The collisional quenching of Sr[5s4d(1D2)] by H2and D2over 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(3PJ)](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: kH2= 7 × 10–10 exp (–13 kJ mol–1/RT) cm3 molecule–1s–1 and kD2= 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(3PJ)+ H2 and D2. This is in marked contrast to previous measurements on Ca[4s4p(3PJ)] 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(3DJ)](2.260 eV) and Sr[5s5p(3PJ)] 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(3DJ) and Sr(3PJ).