Kinetic study of Ca(4 3PJ) by time-resolved emission, 4 3P1→ 4 1S0+hν(λ= 657.3 nm), following dye-laser excitation. Spontaneous emission, diffusion and collisional quenching

Abstract

We present a kinetic study of the low-lying electronically excited state of atomic calcium, Ca[4s4p(³P_J)], 1.885 eV above the 4s²(¹S₀) ground state. Ca(4 ³P₁) was generated by dye-laser pulsed excitation at λ= 657.3 nm of calcium vapour [Ca(4 ³P₁)â†� Ca(4 ¹S₀)] at elevated temperatures (1000 K), in equilibrium with solid calcium, in a slow-flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within Ca(4 ³P_J) through collisions, the forbidden time-resolved atomic resonance fluorescence at the same wavelength was monitored using boxcar integration. The decay of Ca(4 ³P_J) in the presence of all the noble gases, He, Ne, Ar, Kr and Xe, was studied, yielding measurements of relative diffusion coefficients. Extrapolation of the diffusional component of the decay in each noble gas to infinite pressure yields an independent value of the mean radiative lifetime for Ca(4 ³P₁)→ Ca(4 ¹S₀)+hν(λ= 657.3 nm), the average of which from the measurements in all the noble gases yields τ_e= 0.34 ± 0.02 ms (error 2σ). This result is compared with values reported from previous measurements, particularly atomic beams and time-resolved measurements. The kinetic analysis also gives rise to data for the collisional quenching of Ca(4 ³P_J), which is found to be insignificant for most of the noble gases [k_He,Ne,Ar,Kr⩽ 4 × 10^–15 cm³ molecule^–1 s^–1(1000 K)], in contrast to measurements from earlier work using phase-shift techniques. Xenon, however, exhibited measurable rates of collisional quenching of Ca(4 ³P_J) characterised by k_Xe= 2.4 ± 0.3 × 10^–14 cm³ molecule^–1 s^–1(1000 K). The time-resolved emission technique is further applied to the study of the removal of Ca(4 ³P_J) by added gases, for which we report (errors 1σ)k_N2=(8.9 ± 0.9)× 10^–13 cm³ molecule^–1 s^–1(1000 K), k_H2=(6.0 ± 0.6)× 10^–14 cm³ molecule^–1 s^–1(1000 K), k_D2=(2.7 ± 0.3)× 10^–14 cm³ molecule^–1 s^–1(1000 K).