Collisional quenching of Ca(43PJ) by H2and D2studied over the temperature range 850–1075 K by time-resolved atomic resonance emission at λ= 657.3 nm [Ca(43P1)→ Ca(41S0)+hν] following pulsed dye-laser excitation

We present a kinetic study of the collisional behaviour of Ca(4 3PJ), 1.888 eV above the 4 1S0 ground state, with hydrogen and deuterium over the temperature range 850–1075 K. Ca(4 3P1) was optically excited by repetitive pulsed dye-laser excitation at λ= 657.3 nm of calcium vapour in equilibrium with the solid at elevated temperatures in a slow-flow system, kinetically equivalent to a static system. Following rapid Boltzmann equilibration within the 4 3PJ spin–orbit manifold, the time-resolved emission at λ= 657.3 nm [Ca(4 3P1)→ Ca(4 1S0)+hν] was monitored photoelectrically using a boxcar integrator interfaced to a microcomputer for kinetic analysis of the atomic emission data in digital form. Absolute second-order rate constants for collisional quenching of Ca(4 3PJ) were determined at ten different temperatures in the range 850–1075 K yielding the following Arrhenius forms: k[Ca(4 3PJ)+H2]/cm3 molecule–1 s–1=[graphic omitted] × 10–9 exp[(–88.3±4.2 kJ mol–1)/RT], k[Ca(4 3PJ)+D2]/cm3 molecule–1 s–1=[graphic omitted] × 10–9 exp [(–92.9±5.0 kJ mol–1)/RT] and constituting the first temperature-dependent investigation for these processes. In both cases the rate data were also fitted to a three-parameter equation of the form kQ=B+A exp (–E/RT). The term B, which may be attributed to physical quenching, was found to be negligible in magnitude compared with the temperature-dependent term. The experimental activation energies were found to be in accord with reaction endothermicities to yield CaH and CaD (X2Σ+, ν″= 0) and the pre-exponential factors with calculated collision numbers. The results are compared with analogous rate data reported by other workers on the temperature-dependence of the quenching of Mg(33PJ) by hydrogen and deuterium. Finally, an investigation of the temperature-dependence of the diffusion of Ca(4 3PJ) in helium is presented in which the diffusion coefficient D[Ca(43PJ)+ He] is found to satisfy a temperature dependence of T1.8±0.04.