A simple experimental method for determining optical second-order polarizabilities of organic molecules for second harmonic generation (SHG) is developed by using a two-level quantum mechanical model. Required values of excited-state dipole moments are obtained from a solvatochromic method based on the theoretical treatment of McRae. Second-order polarizabilities obtained by this method for a series of compounds compare well with those obtained by the conventional EFISH technique. The solvatochromic method has the important advantage that measurements can be made rapidly with simple equipment available in most chemistry laboratories.

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Pommersheim and Chandra (1975) present analytical solutions for the general problem of the optimal policy of batch reactor operation (and tubular reactor operation when catalyst with no slip moves with the reaction fluid) for catalyst decay rates which depend on temperature, activity and concentration.The optimal temperature and corresponding concentration and activity formulas which they present are valid whenever the Legendre-Clebsch (L-C) condition is obeyed. They list three other criteria which they claim are equiualent to the criterion employed by Szepe and Levenspiel (1968) for the case of concentration independent decay. This is criterion (i) ofTable 1, which requires minimizing the final concentration (CAf), or maximizing the final conversion, for fixed run times (t,) and desired final activities (uf).The entire problem is shown in Table 1 with all four criteria stated. Solutions for the optimal policy can be shown to be identical for all four criteria. But for the optimal policy equations to be valid, the appropriate L-C conditions must be shown not to be violated. The L-C conditions may be different for these criteria, as they are derived employing the transversality conditions. The purpose of this note is to elucidate these restrictions on optimal temperature policies for the problem discussed by Pommersheim and Chandra and by example to further define the ranges of applicability of their solution. NTH ORDER SINGLE IRREVERSIBLE REACTIONThe reaction and activity rate functions considered are rA = KA am CAn respectively, can be shown to the independent of the nature of the criteria which specifies the Legendre-Clebsch condition. In order that Equation (2)-(4) specify optimal profiles, the L-C Functions* C.4 (t), 4 t h Y(t)Constraints 6 = C.4 + r.4 (C.!, a, Y) = 0 6 2 = a' + 6 (C,, a, y) = 0 End Conditions and Objectitje FunctionsCriterion (i) Criterion (ii) Criterion (iii) Criterion (iv)

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The equilibrium partial pressures of various vapor species in the system silicon‐carbon‐hydrogen were computed for conditions under which epitaxial normalSiC deposition has been achieved. It was found that in the temperature range 1300°–1700°C the tendency to form solid silicon carbide decreases with increasing temperature and with increasing silicon to carbon atom ratio. The limiting conditions under which single crystalline deposits of α‐normalSiC could be experimentally obtained were those within which normalSiC was computed to be the only thermodynamically stable solid phase. At 1650° single‐phase silicon carbide condenses at normalSi/C ratios ranging from 0.2 to 0.89. The computed results were in good agreement with experiments.

It was found that the etching rate of alpha (hexagonal) silicon carbide in hydrogen at high temperatures (above 1550~varies with the susceptor material on which the inductively heated silicon carbide rests. Molybdenum exhibits the most pronounced effect increasing the etching rate by about one order of magnitude over that on all other susceptors (W, Nb, Ta, C) employed. Finite etching rates were also observed on a-silicon carbide exposed to inert gas atmospheres at elevated temperatures and were attributed to hydrogen residually present in the susceptors. It was concluded that catalytic action of the susceptor material is involved in the etching process.

Single‐crystal layers of α‐(hexagonal) silicon carbide false(normalSiCfalse) were successfully deposited on the [0001] and false[000true1¯false] surfaces of α‐normalSiC substrates from silane and propane in a hydrogen atmosphere and in the 1500°–1650°C temperature range. Above 1650°C the amount of normalSiC deposited was found to be significantly decreased because of increased hydrogen etch rates and diffusion‐limited reactant transfer to the substrate. The growth characteristics are best described by a model in which the surface of the sample is in equilibrium with the reactants diffusing through a boundary layer. Undoped n‐type deposits (grown on p‐type substrates) exhibited a resistivity of 0.40 ohm‐cm and a Hall mobility of approximately 200 cm2/V‐sec at 77°C.