F. G. Ullman

Eddy correlation measurements of methane flux in a northern peatland ecosystem

A pilot study to measure methane flux using eddy correlation sensors was conducted in a peatland ecosystem in north central Minnesota. A prototype tunable diode laser spectrometer system was employed to measure the fluctuations in methane concentration.The logarithmic cospectrum of methane concentration and vertical wind velocity fluctuations under moderately unstable conditions had a peak nearf = 0.10 (wheref is the nondimensional frequency) and was quite similar to the cospectra of water vapor and sensible heat. Daytime methane flux during the first two weeks of August ranged from 120 to 270 mg m-2 day-1. The temporal variation in methane fluxes was consistent with changes in peat temperature and water table elevation. Our results compared well with the range of values obtained in previous studies in Minnesota peatlands.These field observations demonstrate the utility of the micrometeorological eddy correlation technique for measuring surface fluxes of methane. The current state-of-the-art in tunable diode laser spectroscopy makes this approach practical for use in key ecosystems.

Vanishing of the phase transitions and quantum effects in K2SeO4 at high pressure

Abstract The effects of pressure on the dielectric properties and phase transition in K2SeO4 were investigated. The incommensurate-to-commensurate ferroelectric transition vanishes at 0.73 GPa, and the paraelectric-to-incommensurate transition is expected to vanish at ∼ 1.13 GPa. These large pressure effects can be understood in terms of the delicate balance between Coulomb and short-range interactions. The results show the increased influence of quantum effects on approaching the displacive limit (i.e. Tc = O K), and they confirm the existence of a γ = 2 regime in the temperature dependence of the susceptibility, as predicted by recent theories.

Stress dependence of Tc for the incommensurate phase transition in K2SeO4

Abstract We have measured the uniaxial stress dependence of the frequency of the zone center A1 soft mode in the low temperature phase of K2SeO4. The frequency shows a strong decrease with stress. We infer from this a similar decrease of the incommensurate phase transition temperature Tc. This behavior is explained by detailed lattice dynamical calculations for K2SeO4 which reveal the unusual balance between Coulomb and short-range forces in this structure. Our calculations show that this balance is changed by stress in such a way that the quasi-harmonic normal mode frequencies harden drastically. Tc is reduced by this purely volume dependent effect. This unusual balance between Coulumb and short-range forces also provides a natural explanation for the occurence of an incommensurate phase transition in K2SeO4. We infer that similar transitions are likely in isomorphous structures.

AN INTERCOMPARISON OF TWO TUNABLE DIODE LASER SPECTROMETERS USED FOR EDDY CORRELATION MEASUREMENTS OF METHANE FLUX IN A PRAIRIE WETLAND

Abstract An intercomparison was made between two tunable diode laser spectrometers used to measure methane fluxes by the eddy correlation technique at a prairie wetland site. The spectrometers were built by Unisearch Associates Inc. of Concord, Ontario, Canada, and Campbell Scientific Inc. of Logan, Utah, and were models EMS-50 and TGA-100, respectively. The fluxes were found to agree very well with each other in the range of 0 to 42 mg m−2 h−1. The TGA-100 was observed to exhibit offset drifts. Most of the time, when the offset was only slowly changing (as compared to the eddy correlation averaging time), these drifts did not affect the calculated fluxes. There were times, however, when the offset changed fast enough to have a noticeable affect on the fluxes. The EMS-50 also exhibits some drifting of the measured concentration. It was, however, much slower and of smaller amplitude than the drift seen in the TGA-100.

Optic-mode coupling in ferroelectric gadolinium molybdate☆

Abstract Separate measurements of the A1(TO) and A1(LO) Raman spectra of ferroelectric gadolinium molybdate at 80°K and above have elucidated the origin of the anomalous temperature dependence of the two lowest frequency lines in the A1(TO) spectrum. The observed behavior is postulated to be the result of coupling among modes at 44.5, 51.5, and 83 cm−1 (at 80°K). The 44.5 and 83 cm−1 modes become the degenerate, soft zone-boundary modes of the paraelectric phase while the 51.5 cm−1 mode changes to B2 symmetry. The two lowest frequency lines are the same as those observed previously in i.r. absorption.

Uniaxial stress dependence of the incommensurate phase transition in K2SeO4

Abstract The effect of uniaxial stress along the orthorhombic “c” axis of K2SeO4 crystals (c

Light-scattering study of the non-zero wave vector incommensurate soft mode in K2SeO4

Abstract Observations are reported of inelastic light scattering from the non-zero wave vector soft phonon that drives the incommensurate structural phase transition in K 2 SeO 4 . They are shown to correlate with neutron scattering measurements of phonon frequency shifts on cooling to the incommensurate transition temperature. The observed light scattering is suggested to arise from intrinsic disorder.

New measurements of the Raman spectrum of potassium selenate

Abstract The Raman spectrum of K2SeO4 has been re-measured at 140 K and 110 K with neon line calibration and numerical fitting of peak frequencies to obtain ±0.1 cm−1 precision. Although the results at 140 K confirm the measurements of the internal mode peak frequencies reported by Unruh which appear to agree with the factor group predictions for the perfect crystal, arguments are presented to show that the orientationally-disordered selenate model still holds. Additional measurements in the incommensurate phase at 110 K revealed line-splitting similar to those reported for infrared spectra; a comparison with those results is given.

Analysis of the Raman Spectrum of the Higher Frequency Modes of Rb2ZnCl4

The stretching-region (250–330 cm-1) internal modes of Rb2ZnCl4 were studied by Raman-scattering from 190 to 330 K. Four lines, peaking at about 275, 285, 295 and 304 cm-1 are clearly resolved below the normal-to-incommensurate phase transition (307 K in these measurements); above 307 K, the four merge into three and the 285 cm-1 peak is no longer detectable. The derivatives of these spectra were fitted to the derivative of an overlapped, spectrometer-corrected, four-Lorentzian lineshape. The resulting temperature dependence of the integrated intensity of the 285 cm-1 peak decreases by at most a factor of 3 up to 305 K whereas the square of the order parameter in this range decreases by a factor of about 50. Thus, its disappearance above 307 K is as yet unexplained.

Photoconduction, Trapping, and Chemisorption Effects in Sputtered Niobium Oxide Films

Thermally stimulated conductivity (TSC) measurements on reactively sputtered niobium oxide films on polished silica substrates have revealed a single set of traps with a depth of 0.26±0.05 eV, similar to those found in tantalum oxide in previous work. The resistance of the niobium oxide films decreases by a few orders‐of‐magnitude when the film surface is depleted of oxygen and returns to its original value with oxygen readsorption. The TSC measurements indicate the surface oxygen also serves as a trap with a depth of 1.03±0.09 eV. A model is proposed in which singly occupied oxygen‐vacancy traps in the bulk compete with singly ionized oxygen traps at the surface for conduction electrons. The surface traps also act as hole traps and to provide charge compensation for the positively charged vacancies.