John W. Simons

Characterization of several ultraviolet–visible emission lines from a lead hollow-cathode lamp

The Fourier-transform spectra (0.015-cm−1 resolution) of the nonresonant 405.8-, 368.4-, and 364.0-nm lines and the resonant 283.3-nm line from a lead hollow-cathode lamp are reported. The splittings for the various isotopic and nuclear hyperfine transitions for all four lines are found to be consistent with previous measurements and assignments. At the recommended lamp operating current of 4 mA, all peak shapes within all four lines studied were satisfactorily fitted with Gaussian functions corresponding to the same Doppler temperature of 750 ± 30 K, indicating no significant self-absorption of the 283.3-nm resonant line. At a much-elevated lamp current the peaks for the nonresonant lines are increased in intensity and slightly broadened, whereas the peaks for the resonant line have approximately the same intensity as at 4 mA but are no longer Gaussian shaped.

Activation energy for the dehydration of nickel oxalate dihydrate

Extrapolation of rate data for the dehydration of nickel oxalate dihydrate to conditions of zero mass of sample yields data uninhibited by water vapour and an energy of activation of 31·0 kcal mol–1 which is significantly greater than existing values.

Production and decay of ground-state gaseous lead from the 308-nm photodissociation of gaseous lead diiodide

An extensive investigation of the production and decay of ground-state lead atoms from the 308-nm XeCl excimer laser photodissociation of PbI[sub 2](g) is reported. The laser fluence dependences of the Pb(g) and PbI(g) yields along with energetic considerations suggest that Pb(g) is produced by a two-photon process and PbI(g) is produced by a one-photon process. Pb decay profiles as a function of argon pressure, PbI[sub 2](g) number density, laser fluence, and cell temperature were determined. The rates of Pb decay were found to be approximately second order in Pb. The second-order rate constants were found to increase with PbI[sub 2](g) number density, to not change with argon pressure, and to decrease with increasing laser fluence and cell temperature. These results are consistent with a complex mechanism that can be described as the PbI[sub 2](g)-catalyzed recombinations of Pb and PbI with I involving an exothermic preequilibrium, I + PbI[sub 2] = PbI[sub 3], with [Delta]H [approx] [minus]22 [plus minus] 6 kcal/mol. The proposed mechanism is capable of reproducing the observed Pb(g) decay profiles under all conditions studied when reasonably large values of rate coefficients for the various elementary steps are used.

Absorption by ground-state lead atoms of the 283.3-nm resonant line from a lead hollow cathode lamp. An absolute number density calibration

An accurate absolute number density calibration curve for absorption by gaseous lead atoms of the 283.3-nm resonant line from a typical lead hollow cathode lamp is reported. This calibration shows the usual curvature in the Beer-Lambert plot for atomic absorption at moderate to high absorbances that is commonly attributed to self-absorption leading to line reversal in the source and/or preferential absorption at the line center when the absorber temperature is not much greater than the source Doppler temperature. A theoretical calculation utilizing a Doppler-limited Fourier transform spectrum of the 283.3-nm emission from the lamp and a tabulated value of the absorption cross section and accounting for the isotopic and nuclear hyperfine components in both the emission and absorption due to naturally occurring lead quantitatively reproduces the experimental calibration curve without any parameter adjustments. It is found that the curvature in the Beer-Lambert plot has more to do with the fact that the absorbing and emitting atoms are a mixture of isotopes giving several isotopic and nuclear hyperfine transitions at slightly different frequencies than it does with preferential absorption at line centers.