ALBERT

Notes: Rate constants for the Cl+H2 and D2 reactions have been measured at room temperature by the laser photolysis-resonance absorption (LP-RA) technique. Measurements were also performed at higher temperatures using two shock tube techniques: laser photolysis-shock tube (LP-ST) technique with Cl-atom atomic resonance absorption spectrometric (ARAS) detection, over the temperature range 699–1224 K; and higher temperature rates were obtained using both Cl-atom and H-atom ARAS techniques with the thermal decomposition of COCl2 as the Cl-atom source. The combined experimental results are expressed in three parameter form as kH2( ± 15%) = 4.78 × 10−16 T1.58 exp(−1610 K/T) and kD2( ± 20%) = 9.71 × 10−17 T1.75 exp(−2092 K/T) cm3 molecule−1 s−1 for the 296–3000 K range. The present results are compared to earlier direct studies which encompass the temperature ranges 199–1283 (H2) and 255–500 K (D2). These data including the present are then used to evaluate the rate behavior for each reaction over the entire experimental temperature range. In these evaluations the present data above 1300 K was given two times more weight than the earlier determinations.The evaluated rate constants are: kH2( ±14%)=2.52×10−11 exp(−2214 K/T) (199≤T〈354 K), kH2(±17%)=1.57×10−16 T1.72 exp(−1544 K/T) (354≤T≤2939 K), and kD2(±5%)=2.77×10−16 T1.62 exp(−2162 K/T) (255≤T≤3020 K), in molecular units. The ratio then gives the experimental kinetic isotope effect, KIE ≡ (kH2/kD2). Using 11 previous models for the potential energy surface (PES), conventional transition state theoretical (CTST) calculations, with Wigner or Eckart tunneling correction, are compared to experiment. At this level of theory, the Eckart method agrees better with experiment; however, none of the previous PES's reproduce the experimental results. The saddle point properties were then systematically varied resulting in an excellent model that explains all of the direct data. The theoretical results can be expressed to within ±2% as kH2th = 4.59 × 10−16 T1.588 exp(−1682 K/ T) (200≤T≤2950 K) and kD2th=9.20×10−16 T1.459 exp(−2274 K/T) cm3 molecule−1 s−1 (255≤T ≤3050 K). The KIE predictions are also compared to experiment. The "derived'' PES is compared to a new ab initio calculation, and the differences are discussed. Suggestions are noted for reconciling the discrepancies in terms of better dynamics models. © 1994 American Institute of Physics.

Notes: The dorsal fin of the larval and juvenile Oreochromis mossambicus exhibits a unique black spot known as the ‘Tilapia mark’. We traced its development and found that it occupied a specific position in the dorsal fin bounded by rays 15 and 20. Ablation experiments carried out on the larval dorsal fin showed that this spot region constituted a developmental positional field. This positional field in the fin could regenerate and re-establish the spot pattern despite repeated perturbation. The re-establishment of spot was not simply due to fin injury since ablation of the non-spotted region of the dorsal and the tail fin did not result in aggregation of melanophores. We propose that that‘Tilapia mark’ is a result of positional information in operation.

Notes: A method of evaluating interface states for coupled superlattices has been developed. Explicit solutions for these states have been obtained within the framework of the Kronig–Penney model for the case of two semiconductor superlattices coupled by a tunnel barrier. The interface states are shown to be largely dependent on the coupling strength if the constituent superlattices have identical structure parameters, while insensitive to the tunnel barrier thickness if their structure parameters differ. In the latter case, electrons behave as if the system were composed of two decoupled semi-infinite superlattices.

Notes: Rate constants for the thermal decomposition of CH3Cl and the atom–radical reaction O+CH3 have been measured in shock tube experiments. The decomposition of CH3Cl, at three loading pressures, has been carried out between 1663–2059 K with mixtures that varied from 1.25 to 24.9 ppm in Ar. The first-order rate constant k1=1.21×1010 ×exp(−27 838 K/T) s−1 describes the experimental results to within ±29% at the one standard deviation level. The bimolecular rate experiment has then been carried out over the temperature range 1609–2002 K using mixtures of SO2 (49.9 ppm) and CH3Cl (5.14 and 8.2 ppm) in Ar. The technique first involves allowing the thermal decomposition to proceed forming CH3 radicals, and this is then followed by delayed photolysis of SO2 forming the O-atom species. This new method is called the pyrolysis photolysis-shock tube (PyPh-ST) technique. A reaction mechanism had to be used to simulate the measured O-atom profiles for the various experimental conditions, and the bimolecular rate constant was found to be temperature independent with a value of k2=1.4×10−10 cm3 molecule−1 s−1. Reactions (1) and (2) are both theoretically discussed.

Notes: Experiments on the thermal decomposition of N2O have been carried out in reflected shock waves using the atomic resonance absorption spectrometric (ARAS) technique for monitoring the product O atoms. The results provide a calibration for the ARAS system as well as values for the decomposition rate constant, k1, between 1540 and 2500 K. The present results can be represented as k1=5.25×10−10 exp(−27 921 K/T) cm3 molecule−1 s−1. Additionally, rate constants for the reaction, (2) N+NO→N2+O, have been measured by the laser photolysis-shock tube (LP-ST) technique between 1251 and 3152 K. NO serves as both the photolytic source of N(4S) atoms and the reactant molecule. N atoms are monitored by the ARAS method. The results do not show significant temperature dependence and can be represented by k2=(3.7±0.8)×10−11 cm3 molecule−1 s−1 over the experimental temperature range. Values for the reaction, (−2) O+N2→NO+N, can be derived from these results. The results are compared with earlier studies and are then theoretically discussed in terms of transition states taken from an ab initio electronic structure calculation and also from a dispersion force model. Neither model accurately predicts the observed behavior, and this suggests that further theoretical work with trajectory calculations be attempted to assess the importance of recrossing.

Notes: The reactions of Si+ with CH3SiH3, CH3SiD3, C2H6, and CH3CHD2 have been studied in a tandem mass spectrometric apparatus over the kinetic energy range of 1–10 eV laboratory-frame-of reference (LAB). In all systems, the major process is the formation of SiCH+3, as well as SiCH2D+ and SiCHD+2 in the case of the reaction with CH3CHD2. It is shown that in the reaction of Si+ with CH3SiH3 and CH3SiD3, the process is best described as a Walden inversion, while in the reaction with C2H6 and CH3CHD2, the process appears to approximate the spectator stripping model or modified spectator stripping (polarization-reflection model). In the reaction with CH3CHD2, the slight preference of Si+ to strip the CH3 radical rather than the CHD2 radical is shown to be in accord with a cross-sectional energy dependence of approximately E−1.