The bruker high-frequency-EPR system

The design and performance of the first commercial 94 GHz continuous-wave (CW-)/Fourier transform (FT-) EPR and ENDOR spectrometer are described. The spectrometer design is based on a heterodyne microwave bridge using an X-band intermediate frequency (IF), a hybrid magnet system, a variable-temperature, top-loading TeraFlex probehead with a TE₀₁₁ cavity as well as the ELEXSYS-line digital electronics and the Xepr software package. The W-band bridge can be driven by a CW- or pulse-IF unit and delivers a microwave power of 5 mW at 94 GHz. In pulse mode the power is sufficient for a π/2 pulse of 100 ns at a resonatorQ-value of 3000. The magnet system consists of a 6 T split-coil superconducting magnet and a water-cooled room-temperature coil. The main coil can be swept over the full range from 0 to 6 T. The room-temperature coil has a 800 G sweep range around the persistent field of the main magnet. The ENDOR probe features a tuned circuit for¹H nuclei allowing an RF π-pulse of 8 μs with a 200 W amplifier. A broad-band setup is used for other nuclei. The E680 FT-EPR system utilizes the PatternJet pulse programmer and the SpecJet high-speed transient signal averager. The concerted action of these two devices results in a pulse EPR sensitivity equal or higher than in CW-EPR. Selected examples indicating the performance of the 94 GHz CW/FT-EPR and ENDOR systems are shown.     

Radio frequencies in EPR: Conventional and advanced use

This mini-review focuses on various aspects of the application of radio frequency (rf) irradiation in electron paramagnetic resonance (EPR). The development of the electron-nuclear double resonance (ENDOR) technique is briefly described, and we highlight the use of circularly polarized rf fields and pulse ENDOR methodology in one- and two-dimensional experiments. The capability of pulse ENDOR at Q-band is illustrated with interesting experimental examples. Electron spin echo envelope modulation effects induced by an rf field in liquid samples demonstrate another role which rf fields can play. Technical achievements in the design of ENDOR resonators are illustrated by the example of a bridged loop-gap resonator. Finally, the influence of longitudinal rf fields on the dynamics of EPR transitions is explained using a dressed spin resonance treatment.     

A Q-band pulse EPR/ENDOR spectrometer and the implementation of advanced one- and two-dimensional pulse EPR methodology

A versatile high-power pulse Q-band EPR spectrometer operating at 34.5--35.5 GHz and in a temperature range of 4--300 K is described. The spectrometer allows one to perform one- and two-dimensional multifrequency pulse EPR and pulse ENDOR experiments, as well as continuous wave experiments. It is equipped with two microwave sources and four microwave channels to generate pulse sequences with different amplitudes, phases, and carrier frequencies. A microwave pulse power of up to 100 W is available. Two channels form radiofrequency pulses with adjustable phases for ENDOR experiments. The spectrometer performance is demonstrated by single crystal pulse ENDOR experiments on a copper complex. A HYSCORE experiment demonstrates that the advantages of high-field EPR and correlation spectroscopy can be combined and exploited at Q-band. Furthermore, we illustrate how this combination can be used in cases where the HYSCORE experiment is no longer effective at 35 GHz because of the shallow modulation depth. Even in cases where the echo modulation is virtually absent in the HYSCORE experiment at Q-band, matched microwave pulses allow one to get HYSCORE spectra with a signal-to-noise ratio as good as at X-band. Finally, it is shown that the high microwave power, the short pulses, and the broad resonator bandwidth make the spectrometer well suited to Fourier transform EPR experiments.     

