The collisional quenching of Sr[5s4d(1D2)] by H2 and D2 over the temperature range 850–1100 K studied by time-resolved atomic emission following pulsed dye-laser excitation

Abstract

We present a kinetic study of the collisional quenching of Sr[5s4d(¹D₂)], 2.498 eV above the 5s²(¹S₀) electronic ground state, by H₂ and D₂ across the temperature range T= 850–1100 K. Sr(¹D₂) was generated by the repetitive pulsed dye-laser excitation of strontium vapour at λ= 496.1 nm {Sr[5s4d(¹D₂)]â†� Sr[5s²(¹S₀)]} in the presence of excess helium buffer gas and H₂ or D₂ in a slow flow system, kinetically equivalent to a static system. Its exponential decay was monitored in emission at the resonance wavelength using Boxcar integration coupled with computerized analysis. Sr(¹D₂) was also monitored using the more intense bi-exponential atomic emission profiles for Sr[5s5p(³P_J)](1.807 eV), resulting from emission and collisional quenching of Sr(¹D₂), at λ= 689.3 nm {Sr[5s5p(³P₁)]→Sr[5s²(¹S₀)]+hν}. The resulting rate data, which involve considerable experimental scatter that is considered in some detail, can be expressed in the form: k_H2= 7 × 10^–10 exp (–13 kJ mol^–1/RT) cm³ molecule^–1s^–1 and k_D2= 3 × 10^–10 exp (–17 kJ mol^–1/RT) cm³ molecule^–1s^–1 the data clearly indicating low energy barriers and fully in accord with energy transfer on collision, as also found hitherto for Sr(³P_J)+ H₂ and D₂. This is in marked contrast to previous measurements on Ca[4s4p(³P_J)] and Ca[4s3d(¹D₂)] by H₂ and D₂ where collisional removal was totally dominated by chemical reaction. Finally, branching ratios for collisional removal of Sr(¹D₂) into the lower-lying Sr[5s4d(³D_J)](2.260 eV) and Sr[5s5p(³P_J)] states, for which the individual contributions could not be separated, are estimated and found to lie in the range ca. 0.3–0.9 . consideration of the observed low activation energies, reaction ergicities and branching ratio estimates suggest that the collisional quenching of Sr(¹D₂) by H₂ and D₂ is dominated by (E–E, V) transfer resulting in the production of Sr(³D_J) and Sr(³P_J).