F. J. Adrian

Metal deposition from a supported metal film using an excimer laser

A method for the direct writing of metal features from a metal film supported on an optically transparent substrate using a single pulse from a high‐energy excimer laser (193 nm) is presented. The technique eliminates the need for gas‐phase precursors in many cases and is an inherently clean process. Results of copper depositions onto silicon substrates are shown to exemplify the technique and a mechanism for the process is proposed.

Metal deposition at 532 nm using a laser transfer technique

The laser‐induced forward transfer technique in which material is ablatively transferred from a thin film to a target substrate by a pulsed excimer laser has been extended to 532 nm using a frequency‐doubled YAG laser. Cu and Ag have been deposited on fused silica substrates using microscope objectives for focusing, resulting in reductions in feature size over that obtainable with the multimode excimer laser. The photothermal deposition process has been modeled using the one‐dimensional thermal diffusion equation, including a moving solid‐melt boundary, with good agreement between theoretical and experimental results.

On the theory of chemically induced electron polarization (CIDEP): Vector model and an asymptotic solution

It is shown that the stochastic–Liouville model of CIDEP can be cast into the form of a ’’Bloch‐type’’ equation with diffusion. This leads to a generalized vector model of the radical pair mechanism of chemically induced magnetic polarization, which gives a clear picture of the qualitative features of both CIDNP and CIDEP. For the case of simple Brownian motion of the two radicals the differential form of the stochastic–Liouville equations of CIDEP is readily converted into a single integral equation. In the limit of slow singlet–triplet mixing by the magnetic spin Hamiltonian, and with an exchange interaction that decays exponentially with radical separation, this integral equation may be solved exactly. The electron polarization then takes the form of a superposition of an infinite number of ’’contact exchange modes’’.

A novel microwave technique for detection of superconductivity

A new technique for detection of superconductivity which is based upon microwave resistivity is described. This method is similar in both its implementation and execution to the technique of electron‐spin resonance, and, as a consequence, exhibits high sensitivity. An additional benefit of this method is the fact that metal‐insulator phase transitions of materials are recorded only if they have a magnetic field dependence.

ESR investigation of the photoisomerization of OClO to ClOO in an H2SO4 glass: Magnetophotoselection and kinetics

The photoisomerization of OClO (ClO2) to ClOO in a sulfuric acid glass at 20 to 77 K has been reinvestigated using electron spin resonance (ESR) to observe the reactant and product and to quantitatively determine their relative concentrations. Polarized light photolysis yields a partial orientation of the remaining ClO2 molecules (magnetophotoselection) and shows that the optical transition dipole moment lies in the molecular plane and is perpendicular to the ClO2 rotational symmetry axis, as expected for the well known A(2A2)←X(2B1) transition of ClO2. Here, however, the photolysis yields only ClOO, in contrast to the gas phase where ClO is observed suggesting photodissociation. The photoisomerization kinetics are nonexponential and suggest that random variations among the ClO2 trapping sites in the H2SO4 glass have a considerable effect on the probability that a photoexcited ClO2 molecule will isomerize.

Analytic solution of the stochastic‐Liouville model of spin exchange

An analytic solution is obtained for the stochastic‐Liouville model of spin exchange between a pair of radicals undergoing isotropic diffusion in solution and interacting via an exchange interaction that decays exponentialy with radical separation. The resulting spin exchange cross section is the sum of a ‘‘strong encounter’’ term that is approximately equal to the biomolecular reaction cross section and a ‘‘grazing encounter’’ term that is due to encounters in the tail of the exchange interaction. The latter term may range from negligible to twice the former term for plausible values of the diffusion rate and the range of the exchange interaction. It is shown that the theory combined with experimental data on the spin‐exchange contribution to spin–lattice relaxation can provide an estimate of the range of the exchange interaction between two radicals.

Detection of weak link superconductivity by magnetically modulated electrical resistance

Abstract A new technique, based on observing the effect of magnetic field modulation on the DC resistance of superconductors, sensitively detects the presence of weak links in granular high temperature superconductors. The phase detected response to the magnetic modulation shows a peak at T c for both metallic and ceramic superconductors, analogous to that observed in magnetically modulated microwave absorption. A second peak, attributed to weak links, is observed below T c in the ceramic oxide superconductors when tailing is present in the resistance vs. temperature curve.

On distinguishing intrinsic and weak-link responses in magnetically-modulated dissipation in high Tc superconductors

It is noted that, contrary to a recent report, magnetic-field modulation of electromagnetic dissipation in superconductors yields both an intrinsic response indicative of the superconducting transition and extrinsic responses indicative of weak links. Ways of distinguishing between these difference responses are also noted.

Detection of weak link effects in superconducting YBa2Cu3O7−y epitaxial thin films by magnetically modulated resistance

Evidence of weak link effects in epitaxial thin films of YBa2Cu3O7−y with well‐aligned grains is obtained from magnetically modulated resistance measurements. These results are of interest because weak links could provide a mechanism for the residual microwave loss in these types of superconductor thin films which contain predominantly low angle grain boundaries.

Semiempirical structural model of the electric field gradients at oxygen nuclei in YBa2Cu3O7

Abstract The electric field gradients at the oxygen ion sites in YBa 2 Cu 3 O 7 are calculated as a sum of ionic, overlap, and valence terms using a semiemoirical model in which the O ion holes that determine the valence terms are the principal adjustable parameters. Good fits to the experimental results are obtained for reasonable values of fractional hole character in O 2pσ orbitals only.