Field-cycling nuclear quadrupole double resonance experiment. The sample is polarized at H P , NMR is performed at H D and the search for pure quadrupole resonance occurs at zero field. The times for initial saturation, polarization, detection via AFP and irradiation at zero applied field are τ Sat , τP,τD, and τ L , respectively.
Block diagram of the NQDR spectrometer.
(a) Schematic of harmonic suppression filter array. Each filter in the array is a five-element low-pass Chebychev filter. One and only one switch in the filter array is closed at a given time. (b) Schematic of the binary matching and tuning capacitor arrays. Any combination of switches in the capacitor arrays may be closed.
SQUID detected proton NMR signal of ethylene glycol: 50 μL frozen in a quartz bucket under Ar atmosphere. Polarized at 1.0 T for 30 min at 4.5 K, then change field adiabatically to 0.1 T where NMR is performed. (a) SQUID voltage reflecting change in magnetic flux due to magnetic field drift and proton magnetization inversion via AFP. (b) Signal due to change in proton magnetization after a linear correction for field drift. (c) Derivative of the SQUID voltage. Both peaks are integrated and averaged.
PQR spectrum of 17O at natural abundance in diphenyl ether (C6H5–O–C6H5). Measurements were made at 2 K. Resonances ν 1 and ν 2 correspond to 1/2–3/2 and 3/2–5/2 transitions of 17O. The sinusoidally modulated search RF is equivalent to two CW RF separated by a Δf = 20 kHz. This Δf is chosen to approximate the dipolar linewidth of the PQRs of 17O with nearby protons. Time at zero field τ L = 300 s. Data are scaled to correct for relaxation effects implying that the baseline should be centered on 1.0. The observed baseline shift is attributed to RF heating of the sample and is consistent with an unusually strong proton T 1 temperature dependence at low temperatures that we observed.
(a) Parameters for low-pass (LP), 5 element, 50 Ohm, Chebychev filters in the harmonic suppression filter array. The minimum fundamental frequency that will have at least 10 dB of power attenuation of its second harmonic is designated as fmin. The maximum fundamental frequency that will have less than 3 dB of power attenuation is designated by fmax. (b) Parameters for high-pass (HP), 5 element, 50 Ohm, Chebychev filter used to protect dipolar spin system from inadvertent heating by direct RF.
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