Collisional quenching of Ca(4 3PJ) by H2 and D2 studied over the temperature range 850–1075 K by time-resolved atomic resonance emission at λ= 657.3 nm [Ca(4 3P1)→ Ca(4 1S0)+hν] following pulsed dye-laser excitation

Abstract

We present a kinetic study of the collisional behaviour of Ca(4 ³P_J), 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 ³P_J 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 ³P_J) were determined at ten different temperatures in the range 850–1075 K yielding the following Arrhenius forms: k[Ca(4 ³P_J)+H₂]/cm³ molecule^–1 s^–1=[graphic omitted] × 10^–9 exp[(–88.3±4.2 kJ mol^–1)/RT], k[Ca(4 ³P_J)+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 k_Q=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(3³P_J) by hydrogen and deuterium. Finally, an investigation of the temperature-dependence of the diffusion of Ca(4 ³P_J) in helium is presented in which the diffusion coefficient D[Ca(4³P_J)+ He] is found to satisfy a temperature dependence of T^1.8±0.04.