Thermally induced conformational changes of Ca-arachidate Langmuir-Blodgett Films at different compression.

The conformational order in Ca-arachidate Langmuir-Blodgett films on solid glass supports is investigated by means of vibrational sum-frequency generation spectroscopy (VSFG). The symmetric C-H stretching vibrations of both the terminal methyl and the methylene groups are utilized to monitor the chain conformation at various sample temperatures under ambient conditions. At room temperature the film is well-ordered consisting almost entirely of all-trans configured chains. Between 340 and 430 K we observe a marked increase in gauche-defects before oxidative degeneration starts at sample temperatures above 470 K. The temperature dependence of the data is well represented by apparent enthalpy changes for the formation of gauche-defects, sharply increasing with packing density from 29 to 62 kJ/mol; values, which are an order of magnitude larger than those of the gas phase molecule. These large apparent enthalpies do not prevent the formation of a high degree of conformational disorder at elevated temperatures.

Conformational disorder in alkylsiloxane monolayers at elevated temperatures.

Vibrational sum frequency generation spectroscopy is used to characterize octadecylsiloxane monolayers on glass substrates at ambient conditions with a focus on thermally induced conformational disorder. Different modes of the C-H stretching vibrations of the terminal methyl groups and the methylene groups are therefore monitored in the frequency range of 2850-3000 cm(-1). We observe a progressive increase of conformational disorder of the alkyl chains due to gauche defects over the temperature range from 300 to 510 K. The conformational disorder is reversible over a temperature range from 300 to about 410 K. But after heating to temperatures above 410 K, order is not reestablished on the time scale of the experiment. These results suggest that the assumption of an all-trans configuration of the alkyl chains is an over-simplification which increasingly misrepresents the situation for elevated temperatures which are still well below the one at which decomposition starts.

Vibrational dynamics of hydrogen on Ge surfaces.

The vibrational dynamics of the H stretch excitation on the Ge(100)-(2x1) and Ge(111)-(1x1) surfaces has been studied using picosecond IR pump-SFG probe spectroscopy. Moreover, the temperature dependence and an isotope mixture effect are reported. The symmetric stretching mode at 1994 cm(-1) on the Ge(100)-(2x1):H surface shows a single-exponential relaxation with a decay constant of 4.8+/-0.6 ns at 100 K with a strong temperature dependence, while the Ge-H stretch at 1975 cm(-1) on the Ge(111)-(1x1):H surface relaxes four times faster with a 1.3+/-0.2 ns lifetime also exhibiting a weaker temperature dependence. The lifetime decreases with increasing temperature to 1.6 and 0.74 ns at 400 K on Ge(100) and Ge(111), respectively. We find that the decay rate increases by a factor of 3-6 depending on sample temperature when the Ge(100) surface dimers are saturated with an isotope mixture of H and D. Such an effect upon isotope mixing is not observed for the Ge(111) surface. The results suggest for the Ge(100)-(2x1):H system that a decay into three bending mode quanta requires the creation of two-optical phonons to satisfy energy conservation, whereas the decay into four bending quanta requires the annihilation of only one phonon. The three bending quanta process is hence the slower one. However, the decay into four bending quanta shows a strong temperature dependence. For an isotope mixture covered surface a larger number of combinations of low-frequency adsorbate modes exist facilitating a faster decay of the stretching excitation.

Isotope effects in the vibrational lifetime of hydrogen on germanium(100): theory and experiment.

Combining first-principles calculations and sum frequency generation spectroscopy, we elucidate the microscopic details in the relaxation of the stretching vibration of hydrogen adsorbed on Ge(100). The dominant decay channels involve energy transfer from the stretching to the hydrogen bending modes, with the remaining energy difference being transferred to or from substrate phonons. The coupling between stretching and bending modes is treated from first principles using the calculated multidimensional adiabatic potential energy surface, while the coupling to phonons is treated in perturbation theory. For a surface solely saturated with light hydrogen, we calculate a vibrational lifetime of 1.56 ns at 400 K, in good agreement with experiment, and find a similar temperature dependence of the lifetime in both experiment and theory. The calculations show that the stretching energy dissipates to a vibrational state involving four bending quanta of hydrogen, concurrently absorbing a thermally excited surface phonon related to the Ge dimer rocking mode. For a Ge surface saturated with a mixture of H and D, our experiments find that the relaxation rate of the H stretching vibration is markedly increased when compared to a surface saturated with H only. Experimentally, a single decay is observed although H and D atoms will statistically pair on the surface dimers. The vibrational lifetime of the Ge-H stretching mode is up to six times shorter in the presence of adsorbed D atoms. The calculated relaxation rates are consistent with the experimentally observed trend. The theoretical analysis shows that the breaking of symmetry within the Ge surface dimer due to coadsorption of D opens up further relaxation channels that involve absorption or emission of a substrate phonon at various energies. Moreover, the calculations predict an even shorter vibrational lifetime of the Ge-D stretch mode due to efficient coupling to the Ge dimer rocking mode.

