Absolute rate data for reactions of ground-state atomic calcium, Ca(4s2(1S0)), at elevated temperatures determined by time-resolved atomic resonance absorption spectroscopy at λ = 422.7 nm (41P1 ← 41S0)
✍ Scribed by Robert S. Clay; David Husain
- Publisher
- Elsevier Science
- Year
- 1991
- Tongue
- English
- Weight
- 911 KB
- Volume
- 86
- Category
- Article
- ISSN
- 0010-2180
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✦ Synopsis
Reactions of ground-state atomic calcium, Ca(4s2(ISo)), with a large number of collision partners have been investigated by direct spectroscopic measurement using atomic resonance absorption spectroscopy in the timedomain. Ca(41So) was generated by the pulsed irradiation of Cal 2 vapor at elevated temperatures and monitored photoelectrically in the "single-shot mode" using the resonance transition at k = 422.7 nm (Ca(41P0 '--Ca(41So)). Decay profiles in the presence of the gases CH3CI, C2H5C1, CF3CI, CF3Br, CF2CI 2, CHFCI 2, CH3F, CF3H, CF4, SF6, HC1, HBr, N20 and H20 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 (kn) for reactions with these gases in the region of ca. 900 K (errors la):
10 -12 900 C2H5C1 (4.6 :t: 0.4) x 10-12 897 CF3C1 (2.1 ± 0.2) × 10 -12 906 CF3Br (2.5 + 0.1) x 10 11 909 CF2C12 (1.6 ± 0.1) x 10 -11 909 CHFC12 (1.1 ± 0.1) × 10 -I1 898 CH3F (3.4 + 0.2) × 10-12 901 CF3H (4.7 + 0.3) × 10-13 905 CF 4 (2.5 ± 0.2) x 10-15 908 SF 6 (4.1 + 0.2) × 10 -I1 913 HC1 (1.1 ± 0.1) × 10 -11 911 HBr (8.5 _+ 0.4) × 10-12 905 N20 (1.8 __. 0.1) × 10 TM 907 H20 (3.4 ± 0.2) × 10 -12 911
Although the kinetic measurements for the removal of Ca(41So) with H20 showed consistent plots, these yielded a value of kR(T = 911 K) = (3.4 ± 0.2) x 10-12 cm 3 molecule-l 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 H2-O2-N 2 flames. Apart from the rate measurement for Ca + N20 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.