Absolute rate data for reactions of ground-state atomic calcium, Ca(4 s2( 1S0)), at elevated temperatures determined by time-resolved atomic resonance absorption spectroscopy at λ = 422.7 nm (4 1P1 ← 4 1S0)

Abstract

Reactions of ground-state atomic calcium, Ca(4s 2( 1S 0)) , with a large number of collision partners have been investigated by direct spectroscopic measurement using atomic resonance absorption spectroscopy in the time-domain. Ca(4 1 S 0) was generated by the pulsed irradiation of CaI 2 vapor at elevated temperatures and monitored photoelectrically in the “single-shot mode” using the resonance transition at λ = 422.7 nm (Ca(4 1 P 1) ← Ca(4 1 S 0)). Decay profiles in the presence of the gases CH 3Cl, C 2H 5Cl, CF 3Cl, CF 3Br, CF 2Cl 2, CHFCl 2, CH 3F, CF 3H, CF 4, SF 6, HCl, HBr, N 2O and H 2O with excess helium buffer gas were captured in a transient recorder and transferred either directly to a microcomputer or mainly to an XY-recorder for subsequent kinetic analysis. The development of the present method was principally feasible because of the construction here of an extremely intense, high-current hollow cathode atomic resonance source for the calcium transition employed in the investigation. We report the following absolute second-order rate constants ( k R ) for reactions with these gases in the region of ca. 900 K (errors 1 σ): Reactant k R (cm 3 molecule −1 s −1) T (K) CH 3Cl (3.7 ± 0.4) × 10 −12 900 C 2H 5Cl (4.6 ± 0.4) × 10 −12 897 CF 3Cl (2.1 ± 0.2) × 10 −12 906 CF 3Br (2.5 ± 0.1) × 10 −11 909 CF 2Cl 2 (1.6 ± 0.1) × 10 −11 909 CHFCl 2 (1.1 ± 0.1) × 10 −11 898 CH 3F (3.4 ± 0.2) × 10 −12 901 CF 3H (4.7 ± 0.3) × 10 −13 905 CF 4 (2.5 ± 0.2) × 10 −15 908 SF 6 (4.1 ± 0.2) × 10 −11 913 HCl (1.1 ± 0.1) × 10 −11 911 HBr (8.5 ± 0.4) × 10 −12 905 N 2O (1.8 ± 0.1) × 10 −11 907 H 2O (3.4 ± 0.2) × 10 −12 911 Although the kinetic measurements for the removal of Ca(4 1 S 0) with H 2O showed consistent plots, these yielded a value of k R ( T = 911 K) = (3.4 ± 0.2) × 10 −12 cm 3 molecule −1 s −1, which is viewed with caution as it implies a bond dissociation energy of D(CaOH) of a magnitude at the extreme limits of the largest value reported from various analyses on calcium in premixed H 2O 2N 2 flames. Apart from the rate measurement for Ca + N 2O reported by previous workers using laser techniques, these results constitute a new body of absolute rate data for the Ca atom derived from direct monitoring in the time-domain. The results are considered with analogous data reported for alkali atoms and with the few kinetic estimates that have been made for such reactions of Ca where relevant to flame chemistry.