Ionization-induced Effects Research Articles

Spectral broadening and conical emission of near-infrared femtosecond laser pulses in air

Spectral and spatial properties of infrared femtosecond laser pulses with a wavelength between 1.2 and 1.6 μm propagating in air are investigated both, experimentally and theoretically. It is shown that under tight focusing conditions laser pulses experience asymmetric spectral broadening (SB) and a strong blue-shift with respect to the incident laser wavelength. The observed blue-shifted radiation is emitted conically, in contrast to the weaker red-shifted on-axis emission. The SB and frequency shifts were interpreted in terms of ionization-induced effects.

13C structuring shifts for the analysis of model β-hairpins and β-sheets in proteins: diagnostic shifts appear only at the cross-strand H-bonded residues

The present studies have shown that (13)C=O, (13)C(α) and (13)C(β) of H-bonded strand residues in β-hairpins provide additional probes for quantitating the extent of folding in β-hairpins and other β-sheet models. Large differences in the structuring shifts (CSDs) of these (13)C sites in H-bonded versus non-H-bonded sites are observed: the differences between H-bonded and non-H-bonded sites are greater than 1.2 ppm for all three (13)C probes. This prompts us to suggest that efforts to determine the extent of hairpin folding from (13)C shifts should be based exclusively on the observation at the cross-strand H-bonded sites. Furthermore, the statistics suggest the (13)C' and (13)C(β) CSDs will provide the best differentiation with 100%-folded CSD values approaching -2.6 and +3 ppm, respectively, for the H-bonded sites. These conclusions can be extended to edge-strands of protein β-sheets. Our survey of reported (13)C shifts in β-proteins indicates that some of the currently employed random coil values need to be adjusted, particularly for ionization-induced effects.

Ionization-induced effects in amorphous apatite at elevated temperatures

Electron-beam-induced effects in preamorphized Sr2Nd8(SiO4)6O2 were investigated in situ using transmission electron microscopy with 200-keV electrons at temperatures ranging from 380 to 780 K. Within the electron-irradiated area, epitaxial recrystallization was observed from the amorphous/crystalline interface toward the surface, with the rate of recrystallization increasing as temperature increased from 380 to 580 K. Structural contrast features (i.e., O deficient amorphous material), as well as recrystallization, were observed outside of the irradiation area at temperatures from 680 to 780 K. Ionization-induced processes and local nonstoichiometry induced by oxygen migration and desorption are possible mechanisms for the electron-beam- induced recrystallization and for the formation of the structural contrast features, respectively.

Heavy Ion Energy Effects in CMOS SRAMs

The impact of heavy ion energy on SEU and SEL in commercial and radiation-hardened CMOS SRAMs is explored. Provided the ion range is large enough for the ions to reach the sensitive device region, standard low-energy heavy ion testing is conservative with respect to high-energy heavy ions, at least for LETs above the threshold for direct ionization-induced upsets. However, below the threshold LET for direct ionization-induced effects we find some differences between low- and high-energy tests. These differences are attributed to the effects of nuclear reaction-induced secondary particles. Implications for hardness assurance testing and error rate calculations are discussed.

Ionization-induced effects in the soliton dynamics of high-peak-power femtosecond pulses in hollow photonic-crystal fibers

Ionization phenomena are shown to modify the soliton propagation dynamics of high-peak-power laser pulses in hollow-core photonic-crystal fibers (PCFs). Based on the numerical solution of the pulse-evolution equation for a high-peak-power laser field in an ionizing gas medium in a hollow PCF, we demonstrate that hollow PCFs filled with gases having high ionization potentials ${I}_{p}$ can support soliton transmission regimes for gigawatt femtosecond laser pulses. In hollow PCFs filled with low-${I}_{p}$ gases, on the other hand, the ionization-induced change in the refractive index of the gas leads to a blueshifting of soliton transients, pushing their spectrum beyond the point of zero group-velocity dispersion, thus preventing the formation of stable high-peak-power solitons.

Ionization Induced Scattering of Femtosecond Intense Laser Pulses in Cluster Plasmas

The 45° scattering of a femtosecond (60 fs) intense laser pulse with a 20 nm FWHM (the full width at half maximum) spectrum centered at 790 nm has been studied experimentally while focused in argon clusters at intensity-1016 W/cm2. Scattering spectra under different backing pressures and laser-plasma interaction lengths were obtained, which showed spectral blueshifting, beam refraction and complex modulation. These ionization-induced effects reveal the modulation of laser pulses propagating in plasmas and the existing obstacle in laser cluster interaction at high laser intensity and high electron density.

High-temperature and ionization-induced effects in lithium-doped MgO single crystals

Thermal quenching of Li-doped MgO crystals from temperatures in excess of 1300 K results in the formation of ${[\mathrm{Li}]}^{0}$ defects, which are substitutional lithium ions each with a bound hole at a neighboring oxygen site. Unlike ${[\mathrm{Li}]}^{0}$ defects generated at low temperatures by a low dose of ionizing radiation, those formed by quenching are stable against hole release well above room temperature. This stability is much higher than for all previously reported hole defects in alkaline-earth oxides. These stable ${[\mathrm{Li}]}^{0}$ defects have also been produced by high-dose electron irradiations at both 290 and 85 K, with formation cross sections of ${\mathrm{\ensuremath{\sim}}10}^{5}$ and ${10}^{3}$ barns, respectively. The magnitude and temperature dependence of the cross section suggests a radiation-induced diffusion, rather than an elastic collision, process as the production mechanism. These defects anneal at 450 K in the electron-irradiated crystals and 830 K in the quenched ones. A model involving localized regions of high lithium concentration is proposed in order to explain the stability of the bound hole at the ${[\mathrm{Li}]}^{0}$ defect.