Paul A. S. Cruickshank

A kilowatt pulsed 94 GHz electron paramagnetic resonance spectrometer with high concentration sensitivity, high instantaneous bandwidth, and low dead time

We describe a quasioptical 94 GHz kW pulsed electron paramagnetic resonance spectrometer featuring pi/2 pulses as short as 5 ns and an instantaneous bandwidth of 1 GHz in nonresonant sample holders operating in induction mode and at low temperatures. Low power pulses can be as short as 200 ps and kilowatt pulses as short as 1.5 ns with timing resolution of a few hundred picoseconds. Phase and frequency can be changed on nanosecond time scales and complex high power pulse sequences can be run at repetition rates up to 80 kHz with low dead time. We demonstrate that the combination of high power pulses at high frequencies and nonresonant cavities can offer excellent concentration sensitivity for orientation selective pulsed electron double resonance (double electron-electron resonance), where we demonstrate measurements at 1 microM concentration levels.

W-band PELDOR with 1 kW microwave power: molecular geometry, flexibility and exchange coupling.

A technique that is increasingly being used to determine the structure and conformational flexibility of biomacromolecules is Pulsed Electron-Electron Double Resonance (PELDOR or DEER), an Electron Paramagnetic Resonance (EPR) based technique. At X-band frequencies (9.5 GHz), PELDOR is capable of precisely measuring distances in the range of 1.5-8 nm between paramagnetic centres but the orientation selectivity is weak. In contrast, working at higher frequencies increases the orientation selection but usually at the expense of decreased microwave power and PELDOR modulation depth. Here it is shown that a home-built high-power pulsed W-band EPR spectrometer (HiPER) with a large instantaneous bandwidth enables one to achieve PELDOR data with a high degree of orientation selectivity and large modulation depths. We demonstrate a measurement methodology that gives a set of PELDOR time traces that yield highly constrained data sets. Simulating the resulting time traces provides a deeper insight into the conformational flexibility and exchange coupling of three bisnitroxide model systems. These measurements provide strong evidence that W-band PELDOR may prove to be an accurate and quantitative tool in assessing the relative orientations of nitroxide spin labels and to correlate those orientations to the underlying biological structure and dynamics.

Compact Wideband Corrugated Feedhorns With Ultra-Low Sidelobes for Very High Performance Antennas and Quasi-Optical Systems

The corrugated or scalar feedhorn has found many applications in millimeter wave and sub-millimeter wave systems due to its high beam symmetry, relatively low sidelobe levels and strong coupling to the fundamental mode Gaussian beam. However, for applications such as millimeter wave cosmology, space-based experiments, or even high performance imaging, there is a generic requirement to reduce the size of horns whilst maintaining very high levels of performance. In this paper we describe a general analytic methodology for the design of compact dual-profiled corrugated horns with extremely low sidelobe levels. We demonstrate that it is possible to achieve

Reducing standing waves in quasi-optical systems by optimal feedhorn design

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DEER Sensitivity between Iron Centers and Nitroxides in Heme-Containing Proteins Improves Dramatically Using Broadband, High-Field EPR.

dB sidelobe levels, over wide bandwidths with short horns, which we believe represents state-of-the-art performance. We also demonstrate experimentally a simple scalar design that operates over wide bandwidths and can achieve sidelobes of better than

High power pulsed dynamic nuclear polarisation at 94 GHz

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Technologies for sub-ns pulse coherent mm-wave radar

dB, whilst maintaining a frequency independent phase center. This design methodology has been validated experimentally by the successful manufacture and characterization of feedhorns at 94 GHz and 340 GHz for both radar and quasi-optical instrumentation applications.

Force detected electron spin resonance at 94 GHz

Standing waves between transmit and receive feedhorns in quasi-optical systems often limit the achievable performance of mm-wave and sub-mm-wave instrumentation. Even with high performance corrugated feedhorns and perfect frequency independent optics, significant standing waves can occur because of the resonant build-up of higher order modes between feedhorns. In this paper we describe a new design of wideband corrugated feedhorn that significantly reduces standing wave effects, is scalable to any frequency, is shorter than standard horns and is suitable for a wide range of optical configurations. In addition it produces far-field beam patterns with much reduced sidelobes. We will describe the theory behind this new feedhorn design, outline scaling laws and present experimental results confirming the analysis.

The use of composite pulses for improving DEER signal at 94 GHz

This work demonstrates the feasibility of making sensitive nanometer distance measurements between Fe(III) heme centers and nitroxide spin labels in proteins using the double electron–electron resonance (DEER) pulsed EPR technique at 94 GHz. Techniques to measure accurately long distances in many classes of heme proteins using DEER are currently strongly limited by sensitivity. In this paper we demonstrate sensitivity gains of more than 30 times compared with previous lower frequency (X-band) DEER measurements on both human neuroglobin and sperm whale myoglobin. This is achieved by taking advantage of recent instrumental advances, employing wideband excitation techniques based on composite pulses and exploiting more favorable relaxation properties of low-spin Fe(III) in high magnetic fields. This gain in sensitivity potentially allows the DEER technique to be routinely used as a sensitive probe of structure and conformation in the large number of heme and many other metalloproteins.

High-field pulse EPR instrumentation

In this communication we report initial results using high power pulsed techniques at 94 GHz to perform solid state Dynamic Nuclear Polarisation (DNP) on high volume samples. It is shown that excitation with short pulses, comparable to the pi/2 pulse length, at fast repetition rates can result in higher DNP enhancements relative to continuous wave (cw) excitation for the same average power. Peak enhancements are observed at an average power of only a few hundred mW delivered to the sample.