光的碰撞再分布

光的碰撞再分布是２０多年来发展起来的研究原子碰撞过程的新技术，目前已成为研究原子相互作用的强有力的工具。文章简要介绍了碰撞再分布技术的基本原理和典型实验，并给出了应用实例。     

Adaptive behaviour of the spinal cord in the transition from quiet stance to walking

Background

Parkinson??s disease is characterized by a continuous loss of neurons within the substantia nigra (SN) leading to a depletion of dopamine. Within the adult SN as a non-neurogenic region, cells with mainly oligodendrocytic precursor characteristics, expressing the neuro-glial antigen-2 (NG2) are continuously generated. Proliferation of these cells is altered in animal models of Parkinson??s disease (PD). Exercise and environmental enrichment re-increase proliferation of NG2⁺ cells in PD models, however, a possible mechanistic role of dopamine for this increase is not completely understood. NG2⁺ cells can differentiate into oligodendrocytes but also into microglia and neurons as observed in vitro suggesting a possible hint for endogenous regenerative capacity of the SN. We investigated the role of dopamine in NG2-generation and differentiation in the adult SN stimulated by physical activity and environmental enrichment.

Results

We used the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-model for dopamine depletion and analysed newborn cells in the SN at different maturation stages and time points depending on voluntary physical activity, enriched environment and levodopa-treatment. We describe an activity- induced increase of new NG2-positive cells and also mature oligodendrocytes in the SN of healthy mice. Running and enriched environment refused to stimulate NG2-generation and oligodendrogenesis in MPTP-mice, an effect which could be reversed by pharmacological levodopa-induced rescue.

Conclusion

We suggest dopamine being a key regulator for activity-induced generation of NG2-cells and oliogodendrocytes in the SN as a potentially relevant mechanism in endogenous nigral cellular plasticity.     

Phase-conjugation in two-level atoms: comparison of quantum interference theory to exact results

The analytically derived predictions of a recently developed theory on phase conjugate four wave mixing (FWM) in two-level atoms [Phys. Rev. A 60, 1672 (1999)], which interprets many of the signal features as a consequence of quantum interference between the probability amplitudes for photon emission from relevant dressed states, is compared to the results that one obtains from a non-perturbative, numerical calculation of the density matrix equations. The results delineate the regimes and assumptions under which the perturbative results are valid, and it is shown that despite the simplifying assumptions that are invoked for the dressed state theory, many of its predictions are in qualitative agreement with the exact, numerical results.     

The mathematical technique in Fermat's deduction of the law of refraction

This article attempts to explain Fermat's not quite obvious calculations connected with his deduction of the law of refraction in Analysis ad refractiones (1662), and to describe the development which led to these calculations. In 1657 Fermat tried to deduce a law of refraction based on the principle that light follows the quickest path between two given points. He did not succeed because he found that the calculations were too long and tedious. The calculations are indeed complicated, but if Fermat, in 1657, had been willing to accept Descartes' law of refraction he would probably also have seen that it solved his problem. However, Fermat was of the opinion that Descartes' law was wrong and, therefore, he did not expect that solution. Only in 1662, when he succeeded in reducing the calculations substantially, did he realize that they led to the sine law of Descartes.     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

Observation of a 1/t decay law for hot clusters and molecules in a storage ring

The exponential law is valid both for decay from a single quantum state into a continuum and for an ensemble maintained in thermal equilibrium. For statistical decay of an ensemble of isolated systems with a broad energy distribution, the exponential decay is replaced by a 1/t distribution. We present confirmation of this decay law by experiments with cluster anions in a small electrostatic storage ring. Deviations from the 1/t law for such an ensemble give important information on the dynamics of the systems. As examples, we present measurements revealing strong radiative cooling of anions of both metal clusters and fullerenes.     

Radiative cooling of fullerene anions in a storage ring

Thermionic emission from hot fullerene anions, C_N ^-, has been measured in an electrostatic storage ring for even N values from 36 to 96. The decay is quenched by radiative cooling and hence the observations give information on the intensity of thermal radiation from fullerenes. The experiments are analysed by comparison with a simulation which includes the quantisation of photon energy and the statistics of emission. Experiments with heating of the molecules with a laser beam confirm the interpretation of the observations in terms of radiative cooling and give an independent estimate of the cooling rate for C₆₀ ^-. The measured cooling rates agree in general within a factor of two with the prediction from a classical dielectric model of a thermal radiation intensity of ∼ 300 eV/s for C₆₀ at 1 400 K, scaling approximately with the 6th power of the temperature and with the number of atoms in the molecule. Received 12 March 2001 and Received in final form 12 June 2001     

Controlled coupling of a single nitrogen-vacancy center to a silver nanowire

We report on the controlled coupling of a single nitrogen-vacancy (NV) center to a surface plasmon mode propagating along a chemically grown silver nanowire (NW). We locate and optically characterize a single NV center in a uniform dielectric environment before we controllably position this emitter in the close proximity of the NW. We are thus able to control the coupling of this particular emitter to the NW and directly compare the photon emission properties before and after the coupling. The excitation of single plasmonic modes is witnessed and a total rate enhancement by a factor of up to 4.6 is demonstrated.