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The collisional quenching of Sr(5 3PJ) 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 3PJ) 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 3PJ) by hydrogen and deuterium, each at sixteen temperatures within the overall range employed, yield the following Arrhenius forms (1σ errors): kH2= [graphic omitted] × 10–9 exp (–49.4 ± 1.3 kJ mol–1)/RT cm3 molecule–1 s–1 and kD2= [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 3PJ)+ 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 3PJ) by hydrogen and deuterium, where chemical reaction was the dominant mode of removal. The temperature dependence of the diffusion coefficient, D[Sr(5 3PJ)-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 τe= 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 3PJ) in helium indicates that the quenching of Ca(4 3PJ) 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.

The collisional removal of Ca[4s3d(1D2)] by hydrogen and deuterium has been studied in the time-resolved mode in the temperature range 750–1100 K. Ca(4 1D2) was generated by pulsed dye-laser excitation of calcium vapour at λ= 457.5 nm {Ca[4s3d(1D2)]â†� Ca[4s2(1S0)]} and monitored by time-resolved forbidden atomic emission at the resonance wavelength using boxcar integration with computer interfacing. A limited number of observations was also made in emission at λ= 657.3 nm [Ca(4 3P1)→ Ca(4 1S0)+hν] arising from the removal of Ca(4 1D2) by energy pooling and emission of radiation to the states Ca(4 3P2,1). Absolute second-order rate constants for the collisional removal of Ca(4 1D2) by hydrogen and deuterium were determined for nine separate temperatures across the overall range, yielding the following Arrhenius forms: kH2=(5.5 ±1.00.9)× 10–10 exp(–14.4 ± 1.3 kJ mol–1)//RT cm3 molecule–1 s–1 and kD2=(3.9 ± 0.4)× 10–10 exp(–22.2 ± 0.8 kJ mol–1)//RT cm3 molecule–1 s–1.These data are compared with analogous results for Ca(4 3PJ)+ H2, D2. The activation energies for the collisional removal of both optically metastable states with both isotopic reactants are consistent with chemical removal to yield the diatomic hydrides CaH(D)(X2Σ+, ν″= 0). The Arrhenius pre-exponential factors (A) also support chemical removal in all cases, as seen from symmetry arguments based upon the weak spin–orbit coupling approximation. A factors for Ca(43PJ)+ H2, D2 of the order of the collision number are in accord with direct correlation between Ca(43PJ)+ H2, D2 and CaH(D)(X2Σ+)+ H(D); A factors for Ca(4 1D2)+ H2, D2 significantly lower than those are in support of the absence of such adiabatic correlations.

The kinetic study of Mg(3 1P1) and Mg(4 3S1), 4.346 and 5.108 eV above the 3 1S0 ground state, has been investigated as a function of temperature and pressure of the gases He and Kr following pulsed dye-laser excitation of magnesium vapour at λ= 457.1 nm [Mg(3 3P1)â†� Mg(3 1S0)]. Mg(3 3PJ), Mg(3 1P1), and Mg(4 3S1) were monitored in the time-domain by measurements of the atomic emission at λ= 457.1 nm, λ= 285.3 nm [Mg(3 1P1)→ Mg(3 1S0)+hν], and λ(average)= 517.6 nm [Mg(4 3S1)→ Mg(3 3PJ)+hν]. Kinetic measurements on the decay of Mg(3 3PJ) in He and Kr in the range 600–1100 K yielded the temperature dependence of the diffusion coefficients, when expressed in the form D[Mg(3 3PJ)–He] and D[Mg(3 3PJ)–Kr]∝Tn, of n= 1.64 ± 0.09 and 1.65 ± 0.09, respectively. The collisional quenching of Mg(3 3PJ) by Kr has also been investigated in this temperature range, yielding the following alternative forms for the temperature-dependent rate constant: kKr=(2.9 ± 0.7)× 10–15 exp (–3.1 ± 0.5 kJ mol–1/RT) cm3 atom–1 s–1 or kKr=(6.0 ± 0.5)× 10–17T1/2+(1.1 ± 0.8)× 10–16 cm3 atom–1 s–1, the temperature-independent term lying within the error of the measurements. The mean radiative lifetime (τe) of Mg(3 3P1) has been redetermined from all the decay measurements in the range 600–1100 K, yielding the improved result τe= 2.3 ± 0.3 ms. An approximate estimate of kMg= 2 × 10–15 cm3 atom–1 s–1 is reported for the collisional removal of Mg(3 3PJ) by Mg(3 1S0). Energy pooling to yield Mg(3 1P1) has been studied in the range 800–1100 K, the first-order decay profiles for Mg(3 1P1) and Mg(3 3P1) being consistent with Mg(3 3PJ)+ Mg(3 3PJ) annihilation at all temperatures and pressures. Estimates of the ratio of the initial yields of [Mg(3 1P1)]t= 0/[Mg(3 3P1)]t= 0 determined at different temperatures are found to be sensibly consistent with thermodynamic data for the vapour pressure of atomic Mg. Energy pooling to yield Mg(4 3S1) has been investigated from detailed measurements of the decay profiles for this atomic state using the first-order kinetic approximation at ‘long decay times’ in the range 750–1100 K. This has been interpreted quantitatively in terms of a mechanism involving the low-lying states Mg2(3Πg, 3Σ+u), 2.05 and 2.19 eV above the Mg2(X1Σ+g) ground state, and produced from the three-body recombination of Mg(3 3PJ)+ Mg(3 1S0)+ He, Kr which undergoes subsequent reaction with Mg(3 3PJ) to yield Mg(4 3S1), in accord with the earlier proposal of Breckenridge and coworkers. The mechanism is established from the dependence of the decay profiles at λ(average)= 517.6 nm on the pressure of helium and krypton at all temperatures, and by comparison with those at λ= 457.1 nm. This indicates that Mg2(3Πg, 3Σ+u) can only be placed in stationary state in this type of system when T > 1000 K. The temperature dependence of the initial yields of [Mg(4 3S1)]max/[Mg(3 3P1)]t= 0 is also found to be in sensible accord with expectation based on vapour pressure data for Mg(3 1S0).

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.

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).

The collisional quenching of Ca(4 3PJ) and Sr(5 3PJ) has been studied for an equilibrium system coupled by the process Ