Showing papers by "Jack H. Freed published in 1988"

1‐mm wave ESR spectrometer

Abstract: An ESR spectrometer operating at 250‐GHz frequency (1.22‐mm wavelength) and 9‐T magnetic field is described. The utilization of far‐infrared (FIR) technology greatly simplifies its design and performance. Good frequency and field stability are achieved by unique designs which also conveniently permit the magnetic field to be swept. The potential utility of FIR–ESR is illustrated with examples of spectra from polycrystalline and liquid samples. In the latter case the increased spectral sensitivity to motional dynamics is stressed. Several ways in which the FIR–ESR spectrometer may be improved are also discussed.

Two‐dimensional Fourier transform ESR correlation spectroscopy

Abstract: We describe our pulsed two‐dimensional Fourier transform ESR experiment and demonstrate its applicabilty for the double resonance of motionally narrowed nitroxides. Multiple pulse irradiation of the entire nitroxide spectrum enables the correlation of two precessional periods, allowing observation of cross correlations between hyperfine lines introduced by magnetization transfer in the case of a three‐pulse experiment (2D ELDOR), or coherence transfer in the case of a two‐pulse experiment (COSY). Cross correlations are revealed by the presence of cross peaks which connect the autocorrelation lines appearing along the diagonal ω1=ω2. The amplitudes of these cross peaks are determined by the rates of magnetization transfer in the 2D ELDOR experiment. The density operator theory for the experiment is outlined and applied to the determination of Heisenberg exchange (HE) rates in 2,2,6,6‐tetramethyl‐4‐piperidone‐N‐oxyl‐d15 (PD‐tempone) dissolved in toluene‐d8. The quantitative accuracy of this experiment is established by comparison with the HE rate measured from the dependence of the spin echo T2 on nitroxide concentration.

E.S.R. and D.S.C. investigations of phase transitions in polymorphic 4-n-alkoxybenzylidene-4′-n-alkylanilines

Abstract: It is shown that the McMillan parameter M = T SAN/T N1 (where T SAN and T NI are respectively the temperatures of the smectic A to nematic (SAN) and the nematic to isotropic (NI) phase transitions) is useful in analysing the crossover between second and first order behaviour of the SNN transition in the nO.m homologous liquid crystal series (the 4-n-alkoxybenzylidene-4′-n-alkylanilines). Using a phase diagram of orientational ordering versus M for this series, as obtained in this work (from E.S.R. and D.S.C), a symmetric tricritical point with mean field exponent β2 = 1 is demonstrated. In a preliminary study of E.S.R. linewidth parameters B and C of nitroxide spin probes dissolved in members of the nO.m series exhibiting a first order SAN transition, critical-type divergences are observed near this transition. In the case where M is closer to 0.959 (the value at the tricritical point), these divergences appear similar to those previously observed in related nO.m members with a second order SAN t...

The effects of spin in gases

Abstract: Low-density gases, in which atoms are separated by large distances, have long provided an enjoyable playground for physicists. One might suppose the pleasure of the playground would by now have been exhausted by the very simplicity of low-density gases. Recent work by a number of investigators including the author shows that this is not the case low-density gases continue to serve up a rich variety of phenomena as well as counterintuitive surprises. In particular, the macroscopic properties of a gas composed of individual hydrogen or helium atoms can under special circumstances by changed dramatically by quantum-mechanical effects. According to quantum theory, the nucleus of an atom behaves in a way similar to a rotating top, which has angular momentum about its axis of rotation; that is, the nucleus has spin, known more precisely as spin angular momentum. If the atoms of a gas are spin-polarized, so that their nuclei all have their spins pointing in the same direction, the viscosity of the gas can be changed enormously and so can its ability to conduct heat. Quantum-mechanical correlations among the nuclei called spin waves, which up to now had been observed only in certain liquids and solids such as magnets, canmore » also arise. The changes are large enough for one to say the quantum-mechanical effects have caused the gas to take on entirely new properties. In a certain sense it is amazing to think that polarizing the nuclear spins can have any effect on the macroscopic properties of the gas, since the nuclear spins are son weakly coupled to the outside world. Yet the observations are in full agreement with with theory. Moreover, because spin-polarized gases are still fairly simple systems, they can be understood in terms fundamental principles, something that is still not possible to do in the case of liquids and solids.« less