Uniform Field Re-entrant Cylindrical TE$$_{01\text {U}}$$ Cavity for Pulse Electron Paramagnetic Resonance Spectroscopy at Q-band

Uniform field (UF) resonators create a region-of-interest, where the sample volume receives a homogeneous microwave magnetic field (\(B_1\)) excitation. However, as the region-of-interest is increased, resonator efficiency is reduced. In this work, a new class of uniform field resonators is introduced: the uniform field re-entrant cylindrical TE\(_{\text {01U}}\) cavity. Here, a UF cylindrical TE\(_{\text {01U}}\) cavity is designed with re-entrant fins to increase the overall resonator efficiency to match the resonator efficiency maximum of a typical cylindrical TE\(_{011}\) cavity. The new UF re-entrant cylindrical TE\(_{\text {01U}}\) cavity is designed for Q-band (34 GHz) and is calculated to have the same electron paramagnetic resonance (EPR) signal intensity as a TE\(_{011}\) cavity, a 60% increase in average resonator efficiency \(\Lambda _\mathrm{ave}\) over the sample, and has a \(B_1\) profile that is 79.8% uniform over the entire sample volume (98% uniform over the region-of-interest). A new H-type T-junction waveguide coupler with inductive obstacles is introduced that increases the dynamic range of a movable short coupler while reducing the frequency shift by 43% during over-coupling. The resonator assembly is fabricated and tested both on the bench and with EPR experiments. This resonator provides a template to improve EPR spectroscopy for pulse experiments at high frequencies.     

Multifrequency Probe for Pulsed EPR and ENDOR Spectroscopy

The design, construction, and performance of a multifrequency pulsed EPR and ENDOR probe for use at cryogenic temperatures are described. Interchangeable resonators based on a folded strip line design allow variation of the resonance frequency over a range of 5-11 GHz. Variable coupling to the resonator is achieved capacitively via a simple mechanical adjustment which is thermally and mechanically stable. The entire assembly is robust and easily fabricated. Common methods of analyzing the resonator parameters such as the Q-factor and coupling coefficient are discussed quantitatively. Probe performance data and multifrequency pulsed ENDOR spectra are presented.     

Microwave frequency modulation in CW EPR at W-band using a loop-gap resonator

Loop-gap resonator (LGR) technology has been extended to W-band (94GHz). One output of a multiarm Q-band (35GHz) EPR bridge was translated to W-band for sample irradiation by mixing with 59 GHz; similarly, the EPR signal was translated back to Q-band for detection. A cavity resonant in the cylindrical TE011 mode suitable for use with 100 kHz field modulation has also been developed. Results using microwave frequency modulation (FM) at 50 kHz as an alternative to magnetic field modulation are described. FM was accomplished by modulating a varactor coupled to the 59 GHz oscillator. A spin-label study of sensitivity was performed under conditions of overmodulation and gamma2H1(2)T1T2<1. EPR spectra were obtained, both absorption and dispersion, by lock-in detection at the fundamental modulation frequency (50 kHz), and also at the second and third harmonics (100 and 150 kHz). Source noise was deleterious in first harmonic spectra, but was very low in second and third harmonic spectra. First harmonic microwave FM was transferred to microwave modulation at second and third harmonics by the spins, thus satisfying the "transfer of modulation" principle. The loaded Q-value of the LGR with sample was 90 (i.e., a bandwidth between 3 dB points of about 1 GHz), the resonator efficiency parameter was calculated to be 9.3 G at one W incident power, and the frequency deviation was 11.3 MHz p-p, which is equivalent to a field modulation amplitude of 4 G. W-band EPR using an LGR is a favorable configuration for microwave FM experiments.     

1H and31P ENDOR of the isotropic CO2 − signal atg=2.0007 in the EPR spectra of precipitated carbonated apatites

The isotropic EPR signal atg=2.0007 in X-irradiated carbonated apatites, precipitated from aquaeous solutions and dried at 25°C, is investigated with Electron Nuclear Double Resonance (ENDOR). The²³Na,³¹P and¹H ENDOR results indicate that the precursor of this radical is most probably located in an aqueous phase entrapped between the crystallites (the so-called occluded water). Other experimental features, such as the correlation between the appearance of the signal in the EPR spectrum and the presence of some residual (adsorbed and/or occluded) water in the sample seem to strengthen our hypothesis.     

