ALBERT

Notes: Recent electron diffraction and microscopy studies of GaN nucleation layers have shown that faults in the stacking of the close-packed planes result in the coexistence of cubic and hexagonal phases within the layers. Using grazing incidence x-ray scattering, we have quantified the proportion of the cubic and hexagonal phases throughout the nucleation layer. We compare the structure of a 20 nm nucleation layer grown on sapphire by atmospheric pressure metal-organic chemical vapor deposition at 525 °C to that of an identical layer heated to 1060 °C. The fractions of cubic and hexagonal phases in the layers are determined by a comparison of the scattering data with a Hendricks–Teller model. High temperature exposure results in a decrease of the cubic fraction from 0.56 to 0.17. The good agreement with the Hendricks–Teller model indicates that the positions of the stacking faults are uncorrelated. © 1998 American Institute of Physics.

Notes: We have investigated the quantification properties of scanning capacitance microscopy (SCM) by using two dedicated test structures and highlight the response of SCM to changes in dopant density. Our results indicate that contrast reversal occurs and that the SCM output is not always a monotonically increasing signal with decreasing dopant density. Two epitaxially grown staircase structures covering the doping ranges 1014–1020 cm−3 p type and 5×1014–5×1019 cm−3 n type were produced for this study as the turning point in the response function typically occurs at a doping level of around 1017 cm−3. Through the use of a simple simulation model we see that contrast reversal is expected due to a relative shift between the dC/dV curves for different doping levels. The onset of contrast reversal can be adjusted by changing the dc sample bias leading to a shift in the operating position of the SCM, and the significance of this point will be discussed here. © 1998 American Institute of Physics.

Notes: The filling processes of water and cyclohexane in porous silica (with a characteristic pore size of 60 Å) are investigated using the nuclear magnetic resonance (NMR) technique of cryoporometry. In this technique, the liquid was frozen in the pores before the temperature was raised gradually; melting the smallest particles first and then particles of increased size. The volume of the molten liquid present was measured using the height of a T2 spin echo. The experiments were performed with filling fractions ranging from 10% to 100%. The results showed distinctly different behaviors of the fluids, which depended on the surface adhesion. It was found that water (a fluid which wets the pore surface) forms small puddles—much smaller than the smallest pore size—at low filling fractions. These puddles grow in size as more water is added until all the pore volume is filled. Cyclohexane (a non-wetting fluid) on the other hand, does not form small puddles but completely fills the pores with a preference for the smaller pores. Water is found to give more accurate results for the pore size distribution than cyclohexane, in 60 Å silica. © 1998 American Institute of Physics.

Notes: The mechanism of the reaction CH4+O(1D2)→CH3+OH was investigated by ultrafast, time-resolved and state-resolved experiments. In the ultrafast experiments, short ultraviolet pulses photolyzed ozone in the CH4⋅O3 van der Waals complex to produce O(1D2). The ensuing reaction with CH4 was monitored by measuring the appearance rate of OH(v=0,1;J,Ω,Λ) by laser-induced fluorescence, through the OH A←X transition, using short probe pulses. These spectrally broad pulses, centered between 307 and 316 nm, probe many different OH rovibrational states simultaneously. At each probe wavelength, both a fast and a slow rise time were evident in the fluorescence signal, and the ratio of the fast-to-slow signal varied with probe wavelength. The distribution of OH(v,J,Ω,Λ) states, Pobs(v,J,Ω,Λ), was determined by laser-induced fluorescence using a high-resolution, tunable dye laser. The Pobs(v,J,Ω,Λ) data and the time-resolved data were analyzed under the assumption that different formation times represent different reaction mechanisms and that each mechanism produces a characteristic rovibrational distribution. The state-resolved and the time-resolved data can be fit independently using a two-mechanism model: Pobs(v,J,Ω,Λ) can be decomposed into two components, and the appearance of OH can be fit by two exponential rise times. However, these independent analyses are not mutually consistent. The time-resolved and state-resolved data can be consistently fit using a three-mechanism model. The OH appearance signals, at all probe wavelengths, were fit with times τfast(approximate)0.2 ps, τinter(approximate)0.5 ps and τslow(approximate)5.4 ps. The slowest of these three is the rate for dissociation of a vibrationally excited methanol intermediate (CH3OH*) predicted by statistical theory after complete intramolecular energy redistribution following insertion of O(1D2) into CH4. The Pobs(v,J,Ω,Λ) was decomposed into three components, each with a linear surprisal, under the assumption that the mechanism producing OH at a statistical rate would be characterized by a statistical prior. Dissociation of a CH4O* intermediate before complete energy randomization was identified as producing OH at the intermediate rate and was associated with a population distribution with more rovibrational energy than the slow mechanism. The third mechanism produces OH promptly with a cold rovibrational distribution, indicative of a collinear abstraction mechanism. After these identifications were made, it was possible to predict the fraction of signal associated with each mechanism at different probe wavelengths in the ultrafast experiment, and the predictions proved consistent with measured appearance signals. This model also reconciles data from a variety of previous experiments. While this model is the simplest that is consistent with the data, it is not definitive for several reasons. First, the appearance signals measured in these experiments probe simultaneously many OH(v,J,Ω,Λ) states, which would tend to obfuscate differences in the appearance rate of specific rovibrational states. Second, only about half of the OH(v,J,Ω,Λ) states populated by this reaction could be probed by laser-induced fluorescence through the OH A←X band with our apparatus. Third, the cluster environment might influence the dynamics compared to the free bimolecular reaction.

Notes: The collision-induced electronic energy transfer that occurs when I2 in the E(0g+) ion-pair electronic state collides with ground electronic state I2 has been investigated. We prepare I2 in single rotational levels in v=0 of the E state using two-color double resonance laser excitation. The resulting emission spectrum shows that the nearby (ΔTe=−385 cm−1) D(0u+) electronic state is populated. The cross section for collision-induced E→D energy transfer is found to be 18±3 Å2. A range of D state vibrational levels are populated, consistent with a model in which overlap between the initial and final vibrational wave functions is important, but modulated by propensities for small vibrational energy gaps and those energy gaps that are closely matched to the v=0→v=1 energy separation in the I2(X) collision partner. © 2001 American Institute of Physics.