Annual Report 1974-75

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Nuclear quadrupole resonance as a non-destructive testing tool

Nuclear pure quadrupole resonance (NQR) is a resonance technique that provides distinctly different information from that provided by nuclear magnetic resonance (NMR). In NMR the splitting of the energy levels, and therefore the frequency observed, occurs because of the interaction of the nuclear magnetic moment with an external magnetic field. Information about the system under study comes from perturbations on this magnetic interaction. These perturbations lead to a broadening of the line, or to relaxation effects on the interchange of energy between the spins and the lattice, and among the spins. In NQR the primary interaction is between the electric quadrupole moment of a nucleus and the electric field gradient at that nucleus. The field gradient is provided by internal interactions in the sample itself, arising from the chemical bonds, rather than by an external field. Anything that changes the bonding environment, such as tensile stress, will cause shifts in the quadrupole resonance frequency. All nuclei with spin greater than 1/2 have a nuclear quadrupole moment, in addition to their magnetic moment. The nucleus used as an example in this paper is 75As, which has spin 3/2

Optical detection of NMR J-spectra at zero magnetic field

Scalar couplings of the form J I_1 \cdot I_2 between nuclei impart valuable information about molecular structure to nuclear magnetic-resonance spectra. Here we demonstrate direct detection of J-spectra due to both heteronuclear and homonuclear J-coupling in a zero-field environment where the Zeeman interaction is completely absent. We show that characteristic functional groups exhibit distinct spectra with straightforward interpretation for chemical identification. Detection is performed with a microfabricated optical atomic magnetometer, providing high sensitivity to samples of microliter volumes. We obtain 0.1 Hz linewidths and measure scalar-coupling parameters with 4-mHz statistical uncertainty. We anticipate that the technique described here will provide a new modality for high-precision "J spectroscopy" using small samples on microchip devices for multiplexed screening, assaying, and sample identification in chemistry and biomedicine.Comment: 15 pages, 4 Figure

Global lake responses to climate change

Climate change is one of the most severe threats to global lake ecosystems. Lake surface conditions, such as ice cover, surface temperature, evaporation and water level, respond dramatically to this threat, as observed in recent decades. In this Review, we discuss physical lake variables and their responses to climate change. Decreases in winter ice cover and increases in lake surface temperature modify lake mixing regimes and accelerate lake evaporation. Where not balanced by increased mean precipitation or inflow, higher evaporation rates will favour a decrease in lake level and surface water extent. Together with increases in extreme-precipitation events, these lake responses will impact lake ecosystems, changing water quantity and quality, food provisioning, recreational opportunities and transportation. Future research opportunities, including enhanced observation of lake variables from space (particularly for small water bodies), improved in situ lake monitoring and the development of advanced modelling techniques to predict lake processes, will improve our global understanding of lake responses to a changing climate

MEASUREMENT OF THE STARK EFFECT IN A FLYGARF.-RALLE MICROWAVE SPECTROMETER

$^{\ast}$ Work supported by NSF and PRF. $^{1}$ T. J. Balle and W. H. Flygare, Rev. Sci. Inst. 52. 35 (1981).Author Institution: Noyes Chemical Laboratory, University of IllinoisIn the Flygare-Balle Fourier transform spectrometer a microwave pulse is applied to a Fabry-Perot cavlty. synchronized with an expanding jet of a gas mixture from a pulsed supersonic $nozzle.^{1}$ Measurement of the Stark effect in this type of spectrometer has been hampered by the difficulty of producing an adequately homogeneous electric field without degrading spectrometer performance. The common approach to the production of a homogeneous electric field is to use large. parallel metal plates, spaced as closely as possible. This method works poorly in the present case because the plates disturb both the microwave field and the gas expansion. We have built a device which generates a good electric field (line broadening $<0.5^{\ast}$ of the Stark shift) over a volume of $-8 \times B \times 10^{\prime\prime}$, but does not disturb the gas expansion significantly. It consists of two square Plexiglass frames $(13 \times 13^{\prime/\prime} o.d.. 11 \times 11^{\prime\prime} i.d.$ and $1^{\prime\prime}$ thick), joined at the corners by four aluminum rods 2*** long. forming a structure similar to a box kite. Twelve 24 gauge wires are stretched 1*** apart along each of the four long faces. At one end, each pair of adjacent wires is connected by a 5 Mohn resistor and the four corner wires to external terminals. The orientation of the electric field is selected by the connections of the terminals to the HV power supply. The microwave field is unaffected by the assembly at frequencies above 10 GHz. tolerable at 9 GHz, but unusable below 8 GHz. Examples of performance will be presented. Modifications to allow operation at lower frequencies are under consideration