Optical Single-Sideband Modulators and Their Applications

We review our recent work in the implementation of optical single-sideband (OSSB) modulation and in the application of this modulation format to microwave photonics and optical testing systems. We have developed an enhanced OSSB modulator for wide-band operation, the so-called bidirectional OSSB modulator, and a simplification of this scheme for narrow-band applications. These OSSB modulators are based on the use of a standard single-electrode Mach-Zehnder electro-optic modulator (MZ-EOM) and passive fiber-optic components. In both designs, the OSSB operation is shown to be independent of the MZ-EOM bias. Therefore, the optical modulation depth at the output of the device can be enhanced using minimum transmission biasing to provide suppression of the optical carrier. Finally, we demonstrate the application of OSSB modulators to the improvement of microwave/millimeter-wave subcarrier multiplexing fiber-optic links and to narrow-band fiber radio systems. In addition, we analyzed the feasibility and the limitations of an optical vector network analysis technique based on OSSB modulation that can provide sub-picometer wavelength resolution.     

Resonance Raman investigation of the short-time dynamics of the ultraviolet photodissociation of bromoform

We report ultraviolet resonance Raman spectra of bromoform (CHBr₃) in cyclohexane solution. The resonance Raman spectra show significant intensity in the overtones of the nominal Br-C-Br symmetric bend (v ₆), the nominal H-C-Br asymmetric bend (v3), the nominal Br-C-Br symmetric stretch (v ₂) and the nominal Br-C-Br asymmetric stretch (v ₅) vibrational modes suggesting that the short-time photodissociation dynamics have noticeable multidimensional character. The lack of strong combination bands between several of the Franck-Condon active modes suggests that more than one electronic transition contribute to the resonance Raman spectra. We briefly discuss the ultraviolet short-time photodissociation dynamics of bromoform and the potential implications for the secondary photodissociation reactions of the initially formed CHBr₂ radical.     

Energetics,electronic structures and geometries of didehydroazines

When calibrated against the available experimental data for didehydrobenzenes, RB3LYP/cc-pVTZ, QCISD/cc-pVTZ, CCSD(T)/cc-pVTZ, and G3 electronic structure calculations provide reliable predictions of standard enthalpies and singlet—triplet splittings in all possible isomers of didehydroazines that (with a possible exception of 2,6-didehydropyridine) possess singlet ground states. Singlet didehydroazines with larger numbers of nitrogen atoms turn out to be more prone to ring opening, as indicated by the fact that out of the 6, 11, 6 and 3 possible didehydropyridines, didehydrodiazines, didehydrotriazines, and didehydrotetrazines, respectively, 5, 7, 2 and none are actually found. Immediate proximity of the nitrogen atom to the formally triple carbon—carbon bond confers decreased thermodynamic stabilities and smaller singlet—triplet splittings on the species of the 1,2-didehydro type. Some of the aza analogues of singlet 1,3-didehydrobenzene are as stable as their 1,2-didehydro counterparts. The only existing aza analogue of singlet 1,4-didehydrobenzene is 2,5-didehydropyrazine, which is particularly stable and possesses a large singlet—triplet splitting, making it a feasible synthetic target. The present calculations indicate that the experimental standard enthalpies of formation of pyrimidine and pyrazine are in error.     

The vibrations of glyoxal,studied by ‘Multimode’, with a large amplitude motion,using an ab initio potential surface

The procedure ‘Multimode’ was introduced by Carter and Bowman [1998, J. chem. Phys., 108, 43971, to calculate the vibrations of polyatomic molecules using normal coordinates. Recently Carter and Handy have introduced an extension to ‘Multimode’ to include one large amplitude torsional vibration for molecules for which an analytical potential is available. This procedure is now extended to molecules for which ab initio potential data may be calculated. Glyoxal (CHOCHO) is studied using the density functional approach to generate the potential surface data. The full J = 0 vibrational spectra is presented.     

Discrete path sampling

A theoretical framework is developed for the calculation of rate constants by sampling connected pathways composed of local minima and transition states that link them together. The theory is applicable to two-state or effective two-state systems and is applied to permutational or morphological isomerization in a two-dimensional cluster of seven Lennard-Jones atoms, water clusters containing eight and nine molecules, and a cluster of 38 Lennard-Jones atoms, which exhibits a double funnel energy landscape.     

Conformations and thermodynamic properties of sulphur homocycles. II. The fluxional S+ 8 radical cation

According to both UB3LYP/6-311G* and UMP2/6-311G* calculations, the lowest energy conformer of the S⁺ ₈ radical cation possesses C_s symmetry. However, there are three other structures with low relative energies that do not exceed 6.5 kcal mol^?1. These conformers, which are found to be very prone to pseudorotation, are predicted to interconvert readily. The fluxional nature of S⁺ ₈ is expected to facilitate its reactions with organic species where a specific conformation is demanded by steric constraints, which may explain its high reactivity towards PAHs with crowded hydrogen atoms.     

Experimental verification of the Stokes-Einstein relation in liquid Fe—FeS at 5 GPa

Experiments have been performed at 5 GPa on liquid Fe-FeS in order to determine Fe and S self-diffusivity as a function of temperature. The viscosity of the sample was then obtained using the Stokes-Einstein relation. The results are in excellent agreement with previous experiments where the viscosity of a material of the same composition under similar conditions was measured directly. These results support high, near-metallic, values of diffusivity and low viscosity in liquid Fe-S up to a few hundred K above the eutectic temperature, in contrast with some previous studies. Moreover, these results fully confirm the validity of the Stokes-Einstein relation between viscosity and diffusion coefficients for Fe_0.61S_0.39.     

Numerical Solution of Volterra Integral Equations with Continuous or Discontinuous Terms

Convergence criteria are given for the numerical solution ofVolterra integral equations with and without discontinuous terms.Several numerical methods are compared.