Small Atomic displacements Recorded in Bismuth by the Optical Reflectivity of Femtosecond Laser-Pulse Excitations

Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10-5. Analysis shows that a novel effect reported here-an initial negative drop in reflectivity-relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling

Nonlinear mirror based on cross-polarized wave generation

We present a new type of nonlinear mirror based on the generation of a cross-polarized wave through a nonresonant electronic third-order process. It is characterized by a reflection coefficient that depends on the input intensity. Its behavior results from the interference between the nonlinearly generated cross-polarized wave and a /2 phase-retarded wave. This setup has a lot of advantages: it does not require any phase matching, it is achromatic and suitable for femtosecond pulses, linear losses are easily adjustable, and the overall behavior is predictable. The device has been experimentally tested using BaF 2 and YVO 4 crystals. OCIS codes: 190.0190, 230.4320, 140.4050. Nonlinear mirrors (NLMs) are known to be used for mode-locking (ML) operation in solid-state lasers and also for other applications, e.g., for pulse reshaping and compression and contrast improvement. In general, NLMs can be divided into two groups. The first group is based on ͑3͒ effects: self-induced ellipse rotation in isotropic media, 1,2 the Kerr lens effect, 3 or interference effects in an external cavity with ͑3͒ media. © 2006 Optical Society of America ͑2͒ cascaded processes. 12 Here, we introduce a new type of NLM based on the generation of a linearly polarized wave cross polarized to the input one. The cross-polarized wave (XPW) generation effect is a four-wave mixing process that depends on the anisotropy of the ͑3͒ tensor. The scheme of the XPW-based NLM is shown i

Parámetros abióticos y biomasa planctónica en la laguna Chascomús (provincia de Buenos Aires)

La Laguna Chascomús ubicada en la cuenca del Río Salado, es una laguna pampásica de características eutróficas, alcalina y con elevado material en suspensión. En el presente trabajo se presentan datos obtenidos a partir de marzo de 2002, con muestreos semanales en superficie y en la zona pelágica, que permitieron seguir el período de inundación (nivel superior a la cota de 8 metros IGM), que se extendió desde abril a junio y donde el pico máximo alcanzó alrededor de 9 metros. El aporte de agua debido a las precipitaciones se tradujo en un efecto de dilución que determinó que la salinidad disminuyera en un tercio el valor promedio de 1,57 g/l obtenido en el año 2000. Por el contrario los valores de fósforo total (promedio 290 μg P/l) y de nitrógeno total (promedio 880 μg N/l) fueron elevados debido a la influencia de desechos cloacales producto del desborde proveniente de la ciudad y de la planta de tratamientos cloacales que por efecto de la inundación alcanza también la laguna. El seston presentó valores bajos respecto a los usuales (promedio 79,8 mg/l) siendo afectado por el fenómeno de dilución donde de 179 mg/l (máximo) desciende a 25,8 mg/l (mínimo). En correspondencia se registraron mayores valores de lectura del disco de Secchi (promedio 21 cm, rango 12-28 cm). La biomasa algal evaluada en términos de clorofila a, fue elevada (promedio 133,3 μg/l) favorecida no sólo por la elevada concentración de nutrientes sino además por el aumento de la transparencia. La biomasa zooplantónica determinada sobre muestra obtenida por arrastre vertical mediante red de 29 μm y por la suma de la biomasa de copépodos, cladóceros y rotíferos, mostró un promedio de 173 μg/ l y la densidad de 440 individuos/l. Las relaciones entre la biomasa del zooplancton con la clorofila a y con el número de bacterias obtenido por recuento, indicarían que la predación se ejercería tanto sobre organismos autótrofos como heterótrofos.Trabajo presentado en II Jornadas sobre Ecología y Manejo de Ecosistemas Acuáticos Pampeanos (EMEAP) (La Plata, 2002)

Chemical bonding in zeolites

A review is given with 28 refs. on the current status of chem. bonding of zeolites and AlPO's. Short range covalent bonding dominates, the tetrahedra have to be considered relatively rigid and the Si-O-Si bond angle flexible. Differences in energy of the SiO2 or AlPO4 polymorphs are small. The relative stability depends on a proper accounting of the small changes in electrostatic energy. The deprotonation energy is also mainly detd. by short range covalent interactions. These are only properly accounted for when full lattice relaxation is included in the calcns. Isomorphous substitution effects are also dominated by changes in covalent interaction energ

La médecine aux îles Tubuai, Polynésie française

Etchepare J. La médecine aux îles Tubuai, Polynésie française. In: Journal de la Société des océanistes, tome 24, 1968. pp. 138-147

A CALCULATION OF THE PHONON DISPERSION CURVES IN TRIGONAL AND MONOCLINIC SELENIUM WITH THE HELP OF A SINGLE VIBRATIONAL POTENTIAL

We present phonon dispersion curves in trigonal Selenium (chains) and zone center phonons in α monoclinic Selenium (rings) calculated with the same internal force constants. Most of their differences in phonon energies can be explained by intermolecular interactions which in both case promote a tetrahedral neihbouring leaving some danglin bonds in monoclinic Se. Long range forces of the Van der Waals type were neglected in first approximation