Unshifted Component Research Articles

Experimental Studies of the Mechanisms for Reflectometer Spectrum Formation for Normal and Oblique Probing

The dynamics of spectra formation was studied experimentally using radar stroboscopic time-of-flight measurements. Both conventional fluctuation reflectometry spectra (obtained with probing normal to the cutoff surface) and Doppler reflectometry spectra were studied. Two mechanisms of spectrum formation were distinguished even at low time resolution that did not allow capturing the spectra formed after single radiation pass to the cutoff surface and back. The first mechanism that manifests itself for the normal probing and works for the unshifted component of the Doppler reflectometry spectrum is typical for the nonlocal formation of the fluctuation reflectometry spectra in small machines. This can be associated with the radiation propagation along the lengthy trajectory between the cutoff surface and the chamber wall, as well as with the small-angle scattering that occurs during this propagation along the trajectory. The second mechanism, which manifests itself during the formation of the bulk of the Doppler reflectometry spectrum (which is dominant in power and is frequency shifted), occurs without a time delay and most likely corresponds to the classical single-pass backscattering near the cutoff surface.

A development of lifetime measurement based on the differential decay curve method

A new development of indirect gating case in the differential decay curve method used for lifetime measurement has been introduced. The gate region was extended from partial shifted peak to both shifted and unshifted components. The statistics of flight and stop peaks in gating spectra was improved obviously. The reliability of this change has been tested by reanalysing the lifetime of $$2^{+}$$ state in $$^{134} \hbox {Ce}$$ . The result of 32.2(33) ps was fit well with the previous published values within the experimental uncertainty. The developed method was also used to analyse the lifetime of $$2_1^{+}$$ state in $$^{138} \hbox {Nd}$$ .

Influence of magnetic-field-caused modifications of trajectories of plasma electrons on spectral line shapes: Applications to magnetic fusion and white dwarfs

A general framework for calculating shapes of hydrogen (or deuterium) spectral lines in strongly-magnetized plasmas with the allowance for spiraling trajectories of perturbing electrons has been developed. It has been shown that in this situation the first order term Φ(1)(B) of the Dyson expansion of the electron broadening operator does not vanish – in distinction to the case of rectilinear trajectories, where the first non-vanishing term appeared only in the second order. An example of the Lyα line has been used to illustrate the effects of the spiraling trajectories. It has been shown that the shape of each of the two σ-components can become a doublet: in addition to the shifted component, there can appear also an unshifted component. Moreover, the shape of each of the two σ-components can also become a triplet: in addition to the shifted and unshifted component, there can appear also a component shifted to the opposite wing of the line. Both the positions and the intensities of the shifted components depend strongly on the magnitude of Φ(1)(B). The primary effect in the entire spectral line is a significant increase of the ratio of the intensity of the central peak to the intensity of any of the two lateral peaks.As practical examples, magnetic fusion plasmas and plasmas of DA white dwarfs have been considered. It is shown that for both of these, physically very different objects, the allowance for the spiraling trajectories of perturbing electrons can be important.

Absolute absorption cross sections from photon recoil in a matter-wave interferometer.

We measure the absolute absorption cross section of molecules using a matter-wave interferometer. A nanostructured density distribution is imprinted onto a dilute molecular beam through quantum interference. As the beam crosses the light field of a probe laser some molecules will absorb a single photon. These absorption events impart a momentum recoil which shifts the position of the molecule relative to the unperturbed beam. Averaging over the shifted and unshifted components within the beam leads to a reduction of the fringe visibility, enabling the absolute absorption cross section to be extracted with high accuracy. This technique is independent of the molecular density, it is minimally invasive and successfully eliminates many problems related to photon cycling, state mixing, photobleaching, photoinduced heating, fragmentation, and ionization. It can therefore be extended to a wide variety of neutral molecules, clusters, and nanoparticles.

Phase Resetting of the Mammalian Circadian Clock Relies on a Rapid Shift of a Small Population of Pacemaker Neurons

