Adsorption of potassium on Cr₂O₃(0001) at ionic and metallic coverages and uv-laser-induced desorption

Translational energy distributions of neutral potassium atoms are reported as a function of potassium coverage after uv-laser-induced desorption from well-characterized adsorption sites on an epitaxial film of Cr2O3(0001)/Cr(110). Measurements using x-ray photoelectron spectroscopy, low-energy electron diffraction, and work-function measurements revealed that potassium adsorbs in a nonmetallic phase for deposition temperatures around 280–300 K allowing only a maximal saturation coverage to be grown for moderate growth rates. Aggregates are observed after deposition at 90 K; at this temperature any layer thickness is obtainable. The uv-laser-induced desorption for these two different phases was studied using excitation energies of 3.5 eV, 5.0 eV, and 6.4 eV and (1+1)-resonantly enhanced multiphonon ionization via the 6p2P state for detection. Desorption of potassium atoms from the nonmetallic phase proves to be ten times [σ(6.4eV)=(2±1)×10−19cm2] more efficient than desorption from metallic potassium aggregates. The mechanism of desorption from the nonmetallic phase appears to be the inverse harpooning process starting with an ion pair followed by a transfer of hot electrons from the substrate to unoccupied potassium states to neutralize the initially positively charged potassium. The maximum of the translational energy distribution (starting at 0.65 eV for low coverages) decreases with increasing potassium coverage and is by a factor of approximately 4 smaller for desorption from large potassium aggregates (0.16 eV). The decrease of the translational energy with increasing coverages for isolated atoms is ascribed to an increasing lateral interaction between the adsorbates and a concomittant smooth change of the ionicity of the atoms from partially ionic to neutral

Mechanism of the UV-laser-induced desorption of CO from Cr₂O₃(0001)

The UV-laser-induced desorption of CO from an epitaxially grown film of Cr2O3(0001) is reported using a (1 + 1′) REMPI (resonance enhanced multiphoton ionisation) detection technique via the B1Σ+ electronic state for the desorbing molecules to obtain information on single rovibrational states. The final state distributions at three different wavelengths (6.4, 5.0 and 3.5 eV) were almost identical showing bimodal velocity flux distributions for v″ = 0 and monomodal distributions for v″ = 1. The desorption cross-sections varied between (5.1 ± 1) × 10−18 cm2 at 3.5 eV and (3.5 ± 1) × 10−17 cm2 at 6.4 eV. The overall behaviour indicates that the excitation mechanism is due to a primary electronic excitation step within the substrate. Possible mechanisms are discussed

Rotational alignment in the UV-laser induced desorption of CO from Cr₂O₃(0001)

The rotational alignment of the laser-induced non-thermal desorption of CO adsorbed on an epitaxially grown film of Cr2O3(0001) has been studied using 193 nm (6.4 eV) laser light and a (1 + 1′)-REMPI technique. At low rotational quantum numbers J the molecules desorb like a ‘helicopter’ (J-vector perpendicular to the surface) while at high J-values a ‘cartwheel’ like motion is preferred (J-vector parallel to the surface). CO is adsorbed in a flat geometry as a single species. The resulting angular momentum alignment in the laser desorption is interpreted qualitatively as a sum of effects due to corrugation of the surface in the initial state and a change of the adsorption geometry in the intermediate state before desorption

Photoinduced processes on alkali covered surfaces: NO desorption from KCr₂O₃(0001)

We have studied the ultraviolet laser induced desorption of NO/Cr203(0001), K/Cr203(0001 ), and the coadsorbate system NO/K/Cr2O3(0001) using resonance enhanced multiphoton ionization spectroscopy for state selective detection of the desorbing species after excitation with nanosecond laser pulses. The goal of our experiments was to study the influence of surface electronic modifications via alkali adsorption on the photodynamics of a simple molecule. The photochemistry of the isolated and the coadsorbate systems is strongly dependent on the coverage of the diverse components. In this paper we shall mainly focus on data for the low coverage regime of potassium. From the two adsorbate species of NO, a chemisorbed and a physisorbed species, we present data of the chemisorbed species. The velocity distributions show a strong dependence on the excitation energy which we interpret on the basis of electron energy loss spectra as being due to surface charge transfer states. This is corroborated with our coadsorption experiments with low coverages of potassium which alter the velocity distributions

Effects of early nutrition and growth on brain volumes, white matter microstructure, and neurodevelopmental outcome in preterm newborns

© 2018 International Pediatric Research Foundation, Inc. BackgroundThis study aimed to investigate the effect of nutrition and growth during the first 4 weeks after birth on cerebral volumes and white matter maturation at term equivalent age (TEA) and on neurodevelopmental outcome at 2 years' corrected age (CA), in preterm infants.MethodsOne hundred thirty-one infants born at a gestational age (GA) <31 weeks with magnetic resonance imaging (MRI) at TEA were studied. Cortical gray matter (CGM) volumes, basal ganglia and thalami (BGT) volumes, cerebellar volumes, and total brain volume (TBV) were computed. Fractional anisotropy (FA) in the posterior limb of internal capsule (PLIC) was obtained. Cognitive and motor scores were assessed at 2 years' CA.ResultsCumulative fat and enteral intakes were positively related to larger cerebellar and BGT volumes. Weight gain was associated with larger cerebellar, BGT, and CGM volume. Cumulative fat and caloric intake, and enteral intakes were positively associated with FA in the PLIC. Cumulative protein intake was positively associated with higher cognitive and motor scores (all P <0.05).ConclusionOur study demonstrated a positive association between nutrition, weight gain, and brain volumes. Moreover, we found a positive relationship between nutrition, white matter maturation at TEA, and neurodevelopment in infancy. These findings emphasize the importance of growth and nutrition with a balanced protein, fat, and caloric content for brain development