Pion-nucleon scattering lengths are directly related to the ground-state level shift and broadening in pionic hydrogen as well as to the pionic deuterium level shift. The level broadening in deuterium measures the strength of pion threshold-production in proton-proton reactions. However, collisional processes during the atomic de-excitation cascade considerably complicate the analysis of X-ray line shapes in order to extract the hadronic broadening. Therefore, additionally the purely electromagnetic twin system muonic hydrogen was studied. Results of these experiments performed at PSI by using a high-resolution crystal spectrometer are discussed in the context with a new analysis approach for the hadronic broadening.

The (3p - 1s) X-ray transition to the muonic hydrogen ground state was measured with a highresolution crystal spectrometer. The assumption of a statistical population of the hyperfine levels of the muonic hydrogen ground state was directly confirmed by the experiment and measured values for the hyperfine splitting can be reported. The measurement supplements studies on line broadening effects induced by Coulomb de-excitation hindering the direct extraction of the pion-nucleon scattering lengths from pionic hydrogen and deuterium X-ray lines.

The (3 p — 1 s ) X-ray transition to the muonic hydrogen ground state was measured with a highresolution crystal spectrometer. The assumption of a statistical population of the hyperfine levels of the muonic hydrogen ground state was directly confirmed by the experiment and measured values for the hyperfine splitting can be reported. The measurement supplements studies on line broadening effects induced by Coulomb de-excitation hindering the direct extraction of the pion-nucleon scattering lengths from pionic hydrogen and deuterium X-ray lines.

The ground-state level shifts and broadenings of the hydrogen isotopes caused by the strong interaction have been redetermined by using a high-resolution crystal spectrometer. An additional measurement of muonic hydrogen reveals properties of the de-excitation cascade of such electrically neutral exotic atoms, in particular Coulomb de-excitation, the understanding of which is essential for the analysis of the hadronic-atom data.

The strong-interaction shift ε 1s πD and broadening Γ 1s πD in pionic deuterium have been determined in a high statistics study of the πD(3p-1s) X-ray transition using a high-resolution crystal spectrometer. The pionic deuterium shift will provide constraints for the pion-nucleon isospin scattering lengths extracted from measurements of shift and broadening in pionic hydrogen. The hadronic broadening is related to pion absorption and production at threshold. The results are ε 1s πD = (−2356 ± 31) meV (repulsive) and Γ 1s πD meV yielding for the complex πD scattering length a πD = [−(24.99±0.33)+i(6.22 −0.26 +0.12 )] × 10−3 m π −1 . From the imaginary part, the threshold parameter for pion production is obtained to be α = (251 −11 +5 ) μb. This allows, in addition, and by using results from pion absorption in 3He at threshold, the determination of the effective couplings g 0 and g 1 for s-wave pion absorption on isoscalar and isovector NN pairs.

The measurement of the pion-nucleon scattering lengths constitutes a high-precision test of the methods of Chiral Pertubation Theory, which is the low-energy approach of QCD. The pion-nucleon s-wave scattering lengths are related to the strong-interaction shift and width of the s-states of the pionic hydrogen atom. Shift and width are determined from the measured energies and line widths of X-ray transitions to the 1s ground state when compared to the calculated electromagnetic values. A new experiment, set up at the Paul-Scherrer-Institut, has completed a first series of measurements.

An unusual but effective way to determine at threshold the dpi<-->NN transition strength alpha is to exploit the hadronic ground-state broadening Gamma(1s) in pionic deuterium, accessible by x-ray spectroscopy. The broadening is dominated by the true absorption channel dpi(-)-->nn, which is related to s-wave pion production pp-->dpi(+) by charge symmetry and detailed balance. Using the exotic atom circumvents the problem of Coulomb corrections to the cross section as necessary in the production experiments. Our dedicated measurement finds Gamma(1s)=(1171(-49)(+23)) meV yielding alpha=(252(-11)(+5)) microb.

