Volume 44, Issue 3
March 1995, pages 201-279
pp 201-209 March 1995
We report Raman scattering from the boehmite,γ-, δ- andα-phases of the alumina gel. Samples are characterized by transmission and scanning electron microscopy, X-ray diffraction and density measurements. The main Raman line in the boehmite phase is red-shifted as well as asymmetrically broadened with respect to that in the crystalline boehmite, signifying the nanocrystalline nature of the gel. Raman signatures are absent in theγ- andδ-phases due to the disorder in cation vacancies. We also show that low frequency Raman scattering from the boehmite phase resembles that from a fractal network, characterized in terms of fraction dimension$$\tilde d$$. Taking Hausdorff dimension D of the boehmite gel to be 2.5 (or 3.0), the value of$$\tilde d$$ is 1.33±0.02 (or 1.44±0.02), which is close to the theoretically predicted value of 4/3.
pp 211-217 March 1995
Thermally stimulated luminescence (TSL) studies of gamma-irradiated uraniumdoped K2Ca2(SO4)3 revealed two glow peaks around 400 K and 435 K. Electron paramagnetic resonance (EPR) studies carried out on these samples have shown the formation of the radical ions SO4−, SO3−, SO2− and O3−. From the study of the thermal stabilities of these radical ions, it was found that the thermal destruction of SO2− and SO4− radical ions are associated with the glow peaks observed around 400 K and 435 K respectively. Uranate ion was identified as the luminescent centre for the observed TSL glow. The trap depth values for the glow peaks have been determined from TSL data.
pp 219-224 March 1995
Polycrystalline samples of KVO3, a member of the pyroxene structural family have been synthesised with high-temperature solid-state reaction technique. Preliminary structural and spectroscopic and detailed dielectric properties have been studied in different conditions. No dielectric anomaly or hysteresis loop has been observed in a wide temperature range (30°C to 450°C). Basic structural and spectroscopic (IR and Raman) studies suggest that at room temperature, KVO3 is centrosymmetric as reported earlier. This and other observations do not support the recent report of ferroelectric phase transition in the compound.
pp 225-229 March 1995
Thermal lens signals in solutions of rhodamine B laser dye in methanol are measured using the dual beam pump-probe technique. The nature of variations of signal strength with concentration is found to be different for 514 and 488 nm Ar+ laser excitations. However, both the pump wavelengths produce an oscillatory type variation of thermal lens signal amplitude with the concentration of the dye solution. Probable reasons for this peculiar behaviour (which is absent in the case of fluorescent intensity) are mentioned.
pp 231-235 March 1995
Pulsed photoacoustic measurements have been carried out in toluene at 532 nm wavelength using a Q-switched frequency doubled Nd:YAG laser. The variation of photoacoustic signal amplitude with incident laser power indicates that at lower laser powers one photon absorption takes place at this wavelength while a clear two photon absorption occurs in this liquid at higher laser powers. The studies made here demonstrate that pulsed photoacoustic technique is simple and effective for the investigation of multiphoton processes in liquids.
pp 237-248 March 1995
Closed-form analysis of pulse evolution in the transient regime in an actively modelocked internally frequency doubled broad-band continuous wave laser is presented. The analysis enables investigation of the effect of nonlinear crystal inside the laser cavity on mode-locked pulse parameters at the fundamental frequency. It is shown that the presence of internal second harmonic crystal broadens the fundamental mode-locked pulse while accelerating the approach of the system to steady-state. The dependence of pulse parameters on the bandwidth of the tuning element, modulation depth of the active mode-locker and conversion efficiency of the frequency doubler is presented in detailed graphical form.
pp 249-256 March 1995
The absorption/gain induced self tapering and uptapering of a self guided Gaussian laser beam when the beam propagates in a saturable nonlinear medium has been discussed. Using the WKBJ and abberationless paraxial ray approximation a beam width equation has been derived. Conditions for tapering/uptapering for both cases i.e., for absorption and gain have been discussed. The beam width of tapered/uptapered beam depends on the rate of absorption/gain, and the length of absorption/gain region along the beam propagation direction. The importance of the present study is pointed out.
pp 257-262 March 1995
We study the evolution of the resonant waves considering its interaction with the nonresonant waves and the plasma particles due to plasma-maser effect. The nonlinear dielectric function of the resonant wave is calculated and is found to consist of two parts: the direct and the polarization coupling terms. On the other hand, the nonlinear dielectric function of the nonresonant wave consists only of the direct coupling term. The significance of our results is discussed.
pp 263-270 March 1995
Experimental results on the measurement of current collected by an electrode immersed in a plasma for a pulsed negative bias are presented. The measured current is compared with a model based on the concept of an expanding capacitor. The scaling laws predicted by the model are verified for the measured current which agree each other. The paper emphasizes the role of displacement current in an expanding ion sheath.
pp 271-277 March 1995
An expression for the static structure factor,g+− (r), of electrons at a distancer from an infinitely heavy positively charged particle in a one component quantum rare plasma has been obtained in linear response theory using an appropriate quantum dielectric function of the rare plasma. The expression is a complicated function of the electron plasma frequency, Debye screening length andr, but reduces to that of classical plasma when quantum corrections are neglected. Forr<rs (2rs being the mean distance between two electrons), the temperature dependentg+− (r) has larger values in quantum case in comparison to that in classical situation and keeps increasing with decrease inr, more so at low temperatures when de-Broglie wavelength becomes larger and a considerable fraction ofrs.
pp 279-279 March 1995 Erratum
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