The collisional quenching of Ca[4s3d(1D2)] by H2 and D2 over the temperature range 750–1100 K studied by time-resolved atomic emission at λ= 457.5 nm {Ca[4s3d(1D2)]→ Ca[4s2(1S0)]+hν} following pulsed dye-laser excitation

Abstract

The collisional removal of Ca[4s3d(¹D₂)] by hydrogen and deuterium has been studied in the time-resolved mode in the temperature range 750–1100 K. Ca(4 ¹D₂) was generated by pulsed dye-laser excitation of calcium vapour at λ= 457.5 nm {Ca[4s3d(¹D₂)]â†� Ca[4s²(¹S₀)]} and monitored by time-resolved forbidden atomic emission at the resonance wavelength using boxcar integration with computer interfacing. A limited number of observations was also made in emission at λ= 657.3 nm [Ca(4 ³P₁)→ Ca(4 ¹S₀)+hν] arising from the removal of Ca(4 ¹D₂) by energy pooling and emission of radiation to the states Ca(4 ³P_2,1). Absolute second-order rate constants for the collisional removal of Ca(4 ¹D₂) by hydrogen and deuterium were determined for nine separate temperatures across the overall range, yielding the following Arrhenius forms: k_H2=(5.5 ±^1.0_0.9)× 10^–10 exp(–14.4 ± 1.3 kJ mol^–1)//_RT cm³ molecule^–1 s^–1 and k_D2=(3.9 ± 0.4)× 10^–10 exp(–22.2 ± 0.8 kJ mol^–1)//_RT cm³ molecule^–1 s^–1.

These data are compared with analogous results for Ca(4 ³P_J)+ H₂, D₂. The activation energies for the collisional removal of both optically metastable states with both isotopic reactants are consistent with chemical removal to yield the diatomic hydrides CaH(D)(X²Σ⁺, ν″= 0). The Arrhenius pre-exponential factors (A) also support chemical removal in all cases, as seen from symmetry arguments based upon the weak spin–orbit coupling approximation. A factors for Ca(4³P_J)+ H₂, D₂ of the order of the collision number are in accord with direct correlation between Ca(4³P_J)+ H₂, D₂ and CaH(D)(X²Σ⁺)+ H(D); A factors for Ca(4 ¹D₂)+ H₂, D₂ significantly lower than those are in support of the absence of such adiabatic correlations.