Photon stimulated ion desorption of H+ ions from GaAs (110)/H2O

The photon stimulated ion desorption yield of H⁺ ions from a H₂O dosed GaAs (110) surface has been measured in the range 18eV ? hυ ? 30eV. There is a direct correspondence between the PSID H⁺ yield, reflectance, and the secondary electron yield spectrum of GaAs (110). The data provides evidence that the initial stages of PSID involve core level (Ga(3d), O(2s)) → conduction band excitation followed by Auger decay.     

Total synthesis of (±)-heliannuol C and E via aromatic Claisen rearrangement

The total synthesis of heliannuol C and E from a common intermediate are described. Key steps in the synthesis include a regioselective aromatic Claisen rearrangement to install the (1-vinyl)-4-methyl-3-pentenyl substituent and regioselective biomimetic 7-endo and 6-exo phenol epoxide cyclizations to form the cyclic ether moieties.     

The kinetics of reactions of nickel clusters with hydrogen and deuterium

The kinetics of reactions of nickel clusters with hydrogen and deuterium are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. The abslute rate constants for the addition of the first H₂ (D₂) molecule to nickel clusters Ni _n (n=7→36 for H₂ andn=7→60 for D₂) have been measured. Rate constants are found to be only weakly dependent onn forn≧14, showing a gradual increase with size that scales approximately withn ^(2/3), i.e., the cluster geometrical cross section. Reaction probabilities for clusters in this size range are approximately 0.6 for H₂ and 0.3 for D₂. Belown=14, there is a stronger dependence of reactivity on size, with Ni₉ being far less reactive than any other cluster studied. These results are compared to bulk nickel studies, and a discussion of possible correlation of reactivity to cluster structure is presented.     

Characterization of organometallic polymers by 13C- and 119Sn-NMR: Configurational/compositional triads in poly(tri-n-butyltin methacrylate/methyl methacrylate)

The 1:1 and 2:1 formulations of the free radical initiated copolymers of methyl methacrylate (MMA) and tri-n-butyltin methacrylate (TBTM), and the homopolymer, poly(TBTM), are characterized by ¹³C- and ¹¹⁹C-NMR structural analyses were performed on the tributyltin-free hydrolyzate, a copolymer of MMA and methacrcylic acid (MAA). Configurational sequencing at the triad level is performed using the α-methyl region of the ¹³C-NMR spectrum. The probability of isotactic (meso) dyad placement at 80°C in the homopolymer (0.19) is determined to be significantly less than the probabilities observed for the copolymers (0.23–0.24). Random compositional sequencing is established for the copolymers through a comparison of the carbonyl regions of the ¹³C-NMR spectra of the hydrolyzates with the carbonyl regions in published spectra of structurally characterized copolymers of MMA and MAA. The ¹¹⁹Sn chemical shift and the tin-carbon J coupling for the polymers are dependent on the solvent employed. This dependence is attributed to electron donor or acceptor interactions between the solvent and the strong Sn? O dipole. The tin-containing copolymers exhibit multiple ¹¹⁹Sn resonances, which appear related to compositional sequencing.     

Protein fluorescence measurements within electrospray droplets

The conformation of cytochrome c molecules within electrospray droplets is investigated by monitoring the laser induced fluorescence of its single tryptophan residue (Trp-59). By increasing the alcohol concentration of the electrosprayed solutions, protein denaturation is induced, giving rise to significant changes in the intensity of the detected fluorescence. Comparison with analogous denaturation experiments in solution provides information about the relative protein conformations and differences between the bulk-solution and droplet environments. Both electrospray-plume and bulk-solution fluorescence measurements using low methanol concentration solutions indicate the presence of folded protein structures. At high methanol content, fluorescence measurements are consistent with the presence of partly denatured or unfolded conformations. At intermediate methanol content, differences are observed between the extent of denaturation in solution and that within the droplets, suggesting electrosprayed proteins have more compact structures than those detected in bulk measurements using solutions of similar composition. This infers that some fraction of the proteins within the droplets have refolded relative to their bulk-solution conformation. Protein denaturation experiments using the low vapor pressure solvent 1-propanol indicate that differences between the droplet and solution measurements are not due to solvent evaporation effects. It is suggested that different droplet conformations are more likely the result of protein diffusion to the droplet surface and effects of the droplet/air interface. To our knowledge, these are the first reported measurements of protein fluorescence within electrospray droplets.     

Stimulated Raman laser excitation of spontaneous resonance Raman scattering

The efficient conversion of the second and third harmonics of a Nd YAG laser to near UV radiation in the 395–500 nm range by stimulated Stokes (and anti-Stokes) Raman scattering (SRS) in a 1 m Raman oscillator containing compressed H₂ or D₂ is used as an excitation source for spontaneous resonance Raman spectroscopy (RRS). SRS excited RR spectra are shown for the anion radical of tetracyanoquinodimethane (TCNQ).     

A study of some parameters relevant to the response of harwell PMMA dosimeters to gamma and electron irradiation

The effects on the radiation response of Harwell polymethylmethacrylate (PMMA) dosimeters of dose-rate, radiation type, temperature during irradiation and post-irradiation storage, and post-irradiation stability, are of importance to the operators of commercial irradiation facilities.

This paper describes recent studies of the effects of some of these parameters on the radiation response of Harwell Red 4034, Amber 3042, and Gammachrome YR Perspex dosimeters, and provides data on batch to batch variation and shelf-life.     

Characterization of organotin species using microbore and capillary liquid chromatographic techniques with an epifluorescence microscope as a novel imaging detector

The novel application of a UV epifluorescence microscope as an imaging detector for microbore and capillary high-performance liquid chromatography (HPLC) is reported. The microscope is focused on an in-line quartz flow cell incorporated down stream of a microbore HPLC column or directly on an optically clear portion of fused-silica capillary columns for analyte detection. The effect of different fluorescent ligand to analyte ratios on detection limits is also reported, as well as the effect of different image volume sizes produced by changes in microscope objective lens magnification power. Determination of relative sensitivities an detection limits for methyl- and butyltin compounds, complexed with fluorescent dyes, reveals that the organotins show decreasing sensitivity as the number of alkyl substituents on the tin atom increases, with minimum detectable amounts of 6-160 pg of analyte-ligand complex.