Spin dynamics in continuous wave and pulsed ENDOR spectroscopy of coals

ENDOR experiments on coals recorded using continuous wave (CW) and pulsed techniques appear to give qualitatively different spectra. A matrix proton signal dominates the ENDOR spectrum of coals recorded in the CW ENDOR experiment while both a matrix and local proton ENDOR signals with huperfine couplings of up to 20 MHz are observed in spectra recorded using pulsed excitation techniques. Analysis of these spectra lead to different implications for the structure of the molecules that host the unpaired electron. Using a combination of pulsed EPR (Electron Spin Echo, FID detected hole burning) and pulsed Electron Nuclear Multiple Resonance (Sub-level relaxation, hyperfine selective ENDOR, EPR sub-spectra) experiments, we investigate the electron and nuclear spin dynamics in order to reconcile the different signal amplitudes observed in the CW and pulsed ENDOR spectra. In the CW ENDOR experiment, the results of the FID detected hole burning experiments prove that the low ENDOR signal intensity can not be attributed to spectral diffusion mechanisms competing with ENDOR mechanisms. Instead, we find that an unfavorable ratio of the electron and nuclear spin relaxation rates results in small local ENDOR signals. The matrix line dominates the spectrum because of the large number of matrix protons. In the pulsed ENDOR experiment, the hyperfine contrast selectivity mechanism suppresses the intensity of the matrix ENDOR signal and enhances the amplitudes of the local ENDOR signals. In addition, the ENDOR signal is not a function of the ratio of the electron and nuclear relaxation rates.     

Characterization of free radicals from vitamin K1 and menadione by 2 mm-band EPR,ENDOR and ESEEM

The anion and cation radicals of vitamin K₁ and its analog menadione were characterized using the magnetic resonance techniques of Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM), and Electron Paramagnetic Resonance (EPR) at X-band and 2 mm-band. Theg-factor anisotropy of the radicals at 2 mm-band allow them to be distinguished from each other in the solid state. Theg-factor matrix of the radical anion of vitamin K₁ is virtually identical with that reported for the reduced A₁ acceptor in green plant photosystem I thus demonstrating that reduced A₁ is the anion radical of vitamin K₁.     

A Homebuilt ESE Spectrometer on the Basis of a High-Power Q-Band Microwave Bridge

We present a Q-band spectrometer which was built recently at the Institute of Physical Chemistry of the University of Stuttgart. It allows us to perform the field-sweep electron spin echo (ESE), pulsed electron–nuclear double resonance (ENDOR), relaxation and electron spin echo envelope modulation experiments both at room and low (down to 1.5 K) temperatures. The spectrometer consists of an electromagnet, digital field controller, pulsed microwave bridge, probehead, cryostat, radio frequency unit, pulse programmer and data acquisition electronics. The Q-band microwave bridge with 10.8 W output power is based on a two-stage IMPATT-diode pulse amplifier. The commercial Varian electromagnet system is controlled by a 24-bit home-built digital controller. The external devices are interfaced to the two PCs via GPIB and LAN. The spectrometer control software was developed in Visual C++. It consists of two programs running synchronously on the control PCs. The spectrometer is equipped with a cylindrical TE₀₁₁ cavity constructed both for ESE and for pulsed ENDOR. The cavity fits into a liquid He cryostat thus allowing low-temperature experiments. An 8-bit data acquisition digitizer is used to collect the echo signals, and the PBESR-PRO-400 digital word generator orchestrates the pulse experiments and sets pulse sequences of the microwave bridge. The spectrometer performance is demonstrated on nitrogen impurities in a polycrystalline synthetic diamond, on silver clusters supported on NaA zeolite and electron-irradiated tooth enamel. Authors' address: Igor Tkach, Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany     

Analysis of the tuning and operation of reflection resonator EPR spectrometers

This paper investigates basic characteristics of the electron paramagnetic resonance (EPR) signal obtained from spectrometers employing reflection resonators. General equations are presented which reveal the phase and amplitude dependence on instrumental parameters of both components of the continuous wave (CW) EPR signal (absorption and dispersion). New phase vector diagrams derived from these general equations are presented for the analysis of the EPR response. The dependence of the phase and absolute value of the CW EPR signal on the local oscillator (LO) phase and on resonator offset and coupling is presented and analyzed. The EPR spectrometer tuning procedures for both balanced and unbalanced heterodyne receivers are analyzed in detail using the new phase diagrams. Extraneous signals at the RF input of the microwave receiver (resulting from circulator leakage and reflections in the resonator transmission line) have been taken into account and analyzed. It is shown that a final tuning condition that corresponds to an extremum of the receiver output as a function of the resonator frequency is necessary and sufficient for the acquisition of pure absorption signal. This condition is universal: it applies to all spectrometer configurations in all frequency ranges. High Frequency EPR spectrometer (130 GHz) data are used to generate experimental phase diagrams that illustrate the theoretical concepts presented in the paper. Conditions are presented under which the absorption signal can be measured with complete suppression of the dispersion, independent of the mutual frequency offset between the microwave source and the EPR sample resonator. Equations describing the approximate relationship between changes of the resonator properties (Q-factor and frequency) and paramagnetic susceptibility are derived and analyzed.     

