Donald P. Land

HREELS investigation of the orientation and dehydrogenation of cyclohexane on Pt[111]

Abstract In this article, we report on the investigation of cyclohexane adsorbed on Pt[111] with high resolution electron energy loss spectroscopy (HREELS). Variation of the angle of incidence of the electron beam is used to determine the orientation of the adsorbed molecule. The symmetry of the adsorbate/surface complex is C3v, which indicates adsorption with the C3 axis of the molecule perpendicular to the surface, i.e., with the plane of the molecule parallel to the surface. Annealing of the surface to different temperatures followed by HREELS of the resultant species shows that dehydrogenation begins by 200 K, but that benzene is not formed until 300 K. Spectra between 200 and 300 K show very little variation, implying a single predominant intermediate. Angular profiles after annealing to 260 K indicates the intermediate/surface complex has low symmetry and lends insight to the possible identity.

A Kinetic Model for β-Amyloid Adsorption at the Air/Solution Interface and Its Implication to the β-Amyloid Aggregation Process

At the air/buffer solution interface the kinetics of adsorption of amyloid beta peptide, Abeta(1-42), whose bulk concentration (submicromolar) is more than 2 orders of magnitude lower than that typically used in other in vitro aggregation studies, has been studied using a Langmuir-Blodgett trough. The pressure-time curves exhibit a lag phase, wherein the surface pressure essentially remains at zero, and a rising phase, corresponding to the Abeta adsorption at the interface. The duration of the lag phase was found to be highly dependent on both the Abeta bulk concentration and the solution temperature. A large activation energy (62.2 +/- 4.1 KJ/mol) was determined and the apparent adsorption rate constant was found to be linearly dependent on the Abeta bulk concentration. Attenuated total reflection-IR spectra of the adsorbed Abeta transferred to a solid substrate and circular dichroism measurements of Abeta in the solution layer near the interface reveal that the natively unstructured Abeta in the bulk undergo a conformation change (folding) to mainly the alpha-helical structure. The results suggest that, prior to the adsorption step, an equilibrium between Abeta conformations is established within the subsurface. The kinetic equation derived from this model confirms that the overall Abeta adsorption is kinetically controlled and the apparent rate constant is proportional to the Abeta bulk concentration. This model also indicates that interfaces such as cell membranes and lipid bilayers may facilitate Abeta aggregation/ fibrillation by providing a thin hydrophobic layer adjacent to the interface for the initial A/beta conformation change (misfolding) and accumulation. Such a preconcentration effect offers a plausible explanation of the fact that Abeta fibrillation occurs in vivo at nanomolar concentrations. Another important biological implication from our work is that Abeta misfolding may occur before its adsorption onto a cell membrane. This general kinetic model should also find applications in adsorption studies of other types of biomolecules whose overall kinetics exhibits a lag phase that is dependent on the bulk concentration of the adsorbate.

The conversion of ethylene to ethylidyne on Pt(111): non-first order kinetics and ensemble effects

Abstract The conversion of ethylene to ethylidyne on Pt(111) has been studied in detail by laser induced thermal desorption (LITD) in conjunction with Fourier transform mass spectrometry (FTMS) and Fourier transform infrared reflection absorption spectroscopy (FT-IRAS). The formation of ethylidyne shows strictly first order kinetics over the entire coverage range. In contrast, this holds only for low initial coverages in case of the decomposition of ethylene, whereas at high coverages a pronounced non-first order behavior is observed. The latter observation can be explained by an ensemble effect which assumes an increased decomposition rate for ethylene molecules in the immediate vicinity of an ensemble of free Pt sites. Using a simple Monte Carlo calculation, not only the measured decomposition rates can be reproduced but, in addition it shows that the reaction proceeds in patches rather than uniformly. This result is in agreement with recent scanning tunnelling microscope (STM) observations.

Dynamics of ion coupling in an FTMS ion trap and resulting effects on mass spectra, including isotope ratios

Abstract We have studied the coupled cyclotron motions of ions at moderate to high density in the ion trap of a Fourier transform mass spectrometer. For the case of two ionic species of similar mass in the ion trap (CO + and N + 2 ) we present image current detection results over a range of ion densities. At low ion densities the ion motions are independent, resulting in two distinct image current frequencies. At higher ion densities the ion motions are strongly coupled. In the high density limit only one of the coupled motions has the symmetry required to generate an image current. Thus, the FT mass spectrum of the CO/N 2 mixture at high density consists of only a single peak at the frequency of the image current active, coupled cyclotron motion. By analyzing the image current transients in detail we can follow the development of the collective normal modes of the ions in the trap. FTMS spectra of naphthalene (C 10 H 8 ) as a function of ion density demonstrate that the coupled ion motions can have a substantial effect on the measurement of 12 C/ 13C isotope ratios.

