Magnetic field effect as a probe of distance-dependent electron transfer in systems undergoing free diffusion

A theoretical analysis is presented of the problem of how distance-dependent electron transfer in photoinduced forward electron transfer followed by geminate backward electron transfer in liquid solution is reflected in the viscosity dependence of the magnetic field effect (MFE) on the efficiency of free radical formation (φ_ce) in such reactions. The stochastic Liouville equation formalism is employed to model the reaction behaviour of distance-distributed, triplet-born radical pairs (RPs) undergoing free diffusion, distance- and spin-dependent backward electron transfer, coherent and incoherent spin evolution in the ps time domain. In comparison with real systems the spin situation is simplified by reducing it to a two state (S, T ₀) problem, yet it is parametrized in a way that allows sensible comparison of the results with those of recent experiments. It is predicted that the MFE on φ_ce exhibits characteristic minima in the MFE versus viscosity curves, and it is verified in detail that this feature is peculiar to the diffusional model with distance-dependent electron transfer, i.e. cannot be reproduced with the simpler (‘exponential’) RP model employing distance-independent rate constants. Thus, the MFE versus viscosity curves are established as a genuine fingerprint of distance-dependent electron transfer. The theoretical results compare favourably with recent experimental results obtained with Ru^III complex/methylviologen RPs.     

Concentration of CO2 dissociation products in a capillary discharge in a CO2-N2-He mixture

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 51, No. 1, pp. 12–16, July, 1989.     

Mechanical vibration measurements using a fibre optic laser-Doppler probe

The feasibility of using a fibre optic for transmitting laser light to a vibrating object and scattered light back to a laser-Doppler unit, has been demonstrated. A new mode of operation of laser-Doppler equipment has been devised with marked improvement over the usual Twyman-Green mode. With the aid of this fibre optic laser-Doppler system both frequency and amplitude information can be obtained from otherwise inaccessible machine components.     

The effect of protein internal cavities on ligand migration and binding in myoglobin

Using Fourier Transform Infrared Spectroscopy at cryogenic temperatures, we have studied carbon monoxide (CO) migration in the interior of sperm whale myoglobin and binding to teh heme iron. The effect of protein internal cavities was examined by comparing the wild-type protein with mutants in which the cavities were blocked by bulky amino acid sidechains. After photodissociation at 3 K, CO ligands reside in the primary docking site (B) from where they can migrate to other sites (C, D) that were identified as the Xe4 and Xe1 cavities. These studies were complemented by flash photolysis experiments at room temperature, which revealed that the protein cavities enable efficient excape of ligands from the protein after dissociation from the heme iron.     

MRI measurements of CO2 hydrate dissociation rate in a porous medium

After obtaining experimental data of CO₂ hydrate formation and dissociation in a porous medium using magnetic resonance imaging (MRI), the purpose of this study was to analyze the different dissociation rate of CO₂ hydrate using two heating rates. Images were obtained by using a fast spin-echo sequence, and the field of view was set to 40×40×40 mm. The vessel pressure was monitored during hydrate formation and dissociation, which was used to compare with MRI mean intensity. The result indicated that the MRI could visualize hydrate formation and dissociation, and the MRI mean intensity of water was in good agreement with the vessel pressure changes. The hydrate formation and dissociation rates were also quantified using the MRI mean intensity of water. The experimental results showed that the higher heating rate caused the rapid hydrate dissociation.     

Spin-orbit coupling and dissociation of CO2 molecules

Potential surfaces of the CO₂ molecule for the ground and excited ³ B ₂, ¹ B ₂ electronic states are calculated by quantum chemistry methods. The calculation of the spin-orbit coupling in the molecule shows a large the matrix element, which removes the prohibition for the dissociation-recombination process CO₂(X ¹Σ) + M ? CO(X ¹Σ) + O(³ P) + M. The barrier on the potential curve for ³ B ₂, the energy of which exceeds the limit of dissociation into components in the ground states, explains the data on the dissociation and recombination energies measured in experiments with shock tubes. The absorption cross section of CO₂ molecules in the UV spectral region measured at high temperatures allowed us to plot branches of potential curves near their minima for two upper singlet states assigned to the ¹ B ₂ and ¹ A ₂ symmetry.     

Stabilization of polar ZnO surfaces: validating microscopic models by using CO as a probe molecule

The determination of the structure of inhomogeneous metal-oxide surfaces represents a formidable task. With the present study, we demonstrate that using the binding energy of a probe molecule, CO, is a reliable tool to validate structural models for such complex surfaces. Combining several types of first-principles calculations with advanced molecular beam methods, we are able to provide conclusive evidence that the polar O-terminated surface of ZnO is either reconstructed or hydrogen covered. This finding has important consequences for the ongoing discussion regarding the stabilization mechanism of the electrostatically unstable ("Tasker type 3") polar ZnO surfaces.     

