Light emission from porous silicon single and multiple cavities

Experimental and theoretical techniques are used to examine the effects of microstructuring on the optical properties of multilayer, single and multiple microcavity structures fabricated from porous silicon. Measurements of the reflectivity and photoluminescence spectra of three multilayer samples are presented. The results are modelled using a transfer matrix technique including a negative absorption term to represent the effect of spontaneous emission which gives luminescence. The emitted light is strongly controlled by the optical modes of the structures and very good agreement is observed between theory and experiment.     

Pionic x-ray transitions in 3He

Pionic ³He atoms have been produced in a gaseous target. Energies and strong absorption broadening of the 2 → 1 and the 3 → 1 transition lines have been measured. The 1s level is shifted by 44 ± 5 eV by the (attractive) strong interaction, and the natural 1s level width is 42 ± 14 eV.     

Chemically activated methylcyclobutane from the 337.1 nm photolysis of ketene in the presence of cyclobutane. Dependence of S/D on extent of photolysis

The 3371 A photolysis of ketene/cyclobutane/oxygen mixtures gave stabilization/decomposition ratios for chemically activated methylcyclobutane that varied with the extent of photolysis. Experiments designed to circumvent this complication gave a 10 ± 2 kcal/mole collisional deactivation step size for cyclobutane.     

Electron attachment step in electron capture dissociation (ECD) and electron transfer dissociation (ETD)

We have made use of classical dynamics trajectory simultions and ab initio electronic structure calculations to estimate the cross sections with which electrons are attached (in electron capture dissociation (ECD)) or transferred (in electron transfer dissociation (ETD)) to a model system that contained both an S-S bond that is cleaved and a -NH(3)(+) positively charged site. We used a Landau-Zener-Stueckelberg curve-crossing approximation to estimate the ETD rates for electron transfer from a CH(3)(-) anion to the -NH(3)(+) Rydberg orbital or the S-S sigma* orbital. We draw conclusions about ECD from our ETD results and from known experimental electron-attachment cross sections for cations and sigma-bonds. We predict the cross section for ETD at the positive site of our model compound to be an order of magnitude larger than that for transfer to the Coulomb-stabilized S-S bond site. We also predict that, in ECD, the cross section for electron capture at the positive site will be up to 3 orders of magnitude larger than that for capture at the S-S bond site. These results seem to suggest that attachment to such positive sites should dominate in producing S-S bond cleavage in our compound. However, we also note that cleavage induced by capture at the positive site will be diminished by an amount that is related to the distance from the positive site to the S-S bond. This dimunition can render cleavage through Coulomb-assisted S-S sigma* attachment competitive for our model compound. Implications for ECD and ETD of peptides and proteins in which SS or N-C(alpha) bonds are cleaved are also discussed, and we explain that such events are most likely susceptible to Coulomb-assisted attachment, because the S-S sigma* and C=O pi* orbitals are the lowest-lying antibonding orbitals in most peptides and proteins.     

Measurement of the parity mixing of the 501 keV transition in180Hf by use of a backscattering polarimeter

The circular polarization of the 501 keV gamma radiation in¹⁸⁰Hf was remeasured by use of a Compton backscattering polarimeter with the single pulse counting technique. The resultP _c=?(1.9±0.6) · 10^?3 confirms the strong parity mixing which was previously reported by other groups.     

Chemically activated 1,1-dimethylcyclopropane from photolysis of isobutene–neopentane–diazomethane–oxygen mixtures

A study of the structural isomerization rate of chemically activated 1,1-dimethylcyclopropane from singlet methylene addition to the double bond of isobutene is reported. Singlet methylenes were produced from the 4358- and 3660-Å photolysis of diazomethane in the presence of added oxygen. Theoretical rates calculated via RRKM theory are in excellent agreement with experiment for calculations utilizing activated complex structures and critical energies consistent with known thermal Arrhenius parameters, and excitation energies consistent with previous determinations of ΔH(CH₂) + E*(CH₂) = 116.1 and 112.6 kcal/mole for diazomethane photolyses at 3660 and 4358 Å, respectively.     

Neutral current effects in muonic atoms: E1-E2 interferences

We study the parity-violating neutral current effects in muonic atoms which might result from an interference between E1 and E2 amplitudes in 3D → 1S transitions. We conclude that polarization asymmetries may be measurable in heavy atoms (Z ~ 70) exhibiting large nuclear deformations.     

The Imaginary Point Spectrum and Hypercyclicity

Let A be a generator of a C₀-semigroup T(·) on a separable Banach space. Under suitable conditions on the imaginary point spectrum of A, we show that T(·) is hypercyclic.