A kinetic investigation of the collisional behaviour of Sr(5 3PJ) with H2 and D2 over the temperature range 725–1100 K, studied by time-resolved atomic emission at λ= 689.3 nm [Sr(5 3P1)→ Sr(5 1S0)+hν] following pulsed dye-laser excitation

Abstract

The collisional quenching of Sr(5 ³P_J) 1.807 eV above the Sr(5 ¹S₀) ground state, by the molecules hydrogen and deuterium, has been investigated over the temperature range 725–1100 K. Sr(5 ³P₁) was generated by repetitive pulsed dye-laser excitation of strontium vapour at λ= 689.3 nm [Sr(5 ³P₁)â†� Sr(5 ¹S₀)] in the presence of an excess of helium buffer gas, and in a slow-flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within the Sr(5 ³P_J) spin-orbit manifold, Sr(5 ³P₁) was monitored photoelectrically as a function of time at the resonance wavelength using ‘pre-trigger photomultiplier gating’ and boxcar integration with microcomputer interfacing for data analysis. Absolute second-order rate constants for the quenching of Sr(5 ³P_J) by hydrogen and deuterium, each at sixteen temperatures within the overall range employed, yield the following Arrhenius forms (1σ errors): k_H2= [graphic omitted] × 10^–9 exp (–49.4 ± 1.3 kJ mol^–1)/RT cm³ molecule^–1 s^–1 and k_D2= [graphic omitted] × 10^–10 exp (–50.6 ± 1.1 kJ mol^–1)/RT cm³ molecule^–1 s^–1. The activation energies are approximately one-half the reaction endoergicities to yield SrH and SrD in their electronic and vibrational ground states and demonstrate the temperature dependence of physical energy transfer. This is in marked contrast to Mg(3 ³P_J)+ H₂, D₂ where both chemical reaction and physical quenching have been considered to make contributions to overall removal on the assumed basis of a temperature-independent physical quenching rate. It is also in contrast to our recent results for quenching of Ca(4 ³P_J) by hydrogen and deuterium, where chemical reaction was the dominant mode of removal. The temperature dependence of the diffusion coefficient, D[Sr(5 ³P_J)-He], of the form αTⁿ has yielded n= 1.54 ± 0.05, in accord with the simple expectation on the basis of the kinetic theory of perfect gases. A redetermination of the mean radiative lifetime of Sr(5 ³P₁) yields τ_e= 19.1 ± 0.4 µs, in good agreement with previous measurements using dye-laser excitation and with theoretical predictions. Finally, a reanalysis of the previous rate data for the diffusion of Ca(4 ³P_J) in helium indicates that the quenching of Ca(4 ³P_J) by ground-state calcium atoms, Ca(4 ¹S₀), is characterised by a rate constant whose upper bound is 4 × 10^–14 cm³ atom^–1 s^–1.