ALBERT

Notes: The wettability of the rock surface plays a very important role in the transport of fluid in porous media. The combined Amott-USBM method currently used for the determination of these values is not able to produce reliable estimates when the surface has an intermediate wettability. The nuclear magnetic resonance (NMR) technique could provide an efficient method if the parameters affected by the wettability could be established in well defined model systems. To obtain information about the effect of wettability changes upon the NMR parameters, T1 relaxation time measurements were carried out on 1H and 2H at two different magnetic fields. A large number of well defined consolidated and unconsolidated samples, with different degrees of wettability were used in this study. The results obtained showed that the T1 values, as well as the relaxation rate distribution parameters, such as the distribution width, are affected by changes in the wettability. A dependence of the T1 wettability data on the magnetic field intensity was also observed. © 1995 American Institute of Physics.

Notes: Phosphorus is one of several dopants that electronically compensate the native deep donor responsible for the yellow coloration observed in bismuth silicon oxide (BSO). Low-temperature optical absorption measurements of a series of Czochralski-grown P-doped BSO crystals show that ∼0.1–0.15 at. % P is needed in the sample to fully remove the yellow coloration. The absorption cutoff in the fully compensated P-doped sample was at 3.2 eV while compensated Al- and Ga-doped samples cutoff at 3.35 eV. Excitation at 10–15 K with near band-edge light produces photochromic absorption bands. In the lightly-doped (partially bleached) samples these bands were identical to those observed in undoped BSO. In the fully bleached sample a new spectrum was observed. Its major contribution was a band centered near 1.8 eV with a weaker absorption in the blue-green. By comparison with the spectra observed in undoped and in Al-doped material before and after photoexcitation it is believed that the 1.8 eV band is due to the [PO4]− center and that the broad 2.45 eV band observed in Al- and Ga-doped BSO is due to the [BiO4]0 center. © 1995 American Institute of Physics.

Notes: To study the kinetics of metastable defect creation in amorphous hydrogenated silicon we introduce the Constant Degradation Method as a new experimental scheme. In contrast to conventional degradation experiments in which the incident light intensity is constant during light soaking, in this method the photoconductivity of the sample is kept constant by continuously increasing the light intensity. In this case a linear time dependence of the required light intensity and of the resulting defect density is observed experimentally. A detailed analysis of the method shows that the data obtained are in accordance with the assumption of the "bond-breaking'' model, i.e., the metastable defects are created by bimolecular recombination of localized electron-hole pairs. The observed time dependence is at variance with the stretched exponential time dependence predicted for dispersive transport models. Effects of sample heating due to high light intensities and of Fermi level shifts on the observed time dependence are also discussed.

Notes: Excitonic states involved in electronic transport of undoped amorphous hydrogenated silicon (a-Si:H) are observed using spin-dependent photoconductivity (SDPC). Upon light soaking the excitonic signal decreases with regard to the SDPC signal due to recombination via dangling bonds. It has been suggested that excitonic tail-to-tail recombination leads to metastable defect creation in a-Si:H. Our experimental results are shown to be consistent with this model.

Notes: The pure rotational spectrum in the far-infrared and its absolute intensity in the vibrational ground state of CHD3 and CH3D, and the integrated band strength of the N=5 CH-stretching overtone of CHD3 in the near infrared to visible were measured by high-resolution interferometric Fourier transform techniques. The far-infrared data result in permanent electric dipole moments (||μz0||=(5.69±0.14)×10−3 D for CHD3, ||μz0||=(5.57±0.10)×10−3 D for CH3D), consistent with previous experimental data. The integrated N=5 overtone cross section is found to be (0.828±0.068) fm2. The overtone data are used, together with previous data, to derive a new, nine-dimensional, isotopically invariant dipole moment function for CH4 within the chromophore model for the CH chromophore in CHD3. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine-dimensional calculation of expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrödinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the equilibrium CH bond length of methane is re=108.6 pm. The calculated value for the vibrationally averaged permanent dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is μz0=−(6.6±0.4)×10−3 D for CHD3 (with positive z coordinate for the H atom) and μz0=(6.8±0.5)×10−3 D for CH3D (with positive z coordinate for the D atom) in essential agreement with the far-infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane is C−–H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.

