the absorption cross sections of O3 in the 230-850 nm region measured by Fourier-transform spectroscopy

Abstract: Absolute absorption cross sections of O3 were measured in the 230–850 nm (11765–43478 cm−1) region at five different temperatures (203–293 K) using a Fourier-transform spectrometer, at a spectral resolution of 5.0 cm−1 (corresponding to about 0.027 nm at 230 nm and to about 0.36 nm at 850 nm). The spectral accuracy of the data is better than 0.1 cm−1 — about 0.5 pm at 230 nm and about 7.2 pm at 850 nm — validated by recording of I2 absorption spectra in the visible using the same experimental set-up. O3 absorption spectra at different concentrations were recorded at five different sample temperatures in the range 203–293 K, and at each temperature at two total pressures (100 and 1000 mbar) using O2/N2 mixtures as buffer gas. Within the limits of experimental uncertainties, no influence of total pressure on the O3 spectrum was observed in the entire spectral region, as expected from the short lifetimes of the upper electronic states of O3. The temperature dependence of the O3 absorption cross sections is particularly strong in the Huggins bands between 310 and 380 nm, as observed in previous studies. An empirical formula is used to model the temperature dependence of the O3 absorption cross sections between 236 and 362 nm, a spectral region that is particularly important for atmospheric remote-sensing and for photochemical modelling.

Numerical renormalization group approach to Green’s functions for quantum impurity models

Abstract: We present a technique for the calculation of dynamical correlation functions of quantum impurity systems in equilibrium with Wilson's numerical renormalization group Our formulation is based on a complete basis set of the Wilson chain In contrast to all previous methods, it does not suffer from overcounting of excitations By construction, it always fulfills sum rules for spectral functions Furthermore, it accurately reproduces local thermodynamic expectation values, such as occupancy and magnetization, obtained directly from the numerical renormalization group calculations

Ring effect: impact of rotational raman scattering on radiative transfer in earth’s atmosphere

Abstract: One significant limitation to the accuracy of the remote sensing of trace gas constituents in the atmosphere, using UV-visible spectroscopy and scattered sunlight, has often been a reliable knowledge of the so-called Ring effect. In this study it is demonstrated that the filling-in of Fraunhofer and gas absorption features, resulting from Rotational Raman scattering (RRS), explains to high accuracy the Ring effect. A radiative transfer model has been adapted to include RRS and carefully validated by comparison with Ring effect data by other models and from ground-based and satellite data. The analysis of the principle components of the simulated Ring spectra enabled the Fraunhofer and gas absorption filling-in to be separated. This yields a simple, and therefore computational fast, parameterization of the Ring effect suitable for trace gas retrievals. This approach was tested for the retrieval of NO 2 which is considered to be a worst case with respect to absorption feature filling-in for a trace gas retrieved from scattered light. Analysis of the errors in the vertical column of NO 2 derived using differential optical absorption spectroscopy (DOAS) technique indicate that they are dependent on the amount of NO 2 present in the atmosphere when regarding the experimental Ring spectra. This implies that calculated Ring spectra may be superior for DOAS retrievals, compared to the experimentally determined Ring spectra.

Many-body theory of carrier capture and relaxation in semiconductor quantum-dot lasers

Abstract: In quantum-dot laser devices containing a quasi-two-dimensional wetting layer, a pump process initially populates the wetting-layer states. The scattering of carriers from these spatially-extended quasi-two-dimensional states into the quantum-dot states as well as the relaxation of carriers between the quantum-dot levels are studied theoretically. Based on the wave functions for the coupled quantum-dot/wetting-layer system interaction matrix elements are calculated for carrier-carrier Coulomb interaction and carrier-phonon interaction. Scattering rates for various capture and relaxation processes are evaluated under quasiequilibrium conditions. For elevated carrier densities in the wetting layer, Coulomb scattering provides processes with capture (relaxation) times typically faster than $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}\phantom{\rule{0.3em}{0ex}}(1\phantom{\rule{0.3em}{0ex}}\mathrm{ps})$. When energy conservation allows for interaction with LO phonons, comparable rates are obtained.