Observations of focal conic domains in smectic liquid crystals aligned on patterned self-assembled monolayers

Patterned Self-Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of nCBs in their smectic-A phase were created using microcontact printing of functionalized organothiols on gold films. By patterning the surface with homeotropic and planar aligning SAMs, the location and formation of the focal conic domains (FCDs) can be controlled. Polarizing microscopy was used to study the formation of FCDs in circle, stripe and checkerboard pattern geometries. Fluorescent confocal microscopy (FCM) was used for the first time to measure the eccentricity of FCDs that form along a stripe pattern.     

Frequency locking and unlocking in a femtosecond ring laser with application to intracavity phase measurements

Frequency coupling between the counter-propagating pulses inside a femtosecond ring laser gyroscope is studied. It is shown that frequency lock-in results from the counter-propagating pulses overlapping at or near a scattering surface. Nonetheless, the absence of lock-in with a scatterer at the pulse crossing point is demonstrated for the special case of symmetric scattering. The decoupling of the counter-propagating pulses makes possible the measurement of non-reciprocal phase differences of 10^–5 and index differences of 10^–10 over 1 cm. Besides measuring rotations, applications are found in the measurement of small changes in the index of refraction, and electrical transients on the 100 fs time scale.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

The estimation of relaxation times from out-of-phase detected EPR spectra

A method is described by which the saturation-transfer electron paramagnetic resonance technique may be employed to determine values of T₁ and T₂ from experimental spectra. The effect of higher harmonics of the modulation field on the calculated out-of-phase spectrum is computed. The results suggest that the inclusion of higher harmonics is important to ensure that well-characterized values of especially T₂ may be obtained.     

Observation of high soft x-ray drive in large-scale hohlraums at the National Ignition Facility

The first soft x-ray radiation flux measurements from hohlraums using both a 96 and a 192 beam configuration at the National Ignition Facility have shown high x-ray conversion efficiencies of ～85%-90%. These experiments employed gold vacuum hohlraums, 6.4 mm long and 3.55 mm in diameter, heated with laser energies between 150-635 kJ. The hohlraums reached radiation temperatures of up to 340 eV. These hohlraums for the first time reached coronal plasma conditions sufficient for two-electron processes and coronal heat conduction to be important for determining the radiation drive.     

Ionization of carbon acids in liquid ammonia

The acidities of various carbon acids in liquid ammonia (LNH(3)) at room temperature were determined by NMR and rates of D-exchange. There is a reasonable linear correlation of the pK(a)s in LNH(3) with those in water and DMSO of slope 0.7 and 0.8, respectively. Carbon acids with an aqueous pK(a) of less than 12 are fully ionized in liquid ammonia. Nucleophilic substitution of benzyl chloride by carbanions in liquid ammonia generates a Br?nsted β(nuc) = 0.38.     

Anomalous uniform domain in a twisted nematic cell constructed from micropatterned surfaces

We have discovered an optically uniform type of domain that occurs in twisted nematic (TN) cells that are constructed from substrates chemically patterned with stripes via microcontact printing of self-assembled monolayers; such domains do not occur in TN cells constructed from uniform substrates. In such a cell, the azimuthal anchoring at the substrates is due to the elastic anisotropy of the liquid crystal rather than the conventional rubbing mechanism. A model is presented that predicts the relative stability of the twisted and anomalous states as a function of the material and design parameters.     

The kinetics and mechanisms of aromatic nucleophilic substitution reactions in liquid ammonia

The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFBs) readily undergo solvolysis in liquid ammonia and 2-nitrofluorobenzene is about 30 times more reactive than the 4-substituted isomer. Oxygen nucleophiles, such as alkoxide and phenoxide ions, readily displace fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. Using the pK(a) of the substituted phenols in liquid ammonia, the Br?nsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative of the removal of most of the negative charge on the oxygen anion and complete bond formation in the transition state and therefore suggests that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second-order rate constants generate a Br?nsted β(nuc) of 0.36 using either the pK(a) of aminium ion in acetonitrile or in water, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate. Nitrobenzene and diazene are formed as unusual products from the reaction between sodium azide and 4-NFB, which may be due to the initially formed 4-nitroazidobenzene decomposing to give a nitrene intermediate, which may then give diazene or be trapped by ammonia to give the unstable hydrazine which then yields nitrobenzene.