TEA CO2 laser-induced selective molecular dissociation in a cold shock wave

Selective molecular IR multiphoton dissociation (SF₆ was taken as an example) was studied under the nonequilibrium conditions of a cold shock wave (pressure shock) formed upon the interaction of a pulsed gas-dynamically cooled molecular flow with a solid surface. An anomalously large gain (by more than an order of magnitude) in the product yield (compared to the molecular excitation in an unperturbed flow) with a minor selectivity loss (less than 25–30%) was observed.     

Neutrinos as a probe of CP-violation and leptogenesis

Establishing CP-violation in the lepton sector is one of the most challenging future tasks in neutrino physics. The lepton mixing matrix contains one Dirac phase and, if neutrinos are Majorana particles, two additional CP-violating phases. I will review the main theoretical aspects of CP-violation in the lepton sector. Then, I will present the strategies for determining the Dirac and the Majorana CP-violating phases in long-baseline and neutrinoless double beta decay experiments, respectively. Leptonic CP-violation has received recently a lot of attention as it might be at the origin of the baryon asymmetry of the Universe. Within the context of the see-saw mechanism, I will discuss the possible connection between the CP-violating phases measurable at low energy with the ones entering in leptogenesis.     

Anionic photofragmentation of CO: a selective probe of core-level resonances

Anion-yield spectroscopy using x rays is shown to be a selective probe of molecular core-level processes, providing unique experimental verification of shape resonances. For CO, partial anion and cation yields are presented for photon energies near the C K edge. The O- yield exhibits features above threshold related only to doubly excited states, in contrast to cation yields which also exhibit pronounced structure due to the well-known sigma* shape resonance. Because the shape resonance is completely suppressed for O-, anion spectroscopy thus constitutes a highly selective probe, yielding information unobtainable with absorption or electron spectroscopy.     

Quantum states of hydrogen transfer and vibration in malonaldehyde

New ab initio results on the 21D potential energy surface of malonaldehyde and a quantum mechanical treatment of the hydrogen transfer motion and its interaction with all vibrations are presented. An explicit approximate reaction path, close to the minimum energy path but matching the reactive normal mode near equilibrium, allows one to predict the ground state tunnelling frequency even when using small basis sets. With a barrier of 1144 cm^?1 (3.27kcal mol-¹) the tunnelling splitting is predicted to be 22.0cm^?1 for the parent species and 3.8 cm^?1 for the species deuterated in the hydrogen bond, in good agreement with the observed values 21.6 and 2.9 cm^?1, respectively. Predicted energy levels for excited states of the hydrogen transfer motion and for the non-reactive vibrations suggest a re-examination of the vibrational spectra and an extension of the number of vibrational factors in the basis set to improve the results for the vibrationally excited states.     

Erbium as a probe of everything?

Erbium is a lanthanide ion with unique electronic and optical properties. In its trivalent state it is composed of an incompletely filled 4f inner shell and two closed outer shells. By employing these properties in specific material systems, Er can be used to probe point defects, oxygen, OH, Er, radiation defects, network structure, excitons, optical density of states, optical modes, and photonic bandstructure.     

Dipyrenylphosphatidylcholine as a probe of bilayer pressures

We have used dipyrenylphosphatidylcholines (dipyPCs) to study the pressure in the fluid lamellar phase formed by mixtures of fully hydrated dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE). As we increase the DOPE mole fraction at 25°C we observe a linear increase in the ratio of the excimer-to-monomer signal (E/M1). We argue that this observation can be understood in terms of an increase in the lateral pressure in the chain region, i.e., in the bilayer plane. This change itself is driven by the decrease in lateral pressure between headgroups as we add DOPE. We expect the lateral pressure to vary in magnitude as we probe the bilayer at different depths [1]. We have confirmed this by recording E/M1 using di[10-(pyren-1-yl)decanoyl]phosphatidylcholine (10dipyPC) and di[4-(pyren-1-yl)butanoyl]phosphatidylcholine (4dipyPC). We find that in 100% DOPC the E/M1 for 4dipyPC is 2.5 times greater than that for 10dipyPC. The above observations can all be rationalized in terms of changes in the lateral pressure profile. An inverse hexagonal liquid crystalline phase is found in the range 100–83% DOPE [2]. In this region of the phase diagram we observe a quadratic variation in E/M1, with a minimum at 95% DOPE. We hypothesize that this variation reflects the chain stretching that is necessitated by the geometrical packing constraints of the hexagonal phase [3]. Again, we find that the E/M1 for 4dipyPC is greater than that for 10dipyPC, but in this phase only by a factor of two.