The paper describes the design, implementation, and technical characteristics of a portable Q-band microwave (mw) bridge based on the Gunn diode with the potential use for electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) spectroscopies. The mw frequency can be electronically adjusted in the range of 36-38 GHz with the maximal mw output of 120 mW and electronic attenuation of 0-60 dB. The value of the mw frequency can be stabilized and changed via automatic frequency control for direct and alternating current. A self-written Matlab-based program allows tuning and operating the bridge through the RS-485 interface. Examples of the EPR spectra implemented into the magnetic system of the Bruker ESP300 commercial spectrometer are shown

To study the effects of dynamic nuclear polarization (DNP) in the X-band (microwave frequency of 9 GHz), using the capabilities provided by commercial EPR equipment, a part of the EPR spectrometer associated with the excitation and detection of double electron-nuclear resonance signals (ENDOR) has been modernized. Using the developed preamplifier of NMR signals, a homemade "Kazan Nova II" NMR spectrometer was implemented into the radio frequency path of the EPR spectrometer. The tuning and matching circuits made it possible to observe the NMR and DNP signals on protons in the frequency range 14.5-15.2 MHz. The performance of the DNP equipment was tested for a solution of the stable nitroxyl radical TEMPOL in benzene and a crude oil sample. The DNP effects caused by the Overhauser and solid effects were observed. The modernization of the existing EPR equipment creates a basis for further expanding its capabilities to study DNP effects in various systems at different conditions (in the pulsed mode of saturation of the EPR lines, with the temperature lowering, under the action of optical excitation, etc)

Double-quantum (DQ) coherence transfers and signals in two-pulse DQ and five-pulse DQM (double quantum modulation) pulsed EPR sequences, utilized for orientation selectivity and distance measurements in biological systems using nitroxide biradicals, have been calculated here for X-band (9.26 GHz) pulsed EPR (electron paramagnetic resonance) using a rigorous numerical algorithm. It is shown, in general, that both, a finite (selective) pulse, rather than an infinite (non-selective) pulse, and the dipolar interaction between the two nitroxide radicals, are needed to produce non-zero coherence transfers in 0→2 and 2→-1 transitions. Furthermore, the simulations show that there exits orientational selectivity, as exhibited by the large value of the coherence transfer probability, T0→2, for those coupled nitroxides, whose dipolar axes, relative to the external magnetic field, are oriented symmetrically, within a small region, within about ±10° away from the magic angle θ = 54.74° and its supplementary angle θ = 125.26°. It increases monotonically as the amplitude of the irradiation field (B1) decreases. The magnitudes of the coherence transfers in the transitions 0→2 and 2→-1 are found to be about the same. They depend upon both, the amplitude of B1 and the duration of the pulse. As well, they increase significantly with increasing d, as found for d=10.0, 20.0, 30.0 MHz, where d=2D/3, with D being the dipolar-coupling constant. The numerical calculations, using Monte-Carlo averaging, reveal that the Pake doublets occur at ±3d/4$ and ±d for the two-pulse DQ and the five-pulse DQM sequences, respectively, as calculated for d=0.5, 7.0, 10.0, 20.0, 30.0, 40.0, 50.0 MHz. It is seen that for d = 0.5 MHz, considered here, for which the modulation depth can be measured within the dead-time, the dipolar depth of the modulation is ≈100%, which indicates that the DQ and DQM sequences are more efficient for distance measurements as compared to other techniques, e.g., DEER (double electron-electron resonance). The numerical algorithm for the five-pulse DQM sequence presented here is exploited to provide a good fit to the published experimental data. Simulations were also carried out at Ku-band (17.6 GHz), which showed that there occur no orientational selectivity at this band, unlike that at X-band. On the other hand, the signals and their Fourier transforms are found to be relatively more intense at Ku-band