Scale Of Several Tens Research Articles

Picosecond infrared studies of hot holes in p-type Ge

Hot holes in p-type germanium are investigated via picosecond changes of the inter-valence band absorption in the mid-infrared. The transient absorption from the heavy to the light hole band monitored in spectrally and time resolved pump-probe experiments gives evidence of the subpicosecond inter-valence band scattering of photoexcited holes. This process is followed by the cooling of heavy holes on a time scale of several tens of picoseconds. Nonequilibrium split-off holes created by intense picosecond excitation give rise to a short-lived population inversion between optically coupled states in the split-off and the light hole band. A transient gain of 5 cm-1 is observed with probe pulses of a photon energy around 260 meV. The amplification is due to the very high generation rate of split-off holes, the different transient temperatures of split-off and light holes and the exceptionally strong radiative transition probability.

Picosecond energy transfer of vibrationally hot molecules in solution: Experimental studies and theoretical analysis

The cooling of vibrationally hot azulene is studied in different solvents by picosecond spectroscopy. Excitation to the electronic S1 state generates molecules with a vibrationally hot ground state by rapid internal conversion. The subsequent cooling is monitored by the temperature-dependent change of the S0–S1 absorption edge and occurs via interaction with the solvent on a time scale of several tens of picoseconds. A theoretical model of intermolecular energy transfer in the liquid phase is developed. The vibrational excess energy of azulene is transferred to the solvent molecules by isolated binary collisions, where the multimode vibrational system of the molecules is considered explicitly. The dissipation of energy within the solvent is simulated by the macroscopic conduction of heat. The temporal development of the vibrational temperature of the azulene molecules and the concommitant changes of absorption are calculated taking into account the properties of the specific solvent. The results of the theory show quantitative agreement with our data.

Origin and evolution of the large-scale structure of the universe

Ever since the epoch of the spontaneous breaking of grand unification symmetry between the nuclear and electromagnetic interactions, the universe has expanded under the imprint of a spectrum of density fluctuations that is generally considered to have originated in this phase transition. I will discuss various possibilities for the form of the primordial fluctuation spectrum, spanning the range of adiabatic fluctuations, isocurvature fluctuations, and cosmic strings. Growth of the seed fluctuations by gravitational instability generates the formation of large-scale structures, from the scale of galaxies to that of clusters and superclusters of galaxies. There are three areas of confrontation with observational cosmology that will be reviewed. The large-scale distribution of the galaxies, including the apparent voids, sheets and filaments, and the coherent peculiar velocity field on scales of several tens of megaparsecs, probe the primordial fluctuation spectrum on scales that are only mildly nonlinear. Even larger scales are probed by study of the anisotropy of the cosmic microwave background radiation, which provides a direct glimpse of the primordial fluctuations that existed about 106 years or so after the initial big bang singularity. Galaxy formation is the process by which the building blocks of the universe have formed, involving a complex interaction between hydrodynamical and dynamical processes in a collapsing gas cloud. Both by detection of forming galaxies in the most remote regions of the universe and by study of the fundamental morphological characteristics of galaxies, which provide a fossilized memory of their past, can one relate the origin of galaxies to the same primordial fluctuation spectrum that gave rise' to the large-scale structure of the universe.

Transient absorption spectra of a modulation-doped Ga0.47In0.53As/Al0.48In0.52As multiple quantum well structure measured by picosecond infrared pulses

Intersubband and intrasubband scattering of hot electron-hole pairs is studied with picosecond infrared pulses. The carrier distribution function is derived directly from the transient absorption spectra after selective excitation of carriers to the n=1 and 2 subbands. Band filling of the n=1 conduction band results in a strong bleaching of the corresponding interband absorption and in large changes of the refractive index. An absorption change due to the population of the n=2 subbands is not detected, indicating their fast relaxation within less than 3 ps. The hot plasma cools by LO phonon emission on a time scale of several tens of picoseconds.

Femtosecond hot-carrier energy relaxation in GaAs

Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split-off band as well as the heavy- and light-hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.

