Category Archives: Arbitrary Waveform Generator

Pulse-Shaped Dynamic Nuclear Polarization under Magic-Angle Spinning #DNPNMR

Equbal, Asif, Kan Tagami, and Songi Han. “Pulse-Shaped Dynamic Nuclear Polarization under Magic-Angle Spinning.” The Journal of Physical Chemistry Letters 10, no. 24 (December 19, 2019): 7781–88.

Dynamic nuclear polarization (DNP) under magic-angle spinning (MAS) is transforming the scope of solid-state NMR by enormous signal amplification through transfer of polarization from electron spins to nuclear spins. Contemporary MAS-DNP exclusively relies on monochromatic continuous-wave (CW) irradiation of the electron spin resonance. This limits control on electron spin dynamics, which renders the DNP process inefficient, especially at higher magnetic fields and non cryogenic temperatures. Pulse-shaped microwave irradiation of the electron spins is predicted to overcome these challenges but hitherto has never been implemented under MAS. Here, we debut pulse-shaped microwave irradiation using arbitrary-waveform generation (AWG) which allows controlled recruitment of a greater number of electron spins per unit time, favorable for MAS-DNP. Experiments and quantum mechanical simulations demonstrate that pulse-shaped DNP is superior to CW-DNP for mixed radical system, especially when the electron spin resonance is heterogeneously broadened and/or when its spin−lattice relaxation is fast compared to the MAS rotor period, opening new prospects for MAS-DNP.

Amplification of Dynamic Nuclear Polarization at 200 GHz by Arbitrary Pulse Shaping of the Electron Spin Saturation Profile #DNPNMR

Kaminker, Ilia, and Songi Han. “Amplification of Dynamic Nuclear Polarization at 200 GHz by Arbitrary Pulse Shaping of the Electron Spin Saturation Profile.” The Journal of Physical Chemistry Letters 9, no. 11 (June 7, 2018): 3110–15.

Dynamic nuclear polarization (DNP) takes center stage in nuclear magnetic resonance (NMR) as a tool to amplify its signal by orders of magnitude through the transfer of polarization from electron to nuclear spins. In contrast to modern NMR and electron paramagnetic resonance (EPR) that extensively rely on pulses for spin manipulation in the time domain, the current mainstream DNP technology exclusively relies on monochromatic continuous wave (CW) irradiation. This study introduces arbitrary phase shaped pulses that constitute a train of coherent chirp pulses in the time domain at 200 GHz (7 T) to dramatically enhance the saturation bandwidth and DNP performance compared to CW DNP, yielding up to 500-fold in NMR signal enhancements. The observed improvement is attributed to the recruitment of additional electron spins contributing to DNP via the cross-effect mechanism, as experimentally confirmed by two-frequency pump–probe electron–electron double resonance (ELDOR).

Arbitrary waveform modulated pulse EPR at 200GHz

Kaminker, I., R. Barnes, and S. Han, Arbitrary waveform modulated pulse EPR at 200GHz. J Magn Reson, 2017. 279: p. 81-90.

We report here on the implementation of arbitrary waveform generation (AWG) capabilities at approximately 200GHz into an Electron Paramagnetic Resonance (EPR) and Dynamic Nuclear Polarization (DNP) instrument platform operating at 7T. This is achieved with the integration of a 1GHz, 2 channel, digital to analog converter (DAC) board that enables the generation of coherent arbitrary waveforms at Ku-band frequencies with 1ns resolution into an existing architecture of a solid state amplifier multiplier chain (AMC). This allows for the generation of arbitrary phase- and amplitude-modulated waveforms at 200GHz with >150mW power. We find that the non-linearity of the AMC poses significant difficulties in generating amplitude-modulated pulses at 200GHz. We demonstrate that in the power-limited regime of omega1<1MHz phase-modulated pulses were sufficient to achieve significant improvements in broadband (>10MHz) spin manipulation in incoherent (inversion), as well as coherent (echo formation) experiments. Highlights include the improvement by one order of magnitude in inversion bandwidth compared to that of conventional rectangular pulses, as well as a factor of two in improvement in the refocused echo intensity at 200GHz.

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