X波段脉冲电子顺磁共振谱仪的矩形腔与微型平面腔的设计

为了满足脉冲式电子顺磁共振谱仪的需要,设计并制作了连续波谐振腔和脉冲谐振腔. 连续波谐振腔采用矩形谐振腔的设计,而脉冲谐振腔采用了微型平面腔的设计. 在设计阶段,使用Ansoft-HFSS三维电磁仿真软件对2种谐振腔进行模拟计算. 微型平面腔的加工采用了微纳加工技术. 制作完成的谐振腔的参数指标由网络分析仪测定. 实验测得2种谐振腔的参数指标符合理论模拟值,并满足脉冲式电子顺磁共振谱仪的要求.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

Single loop multi-gap resonator for whole body EPR imaging of mice at 1.2 GHz

For whole body EPR imaging of small animals, typically low frequencies of 250-750 MHz have been used due to the microwave losses at higher frequencies and the challenges in designing suitable resonators to accommodate these large lossy samples. However, low microwave frequency limits the obtainable sensitivity. L-band frequencies can provide higher sensitivity, and have been commonly used for localized in vivo EPR spectroscopy. Therefore, it would be highly desirable to develop an L-band microwave resonator suitable for in vivo whole body EPR imaging of small animals such as living mice. A 1.2 GHz 16-gap resonator with inner diameter of 42 mm and 48 mm length was designed and constructed for whole body EPR imaging of small animals. The resonator has good field homogeneity and stability to animal-induced motional noise. Resonator stability was achieved with electrical and mechanical design utilizing a fixed position double coupling loop of novel geometry, thus minimizing the number of moving parts. Using this resonator, high quality EPR images of lossy phantoms and living mice were obtained. This design provides good sensitivity, ease of sample access, excellent stability and uniform B(1) field homogeneity for in vivo whole body EPR imaging of mice at 1.2 GHz.     

High-field DNP and ENDOR with a novel multiple-frequency resonance structure.

We describe a new triply tuned (e(-), (1)H, and (13)C) resonance structure operating at an electron Larmor frequency of 139.5 GHz for dynamic nuclear polarization (DNP) and electron nuclear double-resonance (ENDOR) experiments. In contrast to conventional double-resonance structures, the body of the microwave cavity simultaneously acts as a NMR coil, allowing for increased efficiency of radiofrequency irradiation while maintaining a high quality factor for microwave irradiation. The resonator design is ideal for low-gamma-nuclei ENDOR, where sensitivity is limited by the fact that electron spin relaxation times are on the order of the RF pulse lengths. The performance is demonstrated with (2)H ENDOR on a standard perdeuterated bis-diphenylene-phenyl-allyl stable radical. In DNP experiments, we show that the use of this resonator, combined with a low microwave power setup (17 mW), leads to significantly higher (1)H signal enhancement (epsilon approximately 400 +/- 50) than previously achieved at 5-T fields. The results emphasize the importance of optimizing the microwave B(1) field by improving either the quality factor of the microwave resonator or the microwave power level.     

