Biomedical subjects

Ellen H G Backus

Publications and source records attributed to Ellen H G Backus.

5 recordsLinked to original sources

PubMed2005

Real-time observation of molecular motion on a surface.

The laser-induced movement of CO molecules over a platinum surface was followed in real time by means of ultrafast vibrational spectroscopy. Because the CO molecules bound on different surface sites exhibit different C-O stretch vibrational frequencies, the site-to-site hopping, triggered by excitation with a laser pulse, can be determined from subpicosecond changes in the vibrational spectra. The unexpectedly fast motion--characterized by a 500-femtosecond time constant--reveals that a rotational motion of the CO molecules, rather than pure translation, is required for this diffusion process. This conclusion is corroborated by density functional theory calculations.

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PubMed2005

Surface photochemistry of bromoform on ice: cross section and competing reaction pathways.

The 266 nm photodissociation of bromoform adsorbed on an amorphous solid water (ASW) layer has been investigated for the first time under well-defined ultrahigh vacuum conditions. Time-of-flight (TOF) measurements indicate direct release of gas-phase Br, CHBr2, Br2, and CHBr species, with potential implications for stratospheric chemistry. Furthermore, new, ice-surface-mediated C-C (C2H2Br2) and C-O (CHBrO, CO) species are revealed in postirradiation temperature programmed desorption (TPD) and reflection absorption infrared (RAIR) spectra. A cross section of approximately 5 x 10(-20) cm2 is determined for bromoform photodissociation at 266 nm based on the integrated area of both the TOF spectra of Br and Br2 and the postirradiation TPD curves of CHBr3. The involvement of the free, non-hydrogen-bonded water groups at the ASW surface in the formation of the photoproducts is evident from the RAIRS results.

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PubMed2004

Adsorption and dissociation of NO on stepped Pt (533).

We present an experimental and theoretical investigation of the adsorption, desorption, and dissociation of NO on the stepped Pt (533) surface. By combining temperature programmed desorption and reflection absorption infrared spectroscopy, information about the adsorption sites at different temperatures is obtained. Surprisingly, metastable adsorption structures of NO can be produced through variation of the dosing temperature. We also show that part of the NO molecules adsorbed on the step sites dissociates around 450 K. After dissociation the N atoms can desorb either by combining with an O fragment, or with another N atom, resulting in NO and N(2). The N(2) production can be enhanced by coadsorbing CO on the surface: CO scavenges the oxygen atom, thereby suppressing associative recombinative desorption of N and O atoms. Density functional theory calculations are used to reveal the adsorption energies and vibrational frequencies of adsorbed NO as well as barriers for dissociation of NO and for diffusion of N atoms. The combined experimental results and theoretical calculations reveal that dissociation of NO is the rate limiting step in the formation of N(2).

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PubMed2004

Theory of bulk, surface and interface phase transition kinetics in thin films.

We report a theoretical study of phase transition kinetics in confined two-dimensional systems, motivated by recent experimental results on the amorphous-to-crystalline transition in supported, thin amorphous water films [E.H.G. Backus, M.L. Grecea, A.W. Kleyn, and M. Bonn, Phys. Rev. Lett. (to be published)]. We generalize and extend existing theories to simultaneously describe the converted (crystalline) fractions in the bulk, at the sample-vacuum surface, and at the sample-support interface as a function of time. The general approach presented here results in expressions for the time-dependent converted bulk, surface, and interface fractions, for arbitrary desorption rate from the thin film, nucleation and growth rates and also includes finite nucleation grain size. The converted bulk, surface, and interface fractions are calculated for nucleation of the new phase occurring (i) in the bulk, (ii) at the support-sample interface, and (iii) at the sample surface (sample-vacuum interface), resulting in nine expressions. The results demonstrate the advantage of monitoring bulk, surface and interface fractions simultaneously to make definite statements regarding the location of the nucleation, and to reliably determine the values of the relevant crystallization parameters.

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PubMed2004

Surface crystallization of amorphous solid water.

We demonstrate that the crystallization of thin, supported amorphous solid water layers is initiated at the water surface. This is concluded from the observation of sequential crystallization of amorphous water at the surface, in the bulk, and at the water-support interface. A surface nucleation model quantitatively reproduces the observed transformation kinetics at the three sites.

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