The circadian pacemaker of the suprachiasmatic nuclei (SCN) contains a major pacemaker for 24 h rhythms that is synchronized to the external light-dark cycle. In response to a shift in the external cycle, neurons of the SCN resynchronize with different pace. We performed electrical activity recordings of the SCN of rats in vitro following a 6 hour delay of the light-dark cycle and observed a bimodal electrical activity pattern with a shifted and an unshifted component. The shifted component was relatively narrow as compared to the unshifted component (2.2 h and 5.7 h, respectively). Curve fitting and simulations predicted that less than 30% of the neurons contribute to the shifted component and that their phase distribution is small. This prediction was confirmed by electrophysiological recordings of neuronal subpopulations. Only 25% of the neurons exhibited an immediate shift in the phase of the electrical activity rhythms, and the phases of the shifted subpopulations appeared significantly more synchronized as compared to the phases of the unshifted subpopulations (p<0.05). We also performed electrical activity recordings of the SCN following a 9 hour advance of the light-dark cycle. The phase advances induced a large desynchrony among the neurons, but consistent with the delays, only 19% of the neurons peaked at the mid of the new light phase. The data suggest that resetting of the central circadian pacemaker to both delays and advances is brought about by an initial shift of a relatively small group of neurons that becomes highly synchronized following a shift in the external cycle. The high degree of synchronization of the shifted neurons may add to the ability of this group to reset the pacemaker. The large desynchronization observed following advances may contribute to the relative difficulty of the circadian system to respond to advanced light cycles.

Temporal evolution of chromospheric downflows

At the footpoints of loops spanning a site of flux emergence, earlier investigated in the papers by Solanki et al. (2003) and Lagg et al. (2004), we find large redshifts in the He 1083 nm line coexisting with an almost unshifted component. The speed associated with these redshifts reaches values as high as 40 km/s. We interpret these downflows in the context of several models: the free-fall downflow of matter along vertical field lines (Schmidt et al. 2000), the redshift by downward propagating acoustic waves (Hansteen 1993) and the motion of condensation regions to either side of loop footpoints (Müller et al. 2003). We present the temporal evolution of these redshifts and reconstruct the magnetic field vector in these regions for both the redshifted and the unshifted atmospheric component.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Interaction of light waves of arbitrary polarisation on a photorefractive grating in a cubic gyrotropic crystal in an external alternating electric field

The symmetric two-wave interaction is considered, for small modulation indices of an interference light pattern, on a transmission photorefractive grating with stationary amplitude produced in a crystal of symmetry group 23 in an external alternating electric field. For an arbitrary off-duty ratio of the external field or arbitrary elliptic polarisation of incident light waves, the space-charge field is shown to contain both shifted and unshifted components, while the polarisation state and intensity of a weak light wave change upon the external field switching. The space-charge-field amplitude and the weak-wave intensity gain are analysed as functions of the interaction wavelength, the off-duty ratio of the external field, and the photorefractive-grating spacing for the longitudinal and transverse interaction geometry at a wavelength of 633 nm in a Bi12TiO20 crystal.

Micro-Raman fingerprint of grain boundary in [100] oriented diamond films: Stress distribution and diamond phases

Local distribution of stress state and carbon bonding in diamond films are studied by micro-Raman spectroscopy. Large changes in intensity, line shape, and peak position of the diamond Raman line occur close to the grain boundary. The characteristic feature of such region is a 1326 cm−1 Raman peak, whose detection suggests that defect incorporation at the grain boundary may promote a diamond symmetry modification from a cubic to hexagonal one. Close to intergrain locations, large anisotropic stresses also induce frequency splitting of the Raman line. Suitable stress configurations consistent with the detection of an almost unshifted component and of a largely frequency-changing mode are analyzed.

RAYLEIGH AND BRILLOUIN SCATTERING FROM LIQUID HELIUM

Studies of spontaneous light scattering from liquid 4He and 3He-4He mixtures have been carried out, both above and below the λ point. Digital recordings have been made at very low light levels using a plane Fabry-Perot interferometer of 377 mm spacing. By careful design of scattering cell, spurious flare has been eliminated, and the absence of unshifted scattering in pure 4He below the λ point has been verified (Fig. 1). Above the λ point the Rayleigh component has been studied (Fig. 2) and its intensity relative to the Brillouin lines has been compared with the available thermodynamic data (Fig. 3 and 4). In superfluid mixtures the full five-component spectrum of Rayleigh and Brillouin scattering has now been observed (Fig. 5). The intensity of scattering from propagating fluctuations (second sound) has been measured relative to that from the first sound and is found to be in fair agreement with theoretical expectations. The central unshifted component from concentration fluctuations is of similar intensity to the second-sound lines. However thermodynamic data for a full comparison is as yet unavailable.

A Preliminary Investigation of Brillouin Scattering in a Liquid Crystal

Brillouin spectra have been observed in the liquid-crystal cholesteryl-2-(2-ethoxyethoxy) ethyl carbonate. The frequency shift and the relative intensities of the shifted and unshifted components have been measured as a function of temperature, which was varied through the cholesteric to isotropic transition.