Photoelectron imaging of helium droplets

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.     

Impact Oscillator with Hertz's Model of Contact

Dynamic properties of simple mechanical 1 DOF system containing soft stop is described and analyzed. The proposed general dynamical impact model respects the nonlinearity of the restoring contact force between solid bodies as function of deformation and velocity. It describes the real behavior of impacting system more exact than the piecewise linear model or the Kelvin–Voigt model and even model based on application of constant coefficient of restitution. Free and forced vibrations of system with Hertz's contact are investigated and domains of various types of impact motion, response curves and phase-plane trajectories are presented.     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

Self-swept erbium fiber laser around 1.56 μm

Self-swept erbium fiber laser emitting around 1.56?μm is reported in detail. Both sweep directions were registered: pointing toward longer and shorter wavelengths, redshift and blueshift sweeping, respectively. We describe method of determining the direction of the wavelength drift using the monochromator based optical spectrum analyzer. Possible root for this sweeping regime, i.e., the gain modulation along active fiber, is discussed with the help of a simple model calculating the overall cavity gain that can predict the direction of the laser wavelength sweeping.     

Photoionization dynamics in pure helium droplets

The photoionization and photoelectron spectroscopy of pure He droplets were investigated at photon energies between 24.6 eV (the ionization energy of He) and 28.0 eV. Time-of-flight mass spectra and photoelectron images were obtained at a series of molecular beam source temperatures and pressures to assess the effect of droplet size on the photoionization dynamics. At source temperatures below 16 K, where there is significant production of clusters with more than 10(4) atoms, the photoelectron images are dominated by fast electrons produced via direct ionization, with a small contribution from very slow electrons with kinetic energies below 1 meV arising from an indirect mechanism. The fast photoelectrons from the droplets have as much as 0.5 eV more kinetic energy than those from atomic He at the same photon energy. This result is interpreted and simulated within the context of a "dimer model", in which one assumes vertical ionization from two nearest-neighbor He atoms to the attractive region of the He2+ potential energy curve. Possible mechanisms for the slow electrons, which were also seen at energies below IE(He), are discussed, including vibrational autoionizaton of Rydberg states comprising an electron weakly bound to the surface of a large HeN+ core.     

Short monolithic columns--a breakthrough in purification and fast quantification of tomato mosaic virus

Drawbacks of conventional virus purification methods have led to the development of new, mostly chromatography-based methods. Short monolithic columns are stationary phases intended for purification of large molecules. In this work efficient chromatographic purification of tomato mosaic virus (ToMV) from plant material is described. Based on short monolithic column, the purification process was shortened from 5 days to 2 hours. High viral purity was achieved and recovery of chromatographic step was up to 90%. In addition, these columns enabled preliminary quantification of the virus in just a few minutes, much faster than other quantification methods (e.g. enzyme-linked immunosorbent assay or real-time polymerase chain reaction) which take 1-2 days. These results demonstrate the potential of short monolith column technology for purification and analysis of different viruses.     

Er-doped twin-core fibre coupler as a saturable-absorber-based narrow-band filter for fibre lasers

We have proposed and demonstrated experimentally that an Er-doped twin-core fibre (ErTCF) can be employed as a tracking narrow-band filter in fibre ring lasers for line narrowing and enhancement of the spectral stability. The device mimics a saturable absorber with inhomogeneous broadening. The slow erbium ion saturation dynamics in the ErTCF prevents wavelength hopping whilst it enables the filter to track the operation wavelength on its tuning. We have improved the device operation by utilizing the inherent filtering effect of a twin-core fibre coupler. Presented at the 1st Czech-Chinesse Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998 This research has been in part supported by the Commission of the European Union in the frame of Copernicus programme, Project # 7882 and by the Ministry of Education of the Czech Republic under Contract OC 241.10/1998.     

Concentrating rotaviruses from water samples using monolithic chromatographic supports

Rotaviruses are the leading cause of diarrhoea in infants around the globe and, under certain conditions they can be present in drinking water sources and systems. Ingestion of 10–100 viral particles is enough to cause disease, emphasizing the need for sensitive diagnostic methods. In this study we have optimized the concentration of rotavirus particles using methacrylate monolithic chromatographic supports. Different surface chemistries and mobile phases were tested. A strong anion exchanger and phosphate buffer (pH 7) resulted in the highest recoveries after elution of the bound virus with 1 M NaCl. Using this approach, rotavirus particles spiked in 1 l volumes of tap or river water were efficiently concentrated. The developed concentration method in combination with a real time quantitative polymerase chain reaction assay detected rotavirus concentrations as low as 100 rotavirus particles/ml.     

Phenomena of subharmonic motions of oscillator with soft impacts

The excited, one degree of freedom mechanical system with soft impacts, characterised by triangle hysteresis loop, is investigated using numerical simulation. Small viscous damping is assumed. Phenomena of subharmonic motions are explained by regions of their existence and stability in the plane of dimensionless excitation frequency and static clearances. Bifurcation diagrams are evaluated during quasistationary changes of frequency by constant clearance.