Biomedical subjects

John J L Morton

Publications and source records attributed to John J L Morton.

5 recordsLinked to original sources

PubMed2006

Davies electron-nuclear double resonance revisited: enhanced sensitivity and nuclear spin relaxation.

Over the past 50 years, electron-nuclear double resonance (ENDOR) has become a fairly ubiquitous spectroscopic technique, allowing the study of spin transitions for nuclei which are coupled to electron spins. However, the low spin number sensitivity of the technique continues to pose serious limitations. Here we demonstrate that signal intensity in a pulsed Davies ENDOR experiment depends strongly on the nuclear relaxation time T(1n), and can be severely reduced for long T(1n). We suggest a development of the original Davies ENDOR sequence that overcomes this limitation, thus offering dramatically enhanced signal intensity and spectral resolution. Finally, we observe that the sensitivity of the original Davies method to T(1n) can be exploited to measure nuclear relaxation, as we demonstrate for phosphorous donors in silicon and for endohedral fullerenes N@C(60) in CS(2).

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PubMed2006

Synthesis and reactivity of N@C60O.

The endohedral fullerene epoxide N@C60O was synthesised, isolated by High Performance Liquid Chromatography (HPLC), and characterised by Electron Spin Resonance (ESR). This nitrogen radical displays predominantly axial symmetry characteristics as expected for a monoadduct, evidenced by a zero-field splitting D parameter of 6.6 MHz and an E parameter of 0.5 MHz in powder at 77 K. Photo- and thermally-activated silencing of the nitrogen radical were observed, the latter showing the evolution of a new spin signal during heating at 100 degrees C. We suggest that loss of nitrogen spin is due to coupling with a radical formed by opening of the epoxide ring. This implies that the reaction of C60O with C60 in the solid state proceeds via a radical, rather than ionic, intermediate.

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PubMed2006

Electron spin relaxation of N@C60 in CS2 in CS2.

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S> or =1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60 , indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170 K.

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PubMed2005

High fidelity single qubit operations using pulsed electron paramagnetic resonance.

Systematic errors in spin rotation operations using simple rf pulses place severe limitations on the usefulness of the pulsed magnetic resonance methods in quantum computing applications. In particular, the fidelity of quantum logic operations performed on electron spin qubits falls well below the threshold for the application of quantum algorithms. Using three independent techniques, we demonstrate the use of composite pulses to improve this fidelity by several orders of magnitude. The observed high-fidelity operations are limited by pulse phase errors, but nevertheless fall within the limits required for the application of quantum error correction.

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PubMed2005

A new mechanism for electron spin echo envelope modulation.

Electron spin echo envelope modulation (ESEEM) has been observed for the first time from a coupled heterospin pair of electron and nucleus in liquid solution. Previously, modulation effects in spin-echo experiments have only been described in liquid solutions for a coupled pair of homonuclear spins in nuclear magnetic resonance or a pair of resonant electron spins in electron paramagnetic resonance. We observe low-frequency ESEEM (26 and 52 kHz) due to a new mechanism present for any electron spin with S > 12 that is hyperfine coupled to a nuclear spin. In our case these are electron spin (S = 32) and nuclear spin (I = 1) in the endohedral fullerene N@C(60). The modulation is shown to arise from second-order effects in the isotropic hyperfine coupling of an electron and (14)N nucleus.

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