State-to-state photodissociation of carbonyl sulfide (nu2=0,1/JlM). II. The effect of initial bending on coherence of S(1D2) polarization

Photodissociation studies using ion imaging are reported, measuring the coherence of the polarization of the S((1)D(2)) fragment from the photolysis of single-quantum state-selected carbonyl sulfide (OCS) at 223 and 230 nm. A hexapole state-selector focuses a molecular beam of OCS parent molecules in the ground state (nu2=0mid R:JM=10) or in the first excited bending state (nu2=1mid R:JlM=111). At 230 nm photolysis the Im[a1 (1)(parallel, perpendicular)] moment for the fast S(1D2) channel increases by about 50% when the initial OCS parent state changes from the vibrationless ground state to the first excited bending state. No dependence on the initial bending state is found for photolysis at 223 nm. We observe separate rings in the slow channel of the velocity distribution of S(1D2) correlating to single CO(J) rotational states. The additional available energy for photolysis at 223 nm is found to be channeled mostly into the CO(J) rotational motion. An improved value for the OC-S bond energy D0=4.292 eV is reported.     

Plant delta(15)N associated with arbuscular mycorrhization, drought and nitrogen deficiency

It has long been evident that plant (15)N chiefly reflects the processes which fractionate (15)N/(14)N rather than the (15)N of plant N source(s). It has emerged recently that one of the most important fractionating processes contributing to the whole plant (15)N is the presence/absence, type or species of mycorrhiza, especially when interacting with nutrient deficiency. Ecto- and ericoid mycorrhizas are frequently associated with (15)N-depleted foliar (15)N, commonly as low as -12 per thousand. As shown by the present study, plants having no mycorrhiza, or those infected with various species of arbuscular mycorrhiza (AM)-forming fungi, interact with varying concentrations of soil nitrogen [N] and moisture to enrich plant (15)N by as much as 3.5 per thousand. Hence the lack of a mycorrhiza, or variation in the species of AM-forming fungal associations, can account for about 25% of the usually reported variations of foliar (15)N found in field situations and do so by (15)N enrichment rather than depletion. Copyright 1999 John Wiley & Sons, Ltd.     

Study of the vortex dynamics in single crystals of the high-T c superconductor Tl2Ba2CaCu2O8 via205Tl NMR

The²⁰⁵Tl linewidth in single crystals of the high-T _c superconductor Tl₂Ba₂CaCu₂O₈ (T _c≈105 K) is analyzed for the normal (N) and impurity (I) Tl-sites. On the I-sites Ca has been replaced by Tl. Measurements were performed in a field of 4.7 T. The normal state linewidth on the N and I sites is shown to be mainly determined by a distribution in Knight shifts. To simulate the final increase below 50 K we have studied the vortex lattice in a single superconducting sheet by a molecular dynamics method for systems up to 2000 vortices. The results compare well with the experimental data. This implies that the temperature dependence of the linewidth in this highly anisotropic material is determined by flux dynamics and only marginally by the temperature dependence of the penetration depth.     

Nuclear spin dynamics in the quantum regime of a single-molecule magnet

We show that the nuclear spin dynamics in the single-molecule magnet Mn12-ac below 1 K is governed by quantum tunneling fluctuations of the cluster spins, combined with intercluster nuclear spin diffusion. We also obtain the first experimental proof that-surprisingly-even deep in the quantum regime the nuclear spins remain in good thermal contact with the lattice phonons. We propose a simple model for how T-independent tunneling fluctuations can relax the nuclear polarization to the lattice that may serve as a framework for more sophisticated theories.     

Voltage-modulated millimeter-wave spectroscopy on a polymer diode: mesoscopic charge transport in conjugated polymers

We present a new technique to determine the carrier mobility mu in semiconducting, undoped, conjugated polymers in the millimeter-wave frequency range, 10-500 GHz. This method probes charge conduction on mesoscopic length scales, a regime inaccessible to other transport experiments. The experiment is based on the detection of millimeter-wave absorption of field-induced charge in polymer diodes, and is applied to poly( p-phenylene vinylene). We demonstrate that locally mu can be as high as 10(-4) m(2)/V s, and deduce the typical hopping parameters in the mesoscopic high- mu regions in the material.