Four-wave mixing with non-resonant pump and resonant probe for OH detection in flames

We report a new long-term interferometric stabilization scheme for a delay line to be used for pump–probe spectroscopy on the attosecond time scale. A separate interferometer with a He–Ne laser beam is used to stabilize the delay line with respect to a slow drift in the optical delay induced mainly by ambient temperature changes in the laboratory. The power of the stabilization scheme is demonstrated with the characterization of an attosecond pulse train in the extreme ultraviolet formed through high order harmonic generation of Ti:sapphire laser radiation. We use the RABBITT technique for the attosecond pulse-train characterization in a reaction microscope employed here for photoelectron–photoion-coincidence spectroscopy. PACS 42.65.Ky; 42.65.Re; 07.60.Ly; 07.81.+a; 32.80.Fb     

Multi electron capture processes in slow collisions between X<Superscript>7+</Superscript> (X = N,O and Ne) ions with C<Subscript>60</Subscript>

Electron capture processes in collision between slow X⁷⁺ (X = N, O and Ne) ions and C₆₀ fullerene have been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of the multicharged C₆₀ ^r+ recoil ions and their fragments C_m ⁱ⁺ and the final charge state of the outgoing projectiles X^(q-s)+ ( ). The collision velocity is about 0.4 a.u. The partial cross-sections σ_r ^s , corresponding to r electrons transferred to the projectile with only s electrons stabilized, have been measured. Cross-sections for collisions “inside” and those “outside” the C₆₀ cage have been separated by analyzing the kinetic energy of the outgoing projectile. The mean final charge state for frontal collisions has been measured to 3.1, 2.6 and 2.5 for N⁷⁺, O⁷⁺ and Ne⁷⁺ respectively. These results show the importance of the core effect on the stabilisation processes of captured electrons.     

Optimization of F-doped SnO2 electrodes for organic photovoltaic devices

Fluorine-doped tin oxide (FTO) thin films have been investigated as an alternative to indium tin oxide anodes in organic photovoltaic devices. The structural, electrical, and optical properties of the FTO films grown by pulsed laser deposition were studied as a function of oxygen deposition pressure. For 400 nm thick FTO films deposited at 300°C and 6.7 Pa of oxygen, an electrical resistivity of 5×10⁻⁴ Ω-cm, sheet resistance of 12.5 Ω/□, average transmittance of 87% in the visible range, and optical band gap of 4.25 eV were obtained. Organic photovoltaic (OPV) cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C₆₁-butyric acid methyl ester bulk heterojunctions were prepared on FTO/glass electrodes and the device performance was investigated as a function of FTO film thickness. OPV cells fabricated on the optimum FTO anodes (∼300–600 nm thick) exhibited power conversion efficiencies of ∼3%, which is comparable to the same device made on commercial ITO/glass electrodes (3.4%).     

Mechanical properties of silk: interplay of deformation on macroscopic and molecular length scales

Using an in situ combination of tensile tests and x-ray diffraction, we have determined the mechanical properties of both the crystalline and the disordered phase of the biological nanocomposite silk by adapting a model from linear viscoelastic theory to the semicrystalline morphology of silk. We observe a strong interplay between morphology and mechanical properties. Silk's high extensibility results principally from the disordered phase; however, the crystals are also elastically deformed.     

Freezing coherent field growth in a cavity by the quantum zeno effect

We have frozen the coherent evolution of a field in a cavity by repeated measurements of its photon number. We use circular Rydberg atoms dispersively coupled to the cavity mode for an absorption-free photon counting. These measurements inhibit the growth of a field injected in the cavity by a classical source. This manifestation of the quantum Zeno effect illustrates the backaction of the photon number determination onto the field phase. The residual growth of the field can be seen as a random walk of its amplitude in the two-dimensional phase space. This experiment sheds light onto the measurement process and opens perspectives for active quantum feedback.     

M1 gamma strength for zirconium nuclei in the photoneutron channel

Photoneutron cross sections were measured for 91Zr, 92Zr, and 94Zr near the neutron separation energy with quasimonochromatic gamma rays. The data exhibit some extra components around the neutron threshold. A coherent analysis of the photoneutron data for 92Zr together with the neutron capture on 91Zr based on the microscopic Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation model for the E1 strength has revealed the presence of an M1 resonance at 9 MeV. The microscopic approach systematically shows the same M1 strength in the photoneutron cross section for 91Zr and 94Zr. The total M1 strength is about 75% larger than the strength predicted by the systematics, being qualitatively consistent with the giant M1 resonance observed in the inelastic proton scattering.     

Local crystallization in an oxyfluoride glass doped with Er3+ ions using a continuous argon laser

Local crystalline formation in erbium doped oxyfluoride glass has been obtained under a cw Argon laser irradiation up to 1.8 W pumping power. By exciting at 514 nm, the emission from 800 nm and 850 nm corresponding to the ⁴S_3/2(²H_11/2)→⁴I_13/2 electronic transitions have been analyzed both inside and outside the irradiated area. The changes in the emission spectra indicate that the high power Ar laser irradiation has resulted in a localized desvitrification process. The temperature dependence of the fluorescence intensity ratio of the 800 nm and 850 nm emission bands has been used to determine the temperature of the irradiated zone. Moreover, the average lifetime of the ⁴S_3/2(²H_11/2) thermalized levels have been measured as a function of the excitation spot position. An important decrease is observed at the irradiated area. These results confirm that a localized cristalline phase has been created by the laser action.     

Lee–Yang Problems and the Geometry of Multivariate Polynomials

We describe all linear operators on spaces of multivariate polynomials preserving the property of being non-vanishing in open circular domains. This completes the multivariate generalization of the classification program initiated by Pólya–Schur for univariate real polynomials and provides a natural framework for dealing in a uniform way with Lee–Yang type problems in statistical mechanics, combinatorics, and geometric function theory.