High-resolution gamma-ray radiography produced by a laser-plasma driven electron source

An electron beam from a laser-plasma accelerator is converted into a gamma-ray source using bremsstrahlung radiation in a dense material. The gamma-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this gamma-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a gamma-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.     

Comparison of dephasing times for vibrational and rotational coherent anti-Stokes Raman scattering: implications for velocimetry

Time-domain coherent anti-Stokes Raman scattering experiments have been carried out by probing vibrational and pure rotational lines of nitrogen in the Doppler broadened regime. The theoretical analysis of the transient responses outlines the role of the geometrical effects. For pure rotational CARS, it is shown that the main contribution to the dephasing of the Raman coherence results from the change in direction between the pump and anti-Stokes wave vectors whereas the difference between the modulus of these two wave vectors accounts for dephasing in vibrational CARS. Furthermore, we demonstrate that the range of operation of time-domain CARS velocimetry is extended by probing pure rotational lines. The predictions are validated by experiments which are performed both in a static gas cell and in a Mach 10 supersonic flow. Received: 30 March 2000 / Revised version: 9 June 2000 / Published online: 13 September 2000     

Making molecular balloons in laser-induced explosive boiling of polymer solutions

The effect of the dynamic molecular rearrangements leading to compositional segregation is revealed in coarse-grained molecular dynamics simulations of short pulse laser interaction with a polymer solution in a volatile matrix. An internal release of matrix vapor at the onset of the explosive boiling of the overheated liquid is capable of pushing polymer molecules to the outskirts of a transient bubble, forming a polymer-rich surface layer enclosing the volatile matrix material. The results explain unexpected "deflated balloon" structures observed in films deposited by the matrix-assisted pulsed laser evaporation technique.     

Active mode locking of continuous-wave doubly and singly resonant optical parametric oscillators

We report on what we believe to be the first active mode locking of near-degenerate, doubly and singly resonant cw-pumped optical parametric oscillators (OPOs). We show experimentally that a steady-state regime of short pulses is reached in a few tens of microseconds under cw pumping. The oscillation buildup dynamics of both OPOs is also explored, evidencing an unusual transient behavior in the mode-locking process.     

Semiclassical model for accidental predissociation in diatomic molecules

After a short review of possible quantum treatments, the uniform semiclassical theory of Child (J. Mol. Spectrosc.53, 280 (1974)) is applied to the problem of accidental predissociation in a diatomic molecule where a bound state dissociates into a continuum via a coupling with a near-degenerate second bound state. A complete connection diagram and an analytical expression for the lineshape is provided. This expression is used in the diabatic limit to derive the widths and shifts of the resonances. Checks are made for two test cases for which a simplified quantum model leads to analytical formulas. The familiar case in which each resonance shares the initial width of the perturbing state according to its weight in the configuration interaction wavefunction is verified. In the opposite coupling scheme, the “resonance narrowing” phenomenon predicted by the quantum model is also obtained.     

Lattice dynamics in RbCN

The dispersion of acoustic and optical phonons of rubidium cyanide has been measured at 300 K. The acoustic branches are affected by rotation-translation coupling in a way similar to the findings in KCN. The data are explained by a model which combines a shell model and rotation-translation coupling.     

A primal-dual algorithm for the fermat-weber problem involving mixed gauges

We give a new algorithm for solving the Fermat-Weber location problem involving mixed gauges. This algorithm, which is derived from the partial inverse method developed by J.E. Spingarn, simultaneously generates two sequences globally converging to a primal and a dual solution respectively. In addition, the updating formulae are very simple; a stopping rule can be defined though the method is not dual feasible and the entire set of optimal locations can be obtained from the dual solution by making use of optimality conditions. When polyhedral gauges are used, we show that the algorithm terminates in a finite number of steps, provided that the set of optimal locations has nonepty interior and a counterexample to finite termination is given in a case where this property is violated. Finally, numerical results are reported and we discuss possible extensions of these results.     

Computational and experimental study of the cluster size distribution in MAPLE

A combined experimental and computational study is performed to investigate the origin and characteristics of the surface features observed in SEM images of thin polymer films deposited in matrix-assisted pulsed laser evaporation (MAPLE). Analysis of high-resolution SEM images of surface morphologies of the films deposited at different fluences reveals that the mass distributions of the surface features can be well described by a power-law, Y(N) ∝ N^−t, with exponent −t ≈ −1.6. Molecular dynamic simulations of the MAPLE process predict a similar size distribution for large clusters observed in the ablation plume. A weak dependence of the cluster size distributions on fluence and target composition suggests that the power-law cluster size distribution may be a general characteristic of the ablation plume generated as a result of an explosive decomposition of a target region overheated above the limit of its thermodynamic stability. Based on the simulation results, we suggest that the ejection of large matrix-polymer clusters, followed by evaporation of the volatile matrix, is responsible for the formation of the surface features observed in the polymer films deposited in MAPLE experiments.