Non-CPMG Fast Spin Echo with full signal

The standard Fast Spin Echo sequence used in MR imaging relies on the CPMG condition. A consequence of this condition is that only one component of the transverse magnetization can be measured. To counter this, some phase modulation schemes (XY, MLEV.) for the pulse train have been proposed, but they are useful only over a very restricted range, close to pi, of the refocusing pulse rotation angle. Some other solutions not relying on phase modulation have also been suggested, but they destroy one half the available signal. Revisiting the phase modulation approach, J. Murdoch ("Second SMR Scientific Meeting," p. 1145, 1994) suggested that a quadratic phase modulation could generate a train of classical echoes. We show here that indeed a quadratic phase modulation has a very suitable property: after an adequate change of frame, the dynamic of the system composed of all the protons situated in one pixel can be seen as stationary. If the parameter of the quadratic phase modulation is well chosen, it is then possible to put the dynamic system in a combination of two suitable states and obtain a signal identical to the signal of a classical spin echo, at least for nutation of the refocusing pulse higher than, approximately, two radians.     

High-resolution Fourier spectrometry of the CO infrared emission spectrum: Perturbation analysis of the a′ 3Σ-a3Π system

The strongly perturbed $\text{a′}{}^3\text{Σ}\text{-a}{}^3\text{Π}$ system of CO was recorded under high resolution with a Fourier transform interferometer. Rotational perturbations observed in the $\text{a′}{}^3\text{Σ}\text{v = 0, 1, 3}$ levels, due to an interaction with high levels of the a state, were completely reduced. The treatment required a theoretical model which takes into account an off-diagonal spin-spin parameter and the centrifugal distortion of off-diagonal interactions between ³Π and ³Σ states.     

Intermittency of velocity time increments in turbulence

We analyze the statistics of turbulent velocity fluctuations in the time domain. Three cases are computed numerically and compared: (i) the time traces of Lagrangian fluid particles in a (3D) turbulent flow (referred to as the dynamic case); (ii) the time evolution of tracers advected by a frozen turbulent field (the static case); (iii) the evolution in time of the velocity recorded at a fixed location in an evolving Eulerian velocity field, as it would be measured by a local probe (referred to as the virtual probe case). We observe that the static case and the virtual probe cases share many properties with Eulerian velocity statistics. The dynamic (Lagrangian) case is clearly different; it bears the signature of the global dynamics of the flow.     

Ambient noise cross correlation in free space: theoretical approach

It has been experimentally demonstrated that the Green's function between two points could be recovered using the cross-correlation function of the ambient noise measured at these two points. This paper investigates the theory behind this result in the simple case of a homogeneous medium with attenuation.     

Delayed fracture in porous media

The fracture of porous media subjected to a constant load is studied. Contrary to homogeneous solids in which fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a porous medium can occur with a delay, allowing us to quantify the average lifetime of the unbroken material. We show that the average fracture probability, a key property for risk analysis in civil engineering, is given by the probability of crack nucleation. The nucleation process can be understood qualitatively by calculating the activation energy for crack nucleation, taking into account the porosity of the medium.     

Supramolecular assembly of strongly chemisorbed size- and shape-defined chiral clusters: S- and R-alanine on Cu(110)

The bonding and self-assembly of a chirally organized monolayer of alanine on the Cu(110) surface has been investigated using reflection-absorption infrared spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). This multitechnique approach has enabled an in-depth understanding of the hierarchy of chirality transfer: from a single adsorbed molecule, to size-defined chiral clusters, and then to an overall chiral assembly. The data have indicated that the alanine is in its anionic form, bound to the copper surface through the oxygens of the ionized carboxylate group and the nitrogen of the neutral amino group. Importantly, the methyl group is held away from the surface, resulting in direct chirality transfer into the footprint of the adsorbed alanine molecules, with the local adsorption motif for S-alanine being the mirror image of that created for R-alanine. STM has shown that S-alanine molecules self-organize to form size-defined chiral clusters of six or eight molecules at the surface, interspersed with chiral channels of bare metal. Together, these clusters and channels further self-assemble into a chiral array with one unique chiral domain sustained across the entire surface. A similar chiral assembly, but with the mirror organization, has been observed for R-alanine. Structural models for the individual clusters are proposed, and in conjunction with LEED data, overall models for these chiral phases of both S- and R-alanine have been constructed. Overall, this adsorption system has been found to be both strongly chemisorbed and capable of extensive intermolecular H-bonding, causing stresses that lead not only to the chiral self-organization of molecules but also to a specific self-organization of the empty chiral channels and spaces that intersperse the structure which, in turn, chirally assemble across macroscopic length scales to give a surface with global organizational chirality.     

Lagrangian velocity statistics in turbulent flows: effects of dissipation

We use the multifractal formalism to describe the effects of dissipation on Lagrangian velocity statistics in turbulent flows. We analyze high Reynolds number experiments and direct numerical simulation data. We show that this approach reproduces the shape evolution of velocity increment probability density functions from Gaussian to stretched exponentials as the time lag decreases from integral to dissipative time scales. A quantitative understanding of the departure from scaling exhibited by the magnitude cumulants, early in the inertial range, is obtained with a free parameter function D(h) which plays the role of the singularity spectrum in the asymptotic limit of infinite Reynolds number. We observe that numerical and experimental data are accurately described by a unique quadratic D(h) spectrum which is found to extend from h(min) approximately 0.18 to h(max) approximately 1.