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Ultrafast degenerate four-wave-mixing (FWM) techniques have been enormously successful in providing new information about coherent processes and excitonic effects in semiconductors. One of the techniques that has proven to be extremely powerful is to study the polarization dependence of the FWM process. Typically, these studies have addressed the dependence of the magnitude of the FWM signal (or its spectrum) on the incident polarizations, and there have been very few attempts to monitor the polarization state of the FWM signal itself. Moreover, the attempts that have been made have been restricted to measurements of the direction of polarization (ignoring the ellipticity and degree of polarization) and, therefore, they have not completely specified the polarization state of the emitted light. In addition, these measurements have been restricted to positive delays.

All four of the Stokes parameters of the time-integrated degenerate four-wave mixing signal from a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum well are measured as a function of the relative angle between the linear polarizations of the pump and the probe and of the incident fluence, thus completely characterizing the polarization state of the signal. The degree to which the signal is polarized, the ellipticity, and the orientation of the polarization ellipse are found to be strongly dependent on the orientation of the incident polarizations and on fluence. The principal features can be qualitatively understood in terms of a simple model that includes density-dependent dephasing and local-field corrections.

The complete polarization state of the degenerate four-wave mixing signal from a GaAs/AlxGa1-xAs multiple quantum well is determined by time resolution of all four of its Stokes parameters as a function of the relative angle between the two linear input polarizations. The degree of ellipticity and the orientation of the polarization ellipse are both observed to vary dramatically in time, and the temporal evolution is found to depend strongly on the orientation of the input polarizations. These time-resolved results are shown to be consistent with previous measurements of the time-integrated Stokes parameters and to provide new constraints for physical models. The results are shown to be qualitatively consistent with a phenomenological model requiring the inclusion of both many-body interactions and biexcitonic effects.

$\ensuremath{\gamma}$ radiation from targets of $^{7}\mathrm{Li}$, $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{13}\mathrm{C}$, and $^{16}\mathrm{O}$ bombarded by ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\pi}}^{+}$ at energies between 60 and 300 MeV was uniquely associated with states formed by removing a single nucleon by the ($\ensuremath{\pi},\ensuremath{\pi}N$) reaction. All yields were found to reflect the well-known 3-3 resonance near 180 MeV in the pion-nucleon system. Ratios of yields for ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\pi}}^{+}$ beams were measured for $^{7}\mathrm{Li}$, $^{9}\mathrm{Be}$, and $^{13}\mathrm{C}$. These ratios were very similar to one another and to previous results on $^{12}\mathrm{C}$, $^{14}\mathrm{N}$, and $^{16}\mathrm{O}$ for neutron removal. Several reaction mechanisms based on quasifree processes are tested by comparison to the data, and all are rejected.

Differential cross sections for the (..pi../sup -/,n) reactions in /sup 3/He and /sup 4/He have been measured at energies of 100, 200, and 290 MeV. The angular distributions for /sup 3/He and /sup 4/He are different, with the /sup 4/He characterized by an oscillatory pattern with a dip at a fixed angle of approx. = 70/sup 0/. The forward-angle cross sections show a simple dependence upon momentum transfer for both /sup 3/He and /sup 4/He. These results are discussed in the context of single-nucleon and multinucleon reaction diagrams.

Differential cross sections for the reaction /sup 3/He(..pi../sup -/,..pi../sup 0/)/sup 3/H at energies of 200, 250, 275, and 290 MeV over the angular region 80 /sup 0/ < or approx. = theta/sub c.m./ < or approx. = 150/sup 0/ are presented. Comparison is made with theoretical predictions based on Glauber theory and optical-potential calculations. Neither approach gives a satisfactory description on the data; in particular, the observed energy dependence is much smaller than the theoretical predictions.

Outgoing-nucleon charge exchange has been used to explain reduction in ..pi..-N isospin selectivity in a large number of nucleon-removal experiments. The present work tests the validity of this model by measuring the total cross sections for nucleon removal to the 4.44-MeV, T = 0 and the 15.11-MeV, T = 1 states in /sup 12/C, where analog charge exchange is possible for the latter, but not the former state.

Neon K X-ray spectra resulting from O4+ and Cl4+ bombardment are observed to have dissimilar K alpha satellite intensities. These results are in contrast to similar measurements made using C6+, N6+ and O6+ bombardment.

De-excitation γ-rays have been detected from nuclear states following positive pion induced two-nucleon removal from 12 C and four-nucleon removal from 16 O. Excitation functions for population of discrete states were obtained for positive pions of 75, 125, 150, 200, 225, 250 and 290 MeV. The pion-nucleon (3, 3) resonance is not reflected in the cross sections for either reaction. Implications of the observed energy dependence are discussed.

Oxygen (plus six) and (plus eight) at 30 MeV incident energy have been used to induce Auger electron spectra of Ne. The high instrumental resolution (0.02% FWHM) spectra show specifically that the charge state of the incident oxygen ion considerably affects the distribution of the K-Auger electron yield among the charge states of multiply ionized neon.

Initial high resolution measurements of projectile electron emission spectra for beam gas collisions are reported. The Auger electron decay of beams of O + , O 2+ , and O 3+ at 3, 8, and 18 MeV, respectively, excited by single collisions with He, Ne, and Ar was measured at a lab emission angle of 23.7° with a 36 cm double focussing electrostatic analyzer. Although prompt transitions are found to predominate the spectra, several weak lines are attributed to transitions from metastable states. Spectral features are seen to vary greatly as a function of exciter gas and as a function of projectile charge state and energy.

Total cross sections for the knockout reactions $^{7}\mathrm{Li}({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}n)^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{+}n)^{6}\mathrm{Li}$ to 3.56-MeV ${0}^{+}$ $T=1$ state have been measured at ten energies between 50 and 290 MeV by detecting the direct $\ensuremath{\gamma}$ de-excitation. As found in previous work, the ratio $\frac{\ensuremath{\sigma}({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}n)}{\ensuremath{\sigma}({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{+}n)}$ on the (3,3) resonance is significantly less than the value of 3 expected from free pion-nucleon results.

The K X-ray spectrum from a 30 MeV chlorine ion beam was measured as it was stopped in aluminium. The spectrum exhibits six lines located approximately half way between the Kalpha 1,2 and Kbeta 1,3 lines of chlorine. These lines can be interpreted as Kalpha transitions with various 2p shell vacancies and with the M shell electrons partially stripped.

Targets of Al and Al2O3 were bombarded by 0.8 MeV protons and 3.2 MeV alpha particles and the subsequent X-ray yield measured. The Kα X-ray spectra of Al and Al2O3 show fairly good agreement in both the proton and alpha-induced spectra; however, the structure in the Kβ X-ray spectra deviate from one another in both cases. These results are interpreted as effects due to the chemical bonding of the oxygen atoms to aluminum.

A technique employing integrating analog-to-digital converters to do pulse shape discrimination between neutron and gamma initiated scintillations in liquid scintillators is described. The technique involves a direct comparison of the fast and slow components of the light output from the scintillator.

Four equidistant peaks are observed below the Kα 1,2 line in the X-ray spectra of Na, Al, Si, SiO 2 , Ca, Sc and Ti, produced by oxygen bombardment. With much weaker intensity, these lines are also seen in proton excitation. The lines are interpreted to be due to the excitation of one or more volume plasmons in the target material by the Kα 1,2 X-ray decay.