Iodobenzene on Pd(111) studied by thermal desorption spectroscopy and laser-induced thermal desorption-Fourier transform mass spectrometry

Iodobenzene decomposes on Pd(111) and forms benzene, iodine, adsorbed carbon, and hydrogen. At low initial iodobenzene exposures no iodobenzene desorbs, benzene desorbs at 500 K, and hydrogen desorbs between 500 and 700 K, indicative of decomposition of some of the benzene. At high initial exposures iodobenzene desorbs at about 200 K and benzene desorbs at about 160 K; however, no hydrogen desorbs. Iodine desorbs from Pd(111) around 1000 K. Adsorbed iodine atoms passivate the Pd(111) surface toward the decomposition of iodobenzene and the dehydrogenation of benzene. Experiments with perdeuterated iodobenzene show that the benzene forms from reaction of phenyl with sub-surface hydrogen by 143 K.

A Fourier transform mass spectrometer for surface analysis by laser‐induced thermal desorption of molecular adsorbates

A Fourier transform mass spectrometer (FTMS) for ultrahigh‐vacuum surface studies is described. The instrument incorporates standard surface analysis techniques such as Auger electron spectroscopy (AES), low‐energy electron diffraction (LEED), and Ar ion sputtering, along with laser‐induced thermal desorption (LITD) and thermal desorption spectroscopy (TDS) using FTMS detection, to perform surface analysis of metal samples. The manipulator allows temperature control of the samples between 110 and 1300 K. Using the LITD/FTMS surface reaction intermediates and kinetics are studied for the dehydrogenation of ethylene and cyclohexane on Pt(111). Relative sensitivities between AES and LITD/FTMS are discussed.

Laser-driven thermal reactions of ethylidyne on platinum

Abstract The reactions of ethylene and ethylidyne (CCH3) adsorbed on a Pt(s)(111) surface have been studied by laser-induced thermal desorption mass spectrometry. A small spot on the surface is heated rapidly by a laser pulse; desorbed species are ionized by an electron beam and detected by Fourier transform mass spectrometry. For Ts≲250 K, the main peak in the laser desorption mass spectrum is molecular ethylene. At higher temperatures, when ethylidyne is the dominant surface species, laser desorption produces large peaks at m/z=2 and 26, instead of m/z=27 as might have been expected. This is interpreted to be due to a laser-driven thermal reaction on the surface to produce C2H2(s) plus H (s), with subsequent desorption as C2H2 and H2.

Desulfurization, deoxygenation and denitrogenation of heterocycles by a palladium surface: a mechanistic study of thiophene, furan and pyrrole on Pd(111) using laser-induced thermal desorption with Fourier-transform mass spectrometry

Abstract The following contains a review of recent work, plus new results, from our laboratory using laser-induced thermal desorption with Fourier transform mass spectrometry (LITD/FTMS) to elucidate the decomposition mechanism of thiophene, furan and pyrrole on Pd(111). The results show that all three heterocycles react differently on Pd, despite their similarity in structure. Thiophene decomposes at 300 K via a C4H4 intermediate species, which subsequently hydrogenates and desorbs as 1,3-butadiene. The cleavage between the CS bonds of thiophene results in the deposition of sulfur, which remains on the Pd(111) surface. In direct contrast to thiophene, furan decomposition on Pd(111) is shown to proceed at 300 K via elimination of α-H and CO, leaving a C3H3 species on the surface. Heating to 350 K causes dimerization of the C3 species, forming benzene. In this case, oxygen is removed efficiently from the Pd(111) surface. Preliminary results of pyrrole on Pd(111) indicate that decomposition occurs at around 230 K, a lower temperature than either thiophene or furan. The only reaction product observed is HCN. The data cannot be readily explained by a pathway involving either a C4 or a C3 species, however, our results indicate the presence of some hydrocarbon species that decomposes above 325 K yielding hydrogen and above 500 K to give HCN. Removal of nitrogen from Pd(111) is similar to that of furan, in that the formation and subsequent desorption of HCN removes a significant fraction of nitrogen from the surface.

Thiophene decomposition on Pd(111) forms S and C4 species : a laser-induced thermal desorption/Fourier transform mass spectrometry study

Abstract The data presented here show that Pd(111) can directly activate thiophene decomposition resulting in the deposition of sulfur and the formation of C 4 species, most likely C 4 H 4 or possibly C 4 H 5 , on the surface. Temperature programmed reaction (TPR) studies of a 0.2 L exposure of thiophene show some reversible, but primarily irreversible adsorption. No C- or S-containing reaction products desorb during TPR. However, laser induced thermal desorption (LITD) with Fourier transform mass spectrometry (FTMS) can be used to monitor the surface composition prior to conventional desorption. LITD/FTMS shows that thiophene is stable to approximately 280 K. Above 300 K, 1,3-butadiene is observed. The yield of 1,3-butadiene on the surface, as observed by LITD/FTMS, is estimated to be 30% of the initial thiophene signal.

A comparison of lateral diffusion in supported lipid monolayers and bilayers

Lipid monolayers and bilayers exist in distinct physical states differentiated by the differences in the manner in which translational fluidity relates to their phase transition and how cholesterol influences the two. Work presented here suggests that intra-leaflet diffusion and cholesterol interactions are modulated by the nature of inter-leaflet coupling. Our results also provide an important practical caveat in the comparisons of membrane physical properties deduced using the two, mono- and bilayer, model membrane configurations.