Reaction rate-surface area relationships during the early stages of weathering. II. Data on eight additional feldspars

Dissolution rate data for eight specimens of feldspars have been obtained as functions of mass and exposed surface area in pH 2 and 3 solutions. These measurements suggest that reaction rates are not related to specific surface areas (m 2/g) in any simple fashion, and that the relation expresses itself differently for the alkali feldspars and the plagioclases. The alkali feldspars display rate minima at intermediate size fractions, suggesting that 1) defects associated with exsolution lamella are the primary loci for reaction, and 2) these minerals cleave preferentially along these boundaries, yielding a net reduction in the number of available reaction sites in the finer grain sized fractions. Plagioclase feldspars, as a group, do not display the pronounced rate minima at intermediate size fractions. However, the finer grain-sized fractions do exhibit reduced specific reaction rates, suggesting that the primary sites for the dissolution are not uniformly distributed within the host mineral on the scale of several tens of microns. Two independent approaches are employed to obtain estimates of the precision of individual rate determinations. Replicate rate measurements on related samples have average coefficients of variation of about 8%, with a maximum cv of less than 18%. Variance estimates obtained on the specific rate data from the coarsest size fractions of each mineral yield comparable precision estimates. While there is some uncertainty associated with individual rate determinations, the magnitude of this variance is small compared with discrepancies between field and laboratory-based estimates of absolute rates.

Wüstite (Fe1‐x O): A review of its defect structure and physical properties

Wüstite is a complex nonstoichiometric oxide of iron with the general structural formula VI[Fe2+ 1−3xFe3+ 2x−t □x+t]IVFe3+ tO where 0.04 < x < 0.12 and 2.0 < (x + t)/t < 4.5. Wüstites of a given bulk composition exist in a variety of structure types characterized by differing ratios of octahedral to tetrahedral ferric iron and by differing degrees of long‐ and short‐range order of clustered defects. In addition, most, if not all, wüstites have exsolved magnetite or iron lamellae on the scale of several tens of unit cells. Though wüstite is compositionally intermediate between pure FeO and magnetite, wüstite is not structurally intermediate and thus does not represent a solid solution between the two stoichiometric iron oxides. Physical properties of wüstite, including cation diffusion, electrical conductivity, and magnetic properties, are dependent on defect structure type and exsolution, as well as degree of nonstoichiometry. Simple linear relationships between stoichiometry and molar volume or between stoichiometry and bulk modulus, for example, do not obtain. Experimental studies on wüstite should include data on conditions of synthesis, as well as details on the structural state before and also after the experiments.

The morphology of the Martian surface

Most of the southern hemisphere of Mars is densely cratered and stands 1–3 km above the topographic datum. The northern hemisphere is more sparsely cratered and elevations are generally below the datum. A broad rise, the Tharsis bulge, centered at 14° S, 101° W, is 8000 km across and 10 km above the datum at its summit. The densely cratered terrain has two main components; very ancient crust, nearly saturated with large craters, and younger intercrater plains. In many areas the older unit is fractured and extensively dissected by small channels. The younger intercrater plains are distinctly layered in places and less dissected, less fractured, and less cratered. Both units probably date from very early in the planet's history. Cratered plains cover much of the northern hemisphere and are highly variegated. Those around the large volcanoes are covered with numerous volcanic flows whereas in other areas the plains are featureless except for craters and lunar mare-like ridges. Between 40° N and 60° N the plains are complex with various kinds of striped and patterned ground, low escarpments, and isolated irregularly shaped mesas. Their peculiar morphology has been attributed, in part, to the repeated deposition and removal of volatile-rich debris layers. Along the boundary between the northern plains and the densely cratered terrain to the south, the plains and cratered terrain complexly inter-finger. The old terrain forms the high ground and appears to have undergone mass wasting on a large scale. In several areas, particularly south of Chryse Planitia, the old, cratered surface has collapsed to form chaotic terrain. Large channels, tens of kilometers wide and hundreds of kilometers long, with numerous characteristics suggestive of catastrophic flooding, commonly emerge from the chaotic areas. Much of the area between 50° W and 180° W and 50° N and 50° S is cut by fractures radial to the center of the Tharsis bulge. The equatorial canyon system, Valles Marineris, is radial to the bulge and appears to have formed largely by faulting along the radial fractures, although it has also been extensively modified by various mass wasting and fluvial processes. Most but not all volcanoes are in the Tharsis and Elysium regions. The largest resemble terrestrial shield volcanoes except for scale; the edifices, flow features and calderas are all far larger than their terrestrial counterparts. Most impact craters on Mars are surrounded by layers of ejecta, each with a distil ridge. This unique morphology coupled with other surface characteristics suggests large amounts of ground ice. Layered deposits at both poles appear to be relatively young, volatile-rich, aeolian deposits. The north pole is also surrounded by a continuous belt of dunes several tens of kilometers across. In most other places, aeolian modification of the surface at a scale of several tens of meters appears slight despite annual global dust storms.