Exchange processes which include conformational change, protonation/deprotonation, and binding equilibria are routinely studied by 2D exchange NMR techniques, where information about the exchange of nuclei between environments with different NMR shifts is obtained from the development of cross-peaks. Whereas 2D NMR enables the real time study of millisecond and slower exchange processes, 2D ESR in the form of 2D-ELDOR (two-dimensional electron-electron double resonance) has the potential for such studies over the nanosecond to microsecond real time scales. Cross-peak development due to chemical exchange has been seen previously for semiquinones in ESR, but this is not possible for most common ESR probes, such as nitroxides, studied at typical ESR frequencies because, unlike NMR, the exchanging states yield ESR signals that are not resolved from each other within their respective line widths. But at 95 GHz, it becomes possible to resolve them in many cases because of the increased g-factor resolution. The 95 GHz instrumental developments occurring at ACERT now enable such studies. We demonstrate these new capabilities in two studies: (A) the protonation/deprotonation process for a pH-sensitive imidazoline spin label in aqueous solution where the exchange rate and the population ratio of the exchanging states are controlled by the concentration and pH of the buffer solution, respectively, and (B) a nitroxide radical partitioning between polar (aqueous) and nonpolar (phospholipid) environments in multilamellar lipid vesicles, where the cross-peak development arises from the exchange of the nitroxide between the two phases. This work represents the first example of the observation and analysis of cross-peaks arising from chemical exchange processes involving nitroxide spin labels.

The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane, vesicles can be observed. These 2D ELDOR experiments are performed as a function of mixing time, Tm, i.e., the time between the second and third π/2 pulses, which provides a third dimension. In fact, a fourth dimension may be added by varying the ESR frequency/magnetic field combination. Therefore, (3) it is shown how continuous-wave multifrequency ESR studies enable the decomposition of complex dynamics of, e.g., proteins by virtue of their respective time scales. These studies motivate our current efforts that are directed to extend 2D ELDOR to higher frequencies, 95 GHz in particular (from 9 and 17 GHz), in order to enable multi-frequency 2D ELDOR. This required the development of quasi-optical methods for performing the mm-wave experiments, which are summarized. We demonstrate state-of-the-art 95 GHz 2D ELDOR spectroscopy through its ability to resolve the two signals from a spin probe dissolved in both the lipid phase and the coexisting aqueous phase. As current 95 GHz experiments are restricted by limited spectral coverage of the π/2 pulse, as well as the very short T2 relaxation times of the electron spins, we discuss how these limitations are being addressed.

We report on electron-spin resonance microscopy (ESRM) providing sub-micron resolution (~700nm) with a high spin concentration sample, i.e. lithium phthalocyanine (LiPc) crystal. For biomedical applications of our ESRM, we have imaged samples containing rat basophilic leukemia (RBL) cells as well as cancerous tissue samples with a resolution of several microns using a water soluble spin probe, Trityl_OX063_d24. Phantom samples with the nitroxide spin label, (15)N PDT, were also imaged to demonstrate that nitroxides, which are commonly used as spin labels, may also be used for ESRM applications. ESRM tissue imaging would therefore be valuable for diagnostic or therapeutic purposes. Also, ESRM can be used to study the motility or the metabolism of cells in various environments. With further modification and/or improvement of imaging probe and spectrometer instrumentation sub-micron biological images should be obtainable, thereby providing a useful tool for various biomedical applications.

High-field/high-frequency electron spin resonance (ESR) offers improved sensitivity and resolution compared to ESR at conventional fields and frequencies. However, most high-field/high-frequency ESR spectrometers suffer from limited mm-wave power, thereby requiring long mm-wave pulses. This precludes their use when relaxation times are short, e.g., in fluid samples. Low mm-wave power is also a major factor limiting the achievable spectral coverage and thereby the multiplex advantage of Fourier transform ESR (FTESR) experiments. High-power pulses are needed to perform two-dimensional (2D) FTESR experiments, which can unravel the dynamics of a spin system in great detail, making it an excellent tool for studying spin and molecular dynamics. We report on the design and implementation of a high-power, high-bandwidth, pulsed ESR spectrometer operating at 95 GHz. One of the principal design goals was the ability to investigate dynamic processes in aqueous samples at physiological temperatures with the intent to study biological systems. In initial experiments on aqueous samples at room temperature, we achieved 200 MHz spectral coverage at a sensitivity of 1.1×1010s spins and a dead time of less than 50 ns. 2D-electron-electron double resonance experiments on aqueous samples are discussed to demonstrate the practical application of such a spectrometer.

