Abstract. With the aim to enhance interpretation of fragmentation mechanisms during explosive volcanism from size and shape characteristics of pyrodasts experimental studies have been conducted using remelted volcanic rock (olivine-melilitite). The melt was fragmented and ejected from a crucible by the controlled release of pressurized air volumes (method 1) or by controlled generation of phreatomagmatic explosions (Molten Fuel Coolant Interaction (MFCI); method 2). Both methods were adjusted so that the ejection history of the melt was identical in both cases. The experiments demonstrate that exclusively during MFCI, angular particles in the grain size interval 32 to 130 ptm are generated that show surface textures dominated by cracks and pitting. The physical process of their generation is described as a brittle process acting at cooling rates of > 106 K/s, at stress rates well above 3 GPa/m 2, and during ~700 pts. In this time period the emission of intense shock waves in the megahertz range was detected, releasing kinetic energy of > 1000 J. By both experimental methods, three more types of particles were produced in addition, which could be identified and related to the acceleration and ejection history of the melt: spherical particles, elongated particles, and Pele's hair. Abundance and grain size distribution of these particles were found to be proportional to the rate of acceleration and the speed of ejection but were not influenced by the experimental method used. Pele's hair occurred at ejection speeds of >75 m/s.

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This paper deals with optical second harmonic generation (SHG) in ferroelectric NaNO2 at 1.06 μm fundamental wavelength. The temperature dependence of SHG in the range between 30 and 200 °C is observed for five different radiation geometries, each corresponding to one of the five nonvanishing components of the nonlinear susceptibility χ(2). Measurements are performed on plane‐parallel and wedge‐shaped one‐domain crystals and on plane‐parallel multi‐domain crystals. Information is obtained about the temperature dependence of the coherence length lc = λ/[4 (n(2) – n(1))], of χ(2), and of the domain structure. χ(2) shows a rather strong decrease already in the ferroelectric phase and in general a sharp drop at the Curie temperature. The influence of the domain structure on SHG is discussed in detail.

In a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8 X 8 k·p Hamiltonian matrix for stress Tll[IOO) and field Bll[OOl) and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TIIBII [OOl]. Characteristic features are extracted for inter-and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration. I. INTRODUCfiONThe electronic energy bands of zinc-blende structure semiconductors exhibit significant deviations from the simple isotropic and parabolic shape close to the gap. In both conduction 1 and valence bands 2 one finds strong terms of cubic and quartic order in k and warping. Uniaxial stress induces k-linear terms which shift the band extrema to nonzero wave vectors. 3 For the valence bands k-linear terms exist already in the unstressed state. 4 The band anomalies can be detected in Shubnikov-de Haas oscillations, 3 in spin-depolarization measurements, 5 or in far-infrared and Raman scattering experiments on doped semiconductors. 6 Under the influence of a magnetic field, the spindegenerate conduction bands form two Landau ladders, which, due to the nonparabolicities, are spaced irregularly. The differences in the spacings show up in splittings of the harmonic transition lines. The cyclotron mass and g factor are field dependent. 7 Higher harmonic and spin-flip transitions become allowed. 8 The uppermost valence band is fourfold degenerate at k=O. In a magnetic field, the four bands turn into four entangled Landau ladders with strongly varying separations of the rungs. Magnetooptical transitions can only be identified reliably if by application of uniaxial stress the band degeneracy is lifted and the ladders are pulled apart.Such a program was first pursued by Hensel and Suzuki 9 in the investigation of the valence bands of Ge. For most of their experiments, stress was applied parallel to the magnetic field along the crystal [001], [111], and [110] directions, and microwave absorption was regis-37 tered. The spectra were calculated from an effective-mass Hamiltonian which was formed in the space of the four degenerate valence-band functions of symmetry rt.Given the geometry of the experiments and the diamond lattice structure of Ge, the symmetry groups of this Hamiltonian are C 4 (consisting of rotations about the direction of the magnetic field by multiples of 27T/4), c3, and C 2 , respectively. The symmetries permit a simple labeling scheme for the magnetic states and simplify the numerical calculations.Similar experiments were performed soon afterwards by Ranvaud et al. 10 Since Ranvaud used far-infrared radiation, higher magnetic fields had to be applied than were necessary in the microwave experiments of Hensel and Suzuki. 9 The strong field...

Resonant effects in second harmonic generation (shg) in InSb near the energy gap have been studied by radiating with a high power Q-switched CO2 laser at 10.6 mu m and tuning the energy gap by applying magnetic fields up to 55 kG in samples of carrier concentrations from 8*1014 to 5*1016 cm-3 at a temperature of 12 K. Absolute, independent measurements of the coherence lengths lc+- were made as a function of magnetic field, using circularly polarized radiation and wedge shaped samples, by a modification of the original 'Maker' experiment. These results, together with measurements of SHG +- and absorption coefficients alpha +- have yielded the magnetic field dependence of the second order nonlinear susceptibilities chi +- at 5.3 mu m. Virtual transition processes have been used to formulate the theory for the resonant behaviour of chi and lc and the effect of blocking by conduction band population is demonstrated experimentally.