Erwin L. Hahn

Pure nuclear quadrupole resonance of naturally abundant 17O in organic solids

Abstract A sensitive nuclear double resonance method enables measurement of naturally abundant 17 O nuclear quadrupole resonance transition frequencies in powdered organic compounds. The nuclear quadrupole parameters enable the identification of resonances from CO, COH, and COC groups in a set of compounds.

Nuclear quadrupole resonance detected at 30 MHz with a dc superconducting quantum interference device

A dc superconducting quantum interference device is used as a tuned radio frequency amplifier at liquid helium temperatures to detect pulsed nuclear quadrupole resonance at ∼30 MHz. At a bath temperature of 4.2 K, a total system noise temperature of 6±1 K has been achieved, with a quality factor Q of 2500. A novel Q spoiler, consisting of a series array of Josephson tunnel junctions, reduces the ring‐down time of the tuned circuit after each pulse. The minimum number of Bohr magnetons observable from a free precession signal after a single pulse is ∼2×1016 in a bandwidth of 10 kHz.

Upper Limits to Electric‐Field‐Induced Nuclear Magnetic Dipole—Dipole Couplings in Polar Liquids

The alignment of polar molecules in liquids by an external electric field is investigated by pulsed nuclear magnetic resonance methods. Sensitive rotating‐frame techniques are described for the detection of the expected induced nuclear magnetic dipole—dipole coupling. In no case could an induced coupling be detected. The upper experimental limits for the induced couplings disagree with the predictions of the Lorentz localfield model. For nitrobenzene and p‐nitrotoluene, disagreement is also obtained with the Onsager local‐field model by a reduction of almost an order of magnitude in the expected size of the effect. The negative results for these two compounds are indicative of strong local antiparallel ordering of the molecular electric dipoles. The induced coupling in p‐nitrotoluene, reported by Buckingham and McLauchlan using standard NMR methods, is found to be absent in this investigation, which employs a measuring technique more sensitive by at least one order of magnitude.

Nuclear magnetic resonance with DC SQUID preamplifiers

Describes five experiments illustrating the application of DC SQUID (superconducting quantum interference device) amplifiers to magnetic resonance experiments. The first experiment involved the observation of nuclear spin noise, that is the spontaneous emission of photons from an ensemble of /sup 35/Cl nuclei in the zero polarization state. The second experiment involved the use of the Q-spoiler in conventional NQR (nuclear quadrupole resonance) and NMR measurements in which one applies a large RF pulse to the nuclei to make them precess. The Q-spoiler was then used in an experiment to detect the oscillating electric polarization induced by /sup 35/Cl nuclear quadrupole moments. The fourth experiment involved the extension of the use of the Q-spoiler and SQUID amplifier to NMR, detecting the signal from /sup 119/Sn nuclei at 30 MHz. Finally, a SQUID amplifier was used with an untuned input circuit to detect the low-frequency NMR signal at 55 kHz from /sup 195/Pt nuclei in an applied field of 60 Gauss. >

Noise triggering of radiation damping from the inverted state

Abstract Spontaneous radiation damping in nuclear magnetic resonance spectroscopy of sharp lines is commonly encountered in highly magnetized samples following magnetization inversion. The time delay for the maximum of the hyperbolic secant like signal to develop following inversion is measured and calculated. The distribution function of the measured delay times is Gaussian–Boltzmann and can be used to predict the sensitivity of both signal delay and phase to inversion errors.

Radiation damping with inhomogeneous broadening: limitations of the single bloch vector model

There are several examples in the literature where effects of inhomoge- neous broadening on radiation damping of free precession signals have been described using a phenomenological 1 / T term in the Bloch equations. The inappropriate use of this 2 model is illustrated for specific cases. Correct signal shapes are predicted only when isochromat solutions of Blochs equations are integrated over the spectrum of the inhomo-

Three component spin echo generation by radiation damping

Exploration of the theory for spin–cavity coupling in inhomogeneously broadened spin ensembles shows that all three components of magnetization can be refocused. The conventional spin echo generated by two components of magnetization is a special case of this three component refocusing in the limit of negligible radiation damping. We demonstrate this effect by an analytical theorem, numerical simulation, and experiment.

The nuclear-quadrupole-induced dipole moment of HD

Abstract We calculate in a simple MO way the dipole moment induced in HD by the nuclear quadrupole of the deuteron, as a function of the internuclear separation. We find a reversal of the direction of the dipole with increasing internuclear separation, for which we note indirect experimental evidence. As a check on the accuracy of our result we also calculate the electric field gradient induced at the D nucleus by an external electric field, per unit external field, which for exact theory would be exactly the same function. The form of the function so obtained is indeed similar - although not identical - to that for the dipole moment. The dipole moment at the equilibrium internuclear separation is ≈0.075 atomic units per unit quadrupole moment, directed H−→D+. (For the measured quadrupole moment of the deuteron, this is ≈2×10−11 D.) However, we find the induced field gradient at the deuteron for this separation to be 0.094 atomic units per unit external field, in the opposite direction.

Nuclear spin thermometry below 1 °K

A thermometry technique is described which consists of measuring the magnetization of a system of nuclear spins by means of pulsed nuclear magnetic resonance. Such a thermometer is useful at temperatures below that of pumped liquid helium, which is used as a calibration point. This technique has been employed to detect nuclear spin temperatures as low as 10-5 °K in a specimen of sodium metal; these temperatures were produced by the well known method of nuclear cooling originated in the Clarendon Laboratory. The results of our experiments confirm the conclusions drawn by the original authors, namely that, after isentropic demagnetization, the nuclear spins reach temperatures of the order of microdegrees, while the metallic lattice and the conduction electrons remain at the temperature of the cooled paramagnetic salt reservoir. Spin-lattice relaxation times Ƭ1 have been measured for copper metal at temperatures ranging from 0·02 to 0·35 °K and for platinum metal at temperatures ranging from 0·05 to 2 °K. The copper measurements give T1T = 1·12 s °K ±10% , in good agreement with the results obtained by Anderson & Redfield (1959) in the temperature range 1 to 4·2 °K. An earlier report of an increase in T1T for platinum by a factor of 2 at temperatures below 1 °K is corrected by the present measurements, which show this quantity to be constant at 30 ms °K at temperatures down to 0·07 °K. It is further shown that the erroneous earlier measurements were caused by eddy current heating in the metallic specimens. The problem of eddy current heating, which was significant for all of the experiments reported here, is analyzed in some detail. A complete description is given of the electronic circuitry developed for the present low-frequency pulsed nuclear resonance experiments.