Search Results (5 total)

We have determined the lifetime of the Li-like ₂₂⁴⁸Ti  1s2s2p  ⁴P_5∕2^o level (210.5±13.5  ps) using data from its x-ray decay channel through beam single- and two-foil experiments, coupled to a multicomponent iterative growth and decay analysis. Theoretical lifetime estimates for this zero-nuclear-spin ion lies within the uncertainty range of our experimental results, indicating that blending contributions to this level from the He-like 1s2p  ³P₂^o and 1s2s  ³S₁ levels are eliminated within the current approach. A previously reported discrepancy between experimental and theoretical 1s2s2p  ⁴P_5∕2^o level lifetimes in ₂₃⁵¹V may, as a result, be attributed to hyperfine quenching.

The option of varying thickness of the fixed foil in the beam–two-foil technique has been incorporated in our experimental setup. It gives an opportunity to explore a new mode of this technique. In the current study, we have investigated the lifetime of the 1s2s2p  ⁴P_5∕2^o level in Li-like titanium using this technique. The data showed a dependence of foil thickness of the fixed foil on the interactions of levels produced in the first foil. Average lifetime obtained for the Li-like titanium 1s2s2p  ⁴P_5∕2^o level (200±12  ps) is compared very well with the earlier theoretical and experimental values.

During rapid solidification, solute may be incorporated into the solid phase at a concentration significantly different from that predicted by equilibrium thermodynamics. This process, known as solute trapping, leads to a progressive reduction in the concentration change across the interface as the solidification rate increases. Theoretical treatments of rapid solidification using traditional sharp-interface descriptions require the introduction of separately derived nonequilibrium models for the behavior of the interfacial temperature and solute concentrations. In contrast, phase-field models employ a diffuse-interface description and eliminate the need to specify interfacial conditions separately. While at low solidification rates equilibrium behavior is recovered, at high solidification rates nonequilibrium effects naturally emerge from these models. In particular, in a previous study we proposed a phase-field model of a binary alloy [A. A. Wheeler et al., Phys. Rev. E 47, 1893 (1993)] in which we demonstrated solute trapping. Here we show that solute trapping is also possible in a simpler diffuse interface model. We show that solute trapping occurs when the solute diffusion length D_I/V is comparable to the diffuse interface thickness. Here V is the interface velocity and D_I characterizes the solute diffusivity in the interfacial region. We characterize the dependence of the critical speed for solute trapping on the equilibrium partition coefficient k_E that shows good agreement with experiments by Aziz and co-workers [see M. J. Aziz, Metall. Mater. Trans. A 27, 671 (1996)]. We also show that in the phase-field model, there is a dissipation of energy in the interface region resulting in a solute drag, which we quantify by determining the relationship between the interface temperature and velocity.

The heat capacity of Spectrosil-WF (vitreous silica containing <20 ppm of OH) in the temperature range from 2 to 16 K as well as the Raman measurements are presented. The heat capacity is similar to that of Spectrosil-B (vitreous silica containing 1200 ppm of OH) but is about 20% higher than the Heralux (vitreous silica containing 130–180 ppm of OH). The density of low-frequency vibrational states, g(ν), has been determined. It has been found that the form of g(ν) is nonquadratic, which can be explained on the basis of a soft-potential model. About 110 atoms participate in a soft mode. The Raman and infrared coupling constants have similar dependence on frequency. These are explained on the basis of a single-coupling-constant bond model. It is found that the form of the Raman coupling constant cannot be explained on the basis of Martin-Brenig theory.

Measurements of the elastic and inelastic neutron scattering from vitreous silica in the frequency range 0.3 to 4 THz and with scattering vectors in the range 0.2 to 5.3 Å^-1 are analyzed in conjunction with heat-capacity measurements on the same samples to provide a microscopic description of low-frequency vibrational modes. The results show that additional harmonic excitations coexist with sound waves below 1 THz, and that these excitations correspond to relative rotation of SiO₄ tetrahedra.