Preparation of Graphene with Graphane Areas of Controlled Hydrogen Isotope Composition on Opposite Sides.

Monolayer graphene was prepared on an Ir(111) substrate where it exhibits a 25 × 25 Å(2) moiré pattern. Molecular hydrogen was dosed first, allowing it to dissociate on open areas of the Ir substrate. The generated H atoms formed an intercalated reservoir that can bind to the graphene subsequently. Next, atomic hydrogen was dosed, which binds to the graphene sheet and also initiates the transfer of H from the Ir substrate to the graphene sheet. The opposite sides of the sheet can be hydrogenated with isotope selectivity, as a sequence of difference isotopes, H or D, can be chosen at will in the preparation procedure. Sum-frequency generation spectra prove that as consequence of the dosing sequence, C-H bonds are predominantly pointing toward the Ir substrate side when H2 is dosed first and alternatively toward the vacuum side when D2 is dosed first.

Preparation of submicron-structured alkylsiloxane monolayers using prepatterned silicon substrates by laser direct writing.

A new constructive method for the preparation of laterally structured alkylsiloxane monolayers is demonstrated. Laser direct writing has been used to create oxide patterns on H-terminated Si(100) samples under ambient conditions. Depending on the laser power and the writing speed, oxide structures with a lateral resolution below 500 nm are prepared routinely. The patterned samples are suitable as temporary templates for the preparation of laterally structured octadecylsiloxane monolayers. Prior to immersion in an octadecyltrichlorosilane solution, however, hydration of the samples in water is essential to facilitate a selective coating of the oxidized areas. After coating, atomic force microscopy reveals the formation of octadecylsiloxane islands exclusively on top of the oxide lines.

Arsenic-speciation in arsenate-resistant and non-resistant populations of the earthworm, Lumbricus rubellus.

Arsenic speciation was determined in Lumbricus rubellus Hoffmeister from arsenic-contaminated mine spoil sites and an uncontaminated site using HPLC-MS, HPLC-ICP-MS and XAS. It was previously demonstrated that L. rubellus from mine soils were more arsenate resistant than from the uncontaminated site and we wished to investigate if arsenic speciation had a role in this resistance. Earthworms from contaminated sites had considerably higher arsenic body burdens (maximum 1,358 mg As kg-1) compared to the uncontaminated site (maximum 13 mg As kg-1). The only organo-arsenic species found in methanol/water extracts for all earthworm populations was arsenobetaine, quantified using both HPLC-MS and HPLC-ICP-MS. Arsenobetaine concentrations were high in L. rubellus from the uncontaminated site when concentrations were expressed as a percentage of the total arsenic burden (23% mean), but earthworms from the contaminated sites with relatively low arsenic burdens also had these high levels of arsenobetaine (17% mean). As arsenic body burden increased, the percentage of arsenobetaine present decreased in a dose dependent manner, although its absolute concentration rose with increasing arsenic burden. The origin of this arsenobetaine is discussed. XAS analysis of arsenic mine L. rubellus showed that arsenic was primarily present as As(III) co-ordinated with sulfur (30% approx.), with some As(v) with oxygen (5%). Spectra for As(III) complexed with glutathione gave a very good fit to the spectra obtained for the earthworms, suggesting a role for sulfur co-ordination in arsenic metabolism at higher earthworm arsenic burdens. It is also possible that the disintegration of As(III)-S complexes may have taken place due to (a) processing of the sample, (b) storage of the extract or (c) HPLC anion exchange. HPLC-ICP-MS analysis of methanol extracts showed the presence of arsenite and arsenate, suggesting that these sulfur complexes disintegrate on extraction. The role of arsenic speciation in the resistance of L. rubellus to arsenate is considered.