ALBERT

Notes: The dissociative adsorption of H2 and D2 on Pt(533) (Pt{4(111)×(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D2 from (100) step sites is observed at lowest exposures in TPD (assigned β3) at 375 K. Saturation of this peak at aitch-thetaH=0.14 corresponds to the filling of half of the available four-fold sites at the (100) step edge. At higher coverages, additional desorption takes place from the (111) terraces in a broad peak below 300 K similar to that observed (assigned β1 and β2) for the Pt(111) surface. The incident kinetic energy (Ei), surface temperature (Ts), coverage (aitch-thetaD), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S0) first decreases with increasing kinetic energy over the range 0〈Ei(meV)〈150 (low energy component), and subsequently increases (high energy component). Comparison with D2 dissociation on Pt(111), where (S0) increases linearly with Ei, leads to the conclusion that it is the step sites that are responsible for the low energy component to dissociation on Pt(533). The high energy component is a result of a direct dissociation channel on (111) terraces of the Pt(533) surface. The probability of dissociation through the direct channel on the (111) terraces is found to be independent of Ts. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of Ei where it contributes, is also shown to be independent of Ts. Only at the very lowest value (6.6 meV) of Ei investigated does S0 exhibit a (negative) temperature dependence. A (0.8-aitch-thetaD)2 dependence (where 0.8 is the measured saturation coverage) of S with aitch-thetaD is observed at Ei=180 meV where the direct channel dominates. However, the dependence of S on aitch-thetaD exhibits characteristics similar to those expected for precursor mediated dissociation at Ei=16 meV and Ei=6.6 meV where the low energy channel dominates. The angular dependence S0(Φ) scattering in a plane perpendicular to the step direction is asymmetric about the Pt(533) surface normal at both Ei=6.6 meV and Ei=180 meV. At 180 meV S0(Φ) can be understood by considering direct dissociation at the (111) terrace and (100) step plane. At 6.6 meV, S0 tends to scale with total energy. The observed characteristics of the low energy channel is discussed in the light of models [specifically the role steps and defects, precursors (accommodated and dynamical), and steering] suggested to account for the low energy component for H2/D2 dissociation and exchange on metal surfaces presenting low activation barriers. At lowest energies (Ei=6.6 meV) dissociation through a conventional accommodated precursor takes place. In addition, more significant proportion of sticking in the range 0〈Ei(meV)〈150 takes place through an indirect channel involving an unaccommodated precursor dissociating at step sites, and is unlikely to be accounted for through a steering mechanism. © 2000 American Institute of Physics.

Notes: Rotational excitation of NO scattered from Pt(111) has been measured for incoming energies from 0.3 to 1.6 eV. For an initial energy of 0.3 eV a clear rotational rainbow, which we assign to O-end collisions, is visible at superspecular exit angles. This is very surprising, since NO–Pt(111) is a chemisorption system with a binding energy of 1 eV. Sharp, pronounced rainbows are visible in the range of incoming kinetic energies where the initial sticking coefficient is high. For an initial energy of 0.3 eV the initial sticking coefficient is 0.9, and at this energy the clearest rotational rainbow is observed. In contrast, at an initial energy of 1.6 eV the sticking coefficient is 0.5 and no rotational rainbow is observed. At subspecular exit angles the distributions are indistinguishable from a Boltzmann distribution at all the energies investigated, and show a clear energy dependence. Boltzmann-type distributions at high incident energy indicate a thorough redistribution of the available energy, although they cannot be explained in terms of a simple statistical model. The scattering results are interpreted as indicating a competition between direct scattering from the repulsive wall and indirect scattering via the deep potential well. Most of the direct scattering can be assigned to O-end collisions with the surface, where the binding energy is expected to be much smaller. Indirect scattering becomes more important at higher energies as more molecules are able to escape the well promptly. © 1995 American Institute of Physics.

Notes: SUMMARY. We have been investigating asexual and sexual stages in the life cycle of a ciliated protozoan, Vorticella microstoma, with the object of determining characteristic patterns at the biochemical level. In the course of our studies we have made progress in the chromatographic analysis of acid hydrolysates of the asexual (neuter) and the sexual forms. Our findings demonstrate that it is possible to obtain from the neuter stages ninhydrin-positive patterns which are characteristically different from the patterns derived from the sexual stage. These patterns are believed to be indicative of the biochemical peculiarities of the organisms. Ninhydrin-positive patterns of the sexual stages of Vorticella microstoma are composed of 15 bands, in contrast with 10 bands observed in the patterns of asexual stages. In regard to color, bands 1–10 in the neuters are similar to bands 1–10 in the sexuals. The difference between the two ninhydrin-positive patterns is believed to be significant, in that it indicates the biochemical peculiarities of the respective kinds of Vorticellae. It is suggested that chromatographic analyses may lead to the identification of the substance or substances liberated by the macroconjugant for the purpose of attracting microconjugants. The methods which have been successfully used in this study may be helpful to investigators who utilize protozoa as experimental organisms. We believe the methods and results reported here may have applications in the genetics of protozoa and in biological studies devoted to metazoan subjects.