Notes: We report high level ab initio calculations (treating correlation by second order Möller–Plesset perturbation theory, MP2) of a five-dimensional normal coordinate subspace of the potential and electric dipole hypersurfaces of the Cs conformer of dideuteromethanol, CD2HOH. Accurate vibrational variational calculations are carried out using a discrete variable representation (DVR) for the five anharmonically coupled modes (three coupled CH stretching and bending modes and the OH stretching and high frequency OH bending mode). The overtone spectra of the OH chromophore are calculated and analyzed in detail with respect to their anharmonic resonance dynamics leading to short time intramolecular vibrational redistribution (IVR) via the close resonance coupling of 5νOH (5ν1) with 4νOH+νCH(4ν1+ν2), as previously observed and assigned experimentally. While the assignment of the resonance is confirmed by the ab initio calculation, a sequence of calculations including various subspaces (two-dimensional to five-dimensional) lead to the conclusion that the resonance contains important contributions from coupling to the various bending modes, not just involving the CH– and OH stretching modes. Furthermore, even in the two-dimensional subspace the effective coupling constants k1112 and k1222 characterizing the resonance are not identical with the anharmonic potential constants C1112 and C1222 in the Taylor expansion of the potential, but rather an expansion to sixth order is needed to describe the resonance quantitatively. A similar conclusion holds true with other low order perturbation expansions of the resonance coupling, involving sequences of cubic couplings to other modes. We furthermore predict important resonances between OH stretching and OH bending also involving CH bending modes, which contribute to IVR at higher levels of excitation. © 1999 American Institute of Physics.

Notes: Interaction energies of K+ and NH+4 with the model ligands methyl-propyl-ether, N-ethyl-N-methyl-acetamide, 1,2-dimethoxy-ethane, 2-methoxy-N,N-dimethyl-ethylamine, and ethyl-propionate were determined by performing ab initio SCF-LCAO-MO calculations for about 1000 complex structures. Atom pair potentials were obtained by fitting the calculated interaction energies with an analytical potential. Both the form of the analytical potential and the assignment of atoms to classes, i.e., in groups having the same atom pair potential values, were amended relative to earlier contributions. Previously determined Li+ and Na+ interaction energies were fitted into a consistent form of analytical potential and classification. The pair potentials obtained were applied for the calculation of interaction energies between the ions and 18-crown-6 and the results were compared with corresponding ab initio calculations.

Notes: We report a new mechanism for intramolecular vibrational redistribution (IVR) in CF3CHFI which couples the CH chromophore vibrations through a strong Fermi resonance to the formal CF stretching normal mode (a heavy atom frame mode) involving the trans F-atom across the CC bond. The analysis is made possible by comparing spectroscopic results with extensive ab initio calculations of the vibrational fundamental and overtone spectra in the range extending to 12 000 cm−1. Potential energy and electric dipole moment hypersurfaces are calculated ab initio by second order Møller–Plesset perturbation theory (MP2) on a grid involving the CH stretching, two CH bending modes and one high frequency CF stretching normal mode. The potentials are scaled to obtain agreement between the experimental spectrum and the theoretical spectrum calculated by a discrete variable representation technique on this grid. Both spectra are then analyzed in terms of three-dimensional (3D) and four-dimensional (4D) effective vibrational Hamiltonians including Fermi- and Darling–Dennison-type resonances between the CH stretching mode and the CH bending modes and the CF stretching mode. The interaction between the CH modes and the CF mode is clearly visible in the experimental and calculated (4D) spectra. The effective Fermi resonance coupling constants [ksff′(similar, equals)(40±10) cm−1 and ksaf′(similar, equals)(55±10) cm−1] coupling the CH and CF mode subspaces are of about the same magnitude as the intra-CH chromophore Fermi resonances (ksaa′(similar, equals)56 cm−1 and ksbb′(similar, equals)42 cm−1, coupling CH stretching mode "s" with the two CH bending modes "a" and "b"). The chiral, pseudo-Cs symmetry breaking coupling (ksab′(similar, equals)11 cm−1) is complemented by an equally strong coupling through the CF mode (ksfb′(similar, equals)15 cm−1). It is demonstrated that low order perturbation theoretical analysis using potential constants from a polynomial expansion to represent effective coupling constants gives inadequate results with discrepancies ranging about from factors of 2–5. Time dependent population and wave packet analysis shows essentially complete IVR among the CH chromophore modes within about 100 fs, the 3D and 4D evolutions being similar up to about that time. At longer times of about 250 fs, there is substantial excitation of the CF stretching mode (with initial pure CH stretching excitation). The 4D treatment is then essential for a correct description of the dynamics. © 2000 American Institute of Physics.