The line shape of the (3p − 1s) X-ray transition in muonic hydrogen was measured for the first time with a high-resolution crystal spectrometer. The assumption of a statistical population of the hyperfine levels was directly confirmed by experiment, and a measured value for the hyperfine splitting is reported. An X-ray line broadening due to Doppler effect could be clearly identified and attributed to different Coulomb de-excitation transitions which precede the measured radiative transition. The results allow a decisive test of advanced cascade model calculations and establish an alternative and “model free” method to extract the strong-interaction parameters from pionic hydrogen data.

The (3p - 1s) X-ray transition to the muonic hydrogen ground state was measured with a high resolution crystal spectrometer. A Doppler effect broadening of the X-ray line was established which could be attributed to different Coulomb de-excitation steps preceding the measured transition. The assumption of a statistical population of the hyperfine levels of the muonic hydrogen ground state was directly confirmed by the experiment and measured values for the hyperfine splitting can be reported. The results allow a decisive test of advanced cascade model calculations and establish a method to extract fundamental strong-interaction parameters from pionic hydrogen experiments.

The most recent pionic-hydrogen experiment marks the completion of a whole series of measurements, the main goal of which was to provide conclusive data on pion-nucleon interaction at threshold for comparison with calculations from Chiral perturbation theory. The precision achieved for hadronic shift and broadening of 0.2% and 2%, respectively, became possible by comprehensive studies of cascade effects in hydrogen and other light exotic atoms including results from the last years of LEAR operation. In order to obtain optimum conditions for the Bragg crystal spectrometer, the cyclotron trap II has been used to provide a suitable X-ray source. To characterize the bent crystal spectrometer, the cyclotron trap has been modified to operate as an electron-cyclotron resonance source, which produces with high intensity narrow X-ray transitions in the few keV range originating from highly charged ions.

Summary  In layered semiconductors, short-range interaction with optical phonons is the dominant electron scattering mechanism. Within the limit of weak coupling and moderate anisotropy, the interaction is treated as a perturbation linear in the atomic displacements. the lowest-order correction to the energies of the electron states is calculated, and the exact relaxation time solution of the Boltzmann equation is presented.

Electrical properties of semiconducting MoSe2 single crystals doped with Re and with Nb are described. Impurity conduction can be characterized by three activation energies ∊1 > ∊2 > ∊3. At low temperature and for high doping levels a transition to a metallic state is observed. A band model is suggested which accounts for the measured activation energies and is in agreement with the photoemission results and optical data.

The low temperature luminescence of GaSe has been measured on a series of single crystals, some of which were doped with tin, copper, zinc, iodine, or cadmium. Concentrations and activation energies of the impurities have been determined by electrical measurements. At 4.2 °K, radiative recombination of electron–hole pairs occurs via two channels, whose relative efficiency is governed by the total impurity concentration. The first channel is most efficient in pure crystals. It gives rise to a spectrum dominated by the free exciton lines. The second channel is most efficient in doped crystals. Its spectrum is not sensitive to the chemical nature of the impurities. It is attributed to structural defects which are created by the chemical impurities. On a mesure la luminescence a basse temperature d'une serie de cristaux de GaSe, dont certains etaient dopes a l'etain, au cuivre, au zinc, au cadmium ou a l'iode. Les concentrations et les energies d'activation des impuretes ont ete determinees par des mesures electriques. A 4.2 °K, les recombinaisons radiatives de paires electron–trou suivent deux canaux distincts, dont l'efficacite relative est determinee par la concentration totale en impuretes. Le premier canal, efficace dans les cristaux purs, donne naissance a un spectre domine par les raies de l'exciton libre. Le second canal est efficace dans les cristaux dopes. Son spectre n'est pas sensible a la nature chimique des impuretes. On attribue ce spectre a des defauts de structure engendres par la presence d'impuretes chimiques.