Character of the reaction between molecular hydrogen and silicon dangling bond in amorphous SiO_2

The passivation by diffusing H2 of silicon dangling bond defects (E' centers, induced by laser irradiation in amorphous SiO_2 (silica), is investigated in situ at several temperatures. It is found that the reaction between E' center and H_2 requires an activation energy of 0.38eV, and that its kinetics is not diffusion-limited. The results are compared with previous findings on the other fundamental paramagnetic point defect in silica, the non bridging oxygen hole center, which features completely different reaction properties with H_2. Besides, a comparison is proposed with literature data on the reaction properties of surface E' centers, of E' centers embedded in silica films, and with theoretical calculations. In particular, the close agreement with the reaction properties of surface E' centers with H_2 leads to conclude that the bulk and surface E' varieties are indistinguishable from their reaction properties with molecular hydrogen.Comment: 15 pages, 3 figures, submitted to J. Phys. Chem.

Bleaching of optical activity induced by UV Laser exposure in natural silica

We report experimental data on two types of natural silica, differing for their OH content, irradiated with UV photons (4.66 eV) from a pulsed Nd:YAG laser. Irradiation induces a reduction of the absorption band at 5.12eV and of the associated emissions at 3.14eV and 4.28eV, ascribed to twofold coordinated Ge (=Ge'') centers pre-existing in our samples. The bleaching is mainly due to the post-irradiation conversion of =Ge'' into the paramagnetic H(II) center via trapping of a H atom. Comparison with literature data points out the peculiarities of silica with a low Ge concentration as regards UV induced transformations.Comment: Accepted for publication on Journal of Non crystalline solid

Hydrogen-Related Conversion Processes of Ge-Related Point Defects in Silica Triggered by UV Laser Irradiation

The conversion processes of Ge-related point defects triggered in amorphous SiO2 by 4.7eV laser exposure were investigated. Our study has focused on the interplay between the (=Ge•-H) H(II) center and the twofold coordinated Ge defect (=Ge••). The former is generated in the post-irradiation stage, while the latter decays both during and after exposure. The post-irradiation decay kinetics of =Ge•• is isolated and found to be anti-correlated to the growth of H(II), at least at short times. From this finding it is suggested that both processes are due to trapping of radiolytic H0 at the diamagnetic defect site. Furthermore, the anti-correlated behavior is preserved also under repeated irradiation: light at 4.7eV destroys the already formed H(II) centers and restore their precursors =Ge••. This process leads to repeatability of the post-irradiation kinetics of the two species after multiple laser exposures. A comprehensive scheme of chemical reactions explaining the observed post-irradiation processes is proposed and tested against experimental data.Comment: 25 pages, 7 figures, submitted to Phys. Rev.

Behavior of rod-like polyelectrolytes near an oppositely charged surface

The behavior of highly charged short rod-like polyelectrolytes near oppositely charged planar surfaces is investigated by means of Monte Carlo simulations. A detailed microstructural study, including monomer and fluid charge distribution, and chain orientation, is provided. The influence of chain length, substrate's surface-charge-density and image forces is considered. Due to the lower chain-entropy (compared to flexible chains), our simulation data show that rod-like polyelectrolytes can, in general, better adsorb than flexible ones do. Nonetheless, at low substrate-dielectric-constant, it is found that repulsive image forces tend to significantly reduce this discrepancy.Comment: Updated results - 19 pages - 10 figs - RevTex

Unitary decoupling treatment of a quadratic bimodal CQED model

We consider a two-photon quantum model of radiation-matter interaction between a single two-level atom and a degenerate bimodal high-Q cavity field. Within this tripartite system the explicit construction of two collective radiation modes, one of which freely evolving and the other one quadratically coupled to the matter subsystem, is reported. Meaning and advantages of such a decoupling treatment are carefully discussed.Comment: accepted for publication in Physica Script

Influence of hydrogen on paramagnetic defects induced by UV laser exposure in natural silica

Diffusion limited reactions of point defects were investigated in amorphous SiO2 exposed to UV laser light. Electron spin resonance and in situ absorption measurements at room temperature evidenced the annealing of E' centers and the growth of H(II) centers both occurring in the post-irradiation stage and lasting a few hours. These transients are caused by reactions involving molecular hydrogen H2, made available by dimerization of radiolytic H0.Comment: Submitted to Physica Status Solid

Geometric phase accumulation-based effects in the quantum dynamics of an anisotropically trapped ion

New physical effects in the dynamics of an ion confined in an anisotropic two-dimensional Paul trap are reported. The link between the occurrence of such manifestations and the accumulation of geometric phase stemming from the intrinsic or controlled lack of symmetry in the trap is brought to light. The possibility of observing in laboratory these anisotropy-based phenomena is briefly discussed.Comment: 10 pages. Acta Physica Hungarica B 200

Zeno Dynamics and High-Temperature Master Equations Beyond Secular Approximation

Complete positivity of a class of maps generated by master equations derived beyond the secular approximation is discussed. The connection between such class of evolutions and physical properties of the system is analyzed in depth. It is also shown that under suitable hypotheses a Zeno dynamics can be induced because of the high temperature of the bath.Comment: 9 pages, 2 figure

Stability of E' centers induced by 4.7eV laser radiation in SiO2

The kinetics of E' centers (silicon dangling bonds) induced by 4.7eV pulsed laser irradiation in dry fused silica was investigated by in situ optical absorption spectroscopy. The stability of the defects, conditioned by reaction with mobile hydrogen of radiolytic origin, is discussed and compared to results of similar experiments performed on wet fused silica. A portion of E' and hydrogen are most likely generated by laser-induced breaking of Si-H precursors, while an additional fraction of the paramagnetic centers arise from another formation mechanism. Both typologies of E' participate to the reaction with H_2 leading to the post-irradiation decay of the defects. This annealing process is slowed down on decreasing temperature and is frozen at T=200K, consistently with the diffusion properties of H_2 in silica.Comment: 12 pages, 3 figures, in press on J. Non cryst. solids (2007