David B. Baker

New, compensated Carr-Purcell sequences

Abstract New, compensated Carr-Purcell pulse sequences are reported, all based on x and y phase alternation of the π pulses. The sequences compensate cumulative pulse errors for all three components of magnetization. Applications include the measurement of homonuclear dipole coupling in the presence of chemical shifts and the measurement of heteronuclear dipole coupling in magic-angle spinning experiments (REDOR). The performance of the new pulse sequences is compared experimentally to previously reported schemes.

Novel measurements of nuclear spin cross-relaxation in metal hydrides

Abstract Recent NMR measurements of proton relaxation have shown an unexpected frequency dependence in several metal-hydrogen systems at low temperatures. An explanation proposed by the Ames group involves cross-relaxation of protons to quadrupolar metal nuclei. We test this hypothesis with relaxation experiments involving large amplitude field modulation. The field modulation sweeps the proton resonance through the distribution of metal frequencies. Thus the protons come into contact with more metal nuclei, increasing the proton relaxation rate. Relaxation measurements have been performed on several samples and the proton resonance frequency, the modulation frequency and the modulation amplitude have been varied. Also, proton relaxation rates T1−1 have been measured on slowly spinning samples where the rotation produces periodic variations of the metal quadrupolar frequencies.

Frequency-dependent proton spin relaxation in Zr2PdHx

Abstract The proton nuclear spin-lattice relaxation rates T1−1 of amorphous Zr2PdH2.9 and crystalline Zr2PdH1.9 were examined from 4 to 400 K at several frequencies. The observed temperature dependence is nearly T1−1 ∝ T, but the relaxation rate depends on frequency, unlike the behavior of the expected Heitler- Teller-Korringa mechanism. Furthermore, the relaxation rates in the crystalline and amorphous samples are surprisingly similar. The frequency dependence can be explained by two models: proton cross-relaxation to metal nuclei and low frequency tunneling motions.