Description of pulse propagation in a dispersive medium by use of a pulse quality factor

We investigated how the duration of short laser pulses evolves in a dispersive material, using rms widths and a propagation law based on a pulse quality factor. Experiments were carried out with femtosecond pulses (10 to 25fs at the temporal waist) propagating in bulk fused silica. Excellent agreement was found between theory and experiment. This approach does not require complete characterization of laser pulses and eliminates the need for any assumption regarding the interpretation of autocorrelation traces. The method is of general validity, and it can be applied to pulses of arbitrary shape.     

MR detection of mechanical vibrations using a radiofrequency field gradient

A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials.     

A Comparative Study of Enzymatically and Photochemically Polymerized Artificial Lignin Supramolecular Structures Using Environmental Scanning Electron Microscopy

Environmental scanning electron microscopy images of the self-assembled structures of enzymatically (DHP) and photochemically polymerized (PCP) artificial lignin are herein presented. Differences in the structural organization between DHP and PCP polymer at the supramolecular level were reported. Based on topological information, we proposed a hypothesis about possible new physiological roles of lignin in live plant cells and the ecological significance of possible in-vivo photochemical lignin polymerization. Copyright 2000 Academic Press.     

The ‘fringe method’ and its application to the measurement of deformations,vibrations, contour lines and differences of objects

A variable sensitivity technique using the projection of a fringe pattern is demonstrated for mapping deformations, vibrations, contour lines and differences between two objects. The method does not require laser illumination and gives an opportunity for studying large objects with incoherent light, and is compared with other techniques such as holography or speckle. The limitations likely to be encountered in industrial environments are discussed. The realization and incoherent projection of gratings is also discussed.     

Micro-Raman investigations of the formaldehyde-ice system

Rapidly frozen aqueous solutions containing variable amounts of dissolved formaldehyde (0.1, 5, 7, 10, 15, and 20 mol %) have been analyzed by micro-Raman spectroscopy at ambient pressure and low temperature. The importance of the formladehyde-ice system has been repeatedly quoted in various contexts, such as atmospheric and snowpack chemistry and interstellar and cometary ices. Understanding and characterizing the effects of freezing and the interactions of formaldehyde with ice are therefore of relevant interest. In this study, the distinct vibrational signatures of the oligomers present in the solution and in the frozen ice mixtures have been identified in the 120-4000 cm(-1) spectral range. From the subtle changes of the bands assigned to the CO and CH group frequencies, at least two distinct crystalline phases (pI and pII) are found to coexist with ice at different temperatures. Depending on the cooling-rewarming protocol, pI is found to crystallize in the 163-213 K temperature range. Above approximately 213 K, pI gets transformed irreversibly into pII which is stable up to approximately 234 K. pII is found to interact more strongly with ice than pI, as revealed, for example, by the drop in frequency of the bands assigned to the O-H stretching as pI transforms into pII. It is suggested that pII consists of a hydrogen-bonded network of oligomers and water molecules. On the other hand, it is suggested that the oligomers mainly present in pI interact through weak forces with the surrounding water molecules.     

Dynamic chemical devices: modulation of photophysical properties by reversible, ion-triggered, and proton-fuelled nanomechanical shape-flipping molecular motions

The terpy-derived (terpy=terpyridine) ligand 1 has an extended W shape in which the two appended photoactive pyrenyl groups are held apart. On binding of a zinc(II) ion with a terpy group, ligand 1 is converted into complex 2 whereby it adopts a U shape, thus stacking the aromatic units. This structural modification leads to a very pronounced change in photophysical properties: from a highly fluorescent free ligand to a very weakly emitting complex. The W/U structural switching can be reversibly induced by the addition of a competitive tren ligand, which binds and releases a zinc(II) ion under protonation/deprotonation cycles, thus leading to oscillations in light emission. Therefore, the present system performs periodic modulation of optical output through a nanomechanical shape-flipping motion, triggered by metal ion binding and fuelled by acid-base neutralisation energy. Overall, it represents an ion-triggered opto-mechanical supramolecular device.     

Adjustable cavity for host-guest recognition in cofacial bis-porphyrinic tweezer

The synthesis of a cofacial bis-porphyrinic tweezer bearing a tris-anthracenic spacer is reported. Its behavior as host has been evidenced as well as the ability of its cavity to adjust to guests of various sizes.     

Detection of acoustic waves by NMR using a radiofrequency field gradient

A B(1) field gradient-based method previously described for the detection of mechanical vibrations has been applied to detect oscillatory motions in condensed matter originated from acoustic waves. A ladder-shaped coil generating a quasi-constant RF-field gradient was associated with a motion-encoding NMR sequence consisting in a repetitive binomial 13;31; RF pulse train (stroboscopic acquisition). The NMR response of a gel phantom subject to acoustic wave excitation in the 20-200 Hz range was investigated. Results showed a linear relationship between the NMR signal and the wave amplitude and a spectroscopic selectivity of the NMR sequence with respect to the input acoustic frequency. Spin displacements as short as a few tens of nanometers were able to be detected with this method.     

Medium ring synthesis by radical ipso-substitution

A new approach to eight and nine membered ring synthesis is described in which a radical ipso-substitution reaction features as a key step.