89 research outputs found
Non-linear spectroscopy of rubidium: An undergraduate experiment
In this paper, we describe two complementary non-linear spectroscopy methods
which both allow to achieve Doppler-free spectra of atomic gases. First,
saturated absorption spectroscopy is used to investigate the structure of the
transition in rubidium. Using a slightly
modified experimental setup, Doppler-free two-photon absorption spectroscopy is
then performed on the transition in
rubidium, leading to accurate measurements of the hyperfine structure of the
energy level. In addition, electric dipole selection rules of
the two-photon transition are investigated, first by modifying the polarization
of the excitation laser, and then by measuring two-photon absorption spectra
when a magnetic field is applied close to the rubidium vapor. All experiments
are performed with the same grating-feedback laser diode, providing an
opportunity to compare different high resolution spectroscopy methods using a
single experimental setup. Such experiments may acquaint students with quantum
mechanics selection rules, atomic spectra and Zeeman effect.Comment: 16 pages, 8 figure
Instability driven fragmentation of nanoscale fractal islands
Formation and evolution of fragmentation instabilities in fractal islands,
obtained by deposition of silver clusters on graphite, are studied. The
fragmentation dynamics and subsequent relaxation to the equilibrium shapes are
controlled by the deposition conditions and cluster composition. Sharing common
features with other materials' breakup phenomena, the fragmentation instability
is governed by the length-to-width ratio of the fractal arms.Comment: 5 pages, 3 figures, Physical Review Letters in pres
Palaeontological data about the climatic trends from Chattian to present along the Northeastern Atlantic frontage
Climatic changes that affected the Northeastern Atlantic frontage are analyzed on the basis of the evolution of faunas and floras from the late Oligocene onwards. The study deals with calcareous nannoplankton, marine micro- and macrofaunas, some terrestrial vertebrates and vegetal assemblages. The climate, first tropical, underwent a progressive cooling (North-South thermic gradient). Notable climatic deteriorations (withdrawal towards the South or disappearance of taxa indicative of warm climate and appearance of "cold" taxa) are evidenced mainly during the Middle Miocene and the late Pliocene. Faunas and floras of modern pattern have regained, after the Pleistocene glaciations, a new climatic ranging of a temperate type in the northern part
Homogeneous and heterogeneous clustering in the accretion regime
Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center. These two steps have been investigated separately through the clustering of homogeneous particles Nan and heterogeneous particles NanX in a helium buffer gas (X = (Na2O)2 or (NaOH)2). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production of NanX clusters.
Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus sodium concentration. This process is the onset of the condensation phase transition.
Towards practical large-eddy simulations of complex turbulent flows
Turbulence that occurs in nature, or in engineering flows, is usually not, even approximatively, homogeneous. There are frequent variations of the mean velocity with position. By explicating the scale-by-scale energy budget of nonhomogeneous turbulence, it has been argued that the subgrid-scale (SGS) stress tensor should encompass two types of interactions [1, 2]: (i) between the mean velocity and the resolved fluctuating velocities (the rapid part of the SGS stress) and (ii) among the resolved fluctuating velocities themselves (the slow part of the SGS stress). The rapid part is related to the large-scale distortion, while the slow part is associated with the Kolmogorov’s energy cascade [3]. Interestingly, these developments end up with a shear-improved Smagorinsky model (SISM) [1], for which the SGS viscosity writes
Examining the Econom(etr)ic Relevance of Discretizing Panel Data : an Application to the Belgian Labour Market
The object of this paper is to explore how to use administrative data for econometric purposes when these data measure total annual duration spent in various states. A basic issue addressed is whether the information extracted through categorising continuous data actually defines an economically meaningful concept of an individual position on the labour market. The main findings are that discretization indeed leads interpretable transitions on the labour market, and that the mobility between positions rather low in Belgium
Examining the Econom(etr)ic Relevance of Discretizing Panel Data: an Application to the Belgian Labour Market
Charge Transfer and Dissociation in Collisions of Metal Clusters with Atoms
We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li- 31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer
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