Anion and cation vacancies in CdTe

The cadmium vacancy (V_Cd) and the tellurium vacancy (V_Te) in CdTe are identified by Electron Paramagnetic Resonance (EPR). The EPR spectrum of the singly ionised V_Te reveals cubic (unpertubed) symmetry and the hyperfine structure shows that the unpaired electron is equally spread over the four Cd neighbors. Further figand hyperfine interactions with the more distant neighbors are resolved by Electron Nuclear Double Resonance (ENDOR). The V_Cd is a double acceptor and the EPR spectrum is observed in its singly negative charge state. The symmerty is found to be trigonal, which can be explained in a model in which the hole occupies a dangling bondt ₂ orbital and the orbital degeneracy is removed by a static Jahn-Teller distortion. The hyperfine interaction shows that the hole is localised on one of the four Te neighbors.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

微环芯腔与锥形纳米光纤耦合过程的实验研究

在实验上研究了共振于铯原子跃迁线附近的微环芯腔与锥形纳米光纤的耦合特性。通过精密控制微环芯腔与锥形纳米光纤的相对位置,实现了两者的欠耦合、临界耦合和过耦合的精确控制。当微环芯腔与锥形纳米光纤间距为0.6μm时,系统达到临界耦合,透射率为0.3%±0.3%,耦合效率为99.7%±0.3%。由微环芯腔透射光谱得到微环芯腔的自由光谱区为1067±5GHz,等效腔长为223±1μm,线宽为2.9±0.1GHz,本征品质因数为(6.2±0.6)×10~4。随着微环芯腔与锥形纳米光纤间距的减小,微环芯腔的线宽逐渐增大,共振频率发生红移,频率移动为19.2±0.1GHz。该研究找到了有效控制微环芯腔与锥形纳米光纤耦合状态的方法,为下一步实现微环芯腔与原子间强耦合奠定了实验基础。同时该研究加深了人们对微环芯腔不同耦合状态的认识,为研究欠耦合和过耦合状态提供了实验基础。     

Compact passively q-switched Nd:YAG laser for 2D micromachining

We present a compact passively q-switched diode end pumped Nd:YAG laser at 1064 nm for 2D micromachining. It consists of a 5.5 cm long plano-concave end pumped resonator carrying a Cr:YAG passive q-switch inside the cavity. With an optical conversion efficiency of 46 and 33% the laser emits 1.4 W in CW and 986 mW in q-switched mode at a current of 2.5 A. After using a 2 mm circular aperture the output is seen in TEM₀₀ mode with a single pulse energy of 5 mJ. The laser produced circular holes of diameter 75 μm in 25 μm thick Tantalum foils. Actual results of 1D and 2D machining are shown along with the diffraction patters of the samples.     

Absolute signs of hyperfine coupling constants as determined by pulse ENDOR of polarized radical pairs

A novel method that allows the determination of absolute signs of hyperfine coupling constants in polarized radical pair (RP) pulse electron-nuclear double resonance (ENDOR) spectra is presented, The variable mixing time (VMT) ENDOR method used here leads to a separation of ENDOR transitions originating from different electron spin manifolds by employing their dependence on the time-dependent parameters of the pulse sequence. The simple kinetic model of the RP VMT ENDOR experiment shows very good agreement with the experiments performed on the P ₇₀₀ ^.+ A ₁ ^.- RP in photosystem I. This method relies on the selective excitation of absorptive or emissive lines of one radical in the RP EPR spectrum and therefore requires high spectral resolution. This condition was fulfilled for the system studied at the low-field edge of the RP EPR spectrum obtained at Q-band. The method presented here has a very high sensitivity and does not require any equipment additional to the one used for RP pulse ENDOR. The VMT ENDOR method offers the possibility for selective suppression of signals from different electron spin manifolds.     

Diode-end-pumped CW and actively Q-switched Tm,Ho:YLF ring laser

This paper presents CW and acoustic-optical Q-switched Tm,Ho:YLF laser performance in a laser-diode end-pumping figure-eight ring resonator structure at 77 K. Under CW operation, different transmission of output couplers and different cavity length were used to achieve best characteristics of the laser. The maximum power of 1.85 W is achieved with the threshold of 2.09 W, slope efficiency of 28.14% and optical-to-optical efficiency of 20.3% on the condition of 1 m cavity length and 18% transmission. Under pump power of 4 W, laser characteristics under Q-switched operation with different pulse repeat frequency was investigated. Maximum energy of 5.86 mJ is achieved with pulse width of 171.2 ns, peak power of 34.2 kW and dynamic-static ratio about Q-switch laser of 0.62 at pulse repeat frequency 70 Hz.