Information-theoretic analysis of quantum mechanical reaction cross sections

Information theory is used to analyze quantum mechanical rotation excitation probabilities for the hydrogen exchange reaction. A linear rotational surprisal is found, and the entropy deficiency of the product rotational distribution is computed.     

Experimental and Theoretical Study of High Temperature-Stability and Low-Chirp 1.55 µm Semiconductor Laser with an External Fiber Grating

An experimental and theoretical study of the effect of temperature on the static and dynamic characteristics of packaged external fiber grating semiconductor lasers (FGL) is reported on. Operating in single frequency mode, the laser exhibits high output power (> 8 mW), high temperature stability of operating frequency (-3.4 GHz/K), and low static chirp (-60 MHz/mA). The observed hysteresis in wavelength versus temperature dependence is explained in the frame of a time-domain FGL model accounting for asymmetric nonlinear gain. The laser has low dynamic chirp (~16 MHz/mA) under 2.5 GB/s direct modulation, which is the key factor determining low penalty transmission over 312 km of SSM fiber. Dense WDM transmission performed at 2.6 Gbit/s over 117 km of SSM fiber shows that an FGL-based transmitter is a factor of 7 more tolerant to temperature variations than externally modulated DFB lasers.     

Wave front-ray synthesis for solving the multidimensional quantum Hamilton-Jacobi equation

A Cauchy initial-value approach to the complex-valued quantum Hamilton-Jacobi equation (QHJE) is investigated for multidimensional systems. In this approach, ray segments foliate configuration space which is laminated by surfaces of constant action. The QHJE incorporates all quantum effects through a term involving the divergence of the quantum momentum function (QMF). The divergence term may be expressed as a sum of two terms, one involving displacement along the ray and the other incorporating the local curvature of the action surface. It is shown that curvature of the wave front may be computed from coefficients of the first and second fundamental forms from differential geometry that are associated with the surface. Using the expression for the divergence, the QHJE becomes a Riccati-type ordinary differential equation (ODE) for the complex-valued QMF, which is parametrized by the arc length along the ray. In order to integrate over possible singularities in the QMF, a stable and accurate Mo?bius propagator is introduced. This method is then used to evolve rays and wave fronts for four systems in two and three dimensions. From the QMF along each ray, the wave function can be easily computed. Computational difficulties that may arise are described and some ways to circumvent them are presented.     

Nucleation and Growth Synthesis of Siloxane Gels to Form Functional,Monodisperse, and Acoustically Programmable Particles

Nucleation and growth methods offer scalable means of synthesizing colloidal particles with precisely specified size for applications in chemical research, industry, and medicine. These methods have been used to prepare a class of silicone gel particles that display a range of programmable properties and narrow size distributions. The acoustic contrast factor of these particles in water is estimated and can be tuned such that the particles undergo acoustophoresis to either the pressure nodes or antinodes of acoustic standing waves. These particles can be synthesized to display surface functional groups that can be covalently modified for a range of bioanalytical and acoustophoretic sorting applications.     

Lateral shearing interferometry of high-harmonic wavefronts

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects.     

Intramolecular CH Activation and Metallacycle Aromaticity in the Photochemistry of [FeFe]‐Hydrogenase Model Compounds in Low‐Temperature Frozen Matrices

The [FeFe]‐hydrogenase model complexes [(μ‐pdt){Fe(CO)₃}₂], [(μ‐edt){Fe(CO)₃}₂], and [(μ‐mdt){Fe(CO)₃}₂], where pdt=1,3‐propanedithiolate, edt=1,2‐ethanedithiolate, and mdt=methanedithiolate, undergo wavelength dependent photodecarbonylation in hydrocarbon matrices at 85 K resulting in multiple decarbonylation isomers. As previously reported in time‐resolved solution photolysis experiments, the major photoproduct is attributed to a basal carbonyl‐loss species. Apical carbonyl‐loss isomers are also generated and may undergo secondary photolysis, resulting in β‐hydride activation of the alkyldithiolate bridge, as well as formation of bridging carbonyl isomers. For [(μ‐bdt){Fe(CO)₃}₂], (bdt=1,2‐benzenedithiolate), apical photodecarbonylation results in generation of a 10 π‐electron aromatic FeS₂C₆H₄ metallacycle that coordinates the remaining iron through an η⁵ mode.     

Structure and fragmentation of dimethyl methylphosphonate and trimethyl phosphite

The structures and fragmentation pathways of two isomeric organophosphorus esters, dimethyl methylphosphonate (DMMP) and trimethyl phosphite (TMP) have been determined. The long-lived, low-energy molecular ions of DMMP were found to undergo a keto-to-enol isomerization prior to collision-induced dissociation. This isomerization was established through the comparison of the collision spectra from DMMP, TMP, isotopically labeled DMMP and a model precursor ion. Electron ionization and charge exchange reactions were used to study the isomerization as a function of the internal energy of the molecular ion. The structure of the TMP molecular ion retained the structure of the neutral molecule. The daughter ion spectra of the isomeric fragment ions from DMMP and TMP were used to infer the fragment ion structures. Negative ions of DMMP and TMP were also studied, and their collision spectra were found to be indistinguishable.