A kinetic study of the third-order recombination reaction Sr( 1 S) + O 2 + He 98 k 1 (He) SrO 2 + He has been carried out in a fast-flow reactor in the temperature and pressure range of respectively 303-968 K and 6-12 Torr. Strontium atoms were generated by thermal evaporation of strontium metal pellets and then monitored by atomic absorption spectroscopy (AAS). The Arrhenius plot shows a departure from linearity above 500 K. The modified Arrhenius expression taking into account a temperature dependence of the preexponential factor for the entire temperature range results in the expression k 1 Sr (He) ) [(1.9 ( 0.3) × 10 -23 ]T -1.7 exp-[(-12.1 ( 0.8 kJ mol -1 )/RT] cm 6 molecule -2 s -1 . The best representation of the temperature dependence is given by the polynomial fit log k 1 Sr (He) = -60.83 + 22.44(log T) -3.89(log T) 2 . A lower limit to the bond energy of the SrO 2 ( 3 A 2 ) reaction product is estimated by means of the method based on the expression of the equilibrium constant and the theory of statistical mechanics. A value of D 0 > 244 kJ mol -1 was obtained, which is in the range of the values derived from ab initio calculations.

A kinetic study of the second-order reactions Ca(1S) + N2O(XΣ+) CaO + N2 and Sr(1S) + N2O(X1Σ+) SrO + N2 has been carried out in a fast-flow reactor in the temperature ranges of, respectively, 303−1015 and 303−999 K. The alkaline earth metal atoms were thermally generated from the solid metal pellets. Their decays as a function of the added N2O concentration were followed by means of atomic absorption spectroscopy (AAS) at 422.7 nm for calcium and 460.7 nm for strontium atoms. Both reactions showed a non-Arrhenius behavior that can best be explained by the presence of two reaction product channels, resulting in a rate constant expressed as the sum of two exponential functions:  k 1 Ca = [(1.9 ± 1.4) × 10-8] exp[(−40.6 ± 4.7 kJ mol-1)/(RT)] + [(2.8 ± 0.7) × 10-10] exp[(−14.1 ± 0.7 kJ mol-1)/(RT)] cm3 molecule-1 s-1; k 1 Sr = [(1.1 ± 0.2) × 10-9] exp[(−23.3 ± 1.3 kJ mol-1)/(RT)] + [(1.1 ± 0.1) × 10-10] exp[(−8.8 ± 0.4 kJ mol-1)/(RT)] cm3 molecule-1 s-1. The best fits over the entire temperature range are given by the polynomial expressions log k 1 Ca = −30.12 + 9.78(log T) − 0.99(log T) and log k 1 Sr = −26.02 + 8.40(log T) − 1.02(log T)2. The results will be discussed in view of the literature data on the alkaline earth metal atom + N2O reactions. The experimentally derived energy barriers will be compared with the calculated values on the basis of the semiempirical configuration interaction theory (SECI). Reasonable good correlations were obtained between the barrier heights of the reaction and the promotion energy of the metals involved.

The kinetics of the second-order reactions Mg(1S) + NO2(X2A1) MgO + NO, Ca(1S) + NO2(X2A1) CaO + NO, and Sr(1S) + NO2(X2A1) SrO + NO have been investigated in a fast-flow reactor in the temperature ranges of, respectively, 303−836, 303−916, and 303−986 K. Solid magnesium, calcium, and strontium pellets were thermally evaporated to generate the corresponding alkaline earth metal atoms in the gas phase. Their decays as a function of the added NO2 concentration were followed by means of atomic absorption spectroscopy (AAS) at 285.2 nm for magnesium, 422.7 nm for calcium, and 460.7 nm for strontium atoms. All reactions show an Arrhenius behavior and the rate constants are given by k 1 Mg = [(1.4 ± 0.2) × 10-11] exp(−3.4 ± 0.6 kJ mol-1/RT) cm3 molecule-1 s-1, k 1 Ca = [(1.5 ± 0.6) × 10-9] exp(−2.9 ± 1.2 kJ mol-1/RT) cm3 molecule-1 s-1, and k 1 Sr = [(1.2 ± 0.1)x 10-9] exp(−0.9 ± 0.3 kJ mol-1/RT) cm3 molecule-1 s-1. The results will be discussed in terms of the electron jump mechanism. Since the Mg/NO2 reaction is too slow to proceed via this mechanism, a classical oxygen atom abstraction is suggested. In the case of the Ca/NO2 and the Sr/NO2 reactions, the experimental rate constants are too high to be quantitatively explained by the classical electron jump mechanism. The modified electron jump mechanism which takes into account long distance forces between the reagents gives a better agreement with the experimental values.

A kinetic investigation of the second-order reactions Ca( 1 S) + Cl 2 ( 1 Σ g + ) f(k 1 Ca ) CaCl + Cl and Sr( 1 S) + Cl 2 ( 1 Σ g + ) f(k 1 Sr ) SrCl + Cl was carried out in a fast-flow reactor in the temperature ranges of respectively 303-1038 K and 303-991 K. The calcium and strontium atoms in the gas phase were generated by thermal evaporation of the solid metal pellets. The concentration of the gas-phase metal atoms was followed by means of atomic absorption spectroscopy (AAS) at wavelengths of 422.7 nm for calcium and 460.7 nm for strontium atoms. Both reactions show an Arrhenius behavior and the rate constants are given by k 1 Ca ) [(6.0 ( 0.8) × 10 -10 ]exp(-0.2 ( 0.5 kJ mol -1 /RT)cm 3 molecule -1 s -1 and k 1 Sr ) [(7.3 ( 0.6) × 10 -10 ]exp(-0.4 ( 0.4 kJ mol -1 /RT)cm 3 molecule -1 s -1 . The results are interpreted in terms of the electron-jump mechanism. For both the Ca/Cl 2 and the Sr/Cl 2 reactions, the experimental rate constants are too high to be quantitatively explained by the classical electron-jump mechanism. However, the modified electron-jump mechanism which takes into account long distance forces between the reagents gives a better agreement with the experimental values.