A three-dimensional (3D) electron spin resonance (ESR) microimaging system, operating in pulse mode at 9GHz is presented. This microscope enables the acquisition of spatially resolved magnetic resonance signals of free-radicals in solid or liquid samples with a resolution of up to ∼3.5×7×11.4μm in 20min of acquisition. The detection sensitivity at room temperature is ∼1.2×109spins∕√Hz, which enables the measurement of ∼2×107 spins in each voxel after 60min of acquisition. The resolution and detection sensitivity are the best obtained so far for ESR at ambient conditions of temperature and pressure. This ESR microscope can be employed in the investigation of a variety of samples in the fields of botany, life sciences, and materials science.

An electron spin resonance (ESR) imaging system, capable of acquiring three-dimensional (3D) images with a resolution of ∼10×10×30 μm in a few minutes of acquisition, is presented. This ESR microscope employs a commercial continuous wave ESR spectrometer, working at 9.1 GHz, in conjunction with a miniature imaging probe (resonator+gradient coils), gradient current drivers, and control software. The system can acquire the image of a small (∼1.5×1.5×0.25 mm) sample either by the modulated field gradient method, the projection reconstruction method, or by a combination of the two. A short discussion regarding the resolution of the modulated field gradient method in two-dimensional (2D) and 3D imaging is given. Detailed descriptions of the various system components are provided, along with several examples of 2D and 3D images that demonstrate the capabilities of the system.

NMR microscopy is routinely employed in fields of science such as biology, botany, and materials science to observe magnetic parameters and transport phenomena in small scale structures. Despite extensive efforts, the resolution of this method is limited (>10μm for short acquisition times), and thus cannot answer many key questions in these fields. We show, through theoretical prediction and initial experiments, that ESR microscopy, although much less developed, can improve upon the resolution limits of NMR, and successfully undertake the 1μm resolution challenge. Our theoretical predictions demonstrate that existing ESR technology, along with advanced imaging probe design (resonator and gradient coils), using solutions of narrow linewidth radicals (the trityl family), should yield 64×64 pixels 2D images (with z slice selection) with a resolution of 1×1×10μm at ∼60GHz in less than 1h of acquisition. Our initial imaging results, conducted by CW ESR at X-band, support these theoretical predictions and already improve upon the previously reported state-of-the-art for 2D ESR image resolution achieving ∼10×10μm, in just several minutes of acquisition time. We analyze how future progress, which includes improved resonators, increased frequency of measurement, and advanced pulsed techniques, should achieve the goal of micron resolution.

We have developed a time domain transverse feedback system with the high bandwidth needed to control transverse instabilities when the CESR ee collider is filled with trains of closely spaced bunches. This system is based on parallel digital processors and a stripline driver. It is capable of acting on arbitrary patterns of bunches having a minimum spacing of 14 ns. Several simplifying features have been introduced. A single shorted stripline kicker driven by one power amplifier is used to control both counter-rotating beams. The desired feedback phase is achieved by sampling the bunch position at a single location on two independently selectable beam revolutions. The system adapts to changes in the betatron tune, bunch pattern, or desired damping rate through the loading of new parameters into the digital processors via the CESR control system. The feedback system also functions as a fast gated bunch current monitor. Both vertical and horizontal loops are now used in CESR operation. The measured betatron damping rates with the transverse feedback system in operation are in agreement with the analytical prediction and a computer simulation developed in connection with this work.

We have developed a time domain transverse feedback system with the high bandwidth needed to control transverse instabilities when the CESR e/sup +/e/sup -/ collider is filled with trains of closely spaced bunches. This system is based on parallel digital processors and a stripline driver. It is capable of acting on arbitrary patterns of bunches having a minimum spacing of 14 ns. Several simplifying features have been introduced. A single shorted stripline kicker driven by one power amplifier is used to control both counter-rotating beams. The desired feedback phase is achieved by sampling the bunch position at a single location on two independently selectable beam revolutions. The system adapts to changes in the betatron tune, bunch pattern, or desired damping rate through the loading of new parameters into the digital processors via the CESR control system. The feedback system also functions as a fast gated bunch current monitor. Both vertical and horizontal loops are now used in CESR operation. The measured betatron damping rates with the transverse feedback system in operation are in agreement with the analytical prediction and a computer simulation developed in connection with this work.

Preliminary studies of a high resolution Cornell Electron-positron Storage Ring (CESR) bunch-to-bunch signal processing system are described. In these studies, a prototype inductive-integrating, sampling current monitor is evaluated for improved estimation of bunch lifetimes over a series of planned CESR luminosity upgrades. Initial test data indicate significant performance advantages over conventional electrostatic pickup systems in the areas of linearity, dynamic range and beam position sensitivity. Novel features of the fast bunch current monitor include its wideband passive integrating pickup, diplex filter section and high resolution bunch signal processor (BSP). Evaluation studies in CESR indicate this fast bunch monitor is potentially useful in colliders and other storage rings utilizing bunch-to-bunch or bunch train separations down to 80 nanoseconds. Proposed system revisions to permit fast, high resolution bunch train current measurements in future CESR operations are discussed. Commercial availability of several principal bunch-to-bunch current monitor components is noted.< >

A fast solid-state pulser (PFH) for the Cornell CESR linac injector system is described. Stripline-packaged high-voltage power MOSFET devices are arranged in a novel cascade output topology to achieve extremely low transition times. In the present CESR injector, the PFH driver outputs pulses of 3 nanoseconds FNHM and 20 amperes peak current through an Eimac Y-796 cathode-grid assembly. Recovery time is approximately 10 nsec. Advantages of the MOSFET pulser over its hard-tube predecessor include a substantial increase in peak beam current, an order-of-magnitude reduction of gun interpulse recovery time and precise control of the unequal output pulse amplitudes required for e/sup +/ vs e/sup /njection modes. Reduction of physical size permits collocating the pulser with the linac electron gun assembly to minimize transmission line artifacts arising from unavoidable impedance mismatch over the gun's bias range. Successful implementation of the MOSFET linac gun pulser is an initial step to a future CESR B-factory injector. >

Fourier transform ESR methods have been extended to permit spatially resolved two-dimensional (2D)-ESR experiments. This is illustrated for the case of 2D-electron-electron double resonance (2D-ELDOR) spectra of nitroxides in a liquid that exhibits appreciable cross-peaks due to Heisenberg spin exchange. The use of spin-echo decays in spatially resolved FT-ESR is also demonstrated.

Modern Fourier transform (FT) ESR methods have been combined with fast, high power pulsed magnetic field gradients to enable FT-ESR imaging. Spectral—spatial imaging by frequency and phase encoded FT methods are compared with cw methods. The initial phase encoded results are comparable in quality to those from the well-developed cw methods and further improvements which would enhance FT-ESR imaging are noted.

Methods are described for measurement and stabilization of energy dependent transverse tunes observed in the Cornell synchrotron. For this accelerator, operating over an energy range of 0.15 to 5.5 GeV, tune correction is required to compensate the low energy offset contribution of synchrotron magnetic field errors. Without dynamic correction, synchrotron transport losses and beam instability limit injection rates into the CESR collider and may induce long‐term damage in the CLEO detector. In the system described, a real‐time digitization technique [‘‘MIRABILE’’] has been applied for analysis of tune spectra over the energy ramp cycle. FTM (frequency‐time magnitude) plots derived from rapidly‐digitized time domain data are analyzed to determine appropriate correction waveforms for excitation of vertical and horiozontal correction quadrupoles distributed about the synchrotron ring. A description of the synchrotron tune stabilization system and related operational data are presented.

Two distinct Hall effect regulation systems have been designed for reduction of hysteresis effects in the linac and transfer line magnets used for injection into the Cornell Electron Storage Ring (CESR) synchrotron. One magnetic field probe described integrates a precision Hall sensor element with low-drift preamplification electronics in a temperature-regulated remote sensing module. Magnetic field stability better than five parts in 104 is achieved by a prototype system which includes this module and its associated signal processing electronics. A second Hall effect regulator design utilizes an inexpensive monolithic linear sensor/amplifier device to attain a minimum repeatable accuracy of 0.5%. The radiation and mechanical stress sensitivity of the Hall sensors employed is evaluated to estimate their operational viability in the injector environment. It is concluded that the service lifetime of either Hall probe type will be at least three years in the CESR injector, given an average neutron exposure rate not exceeding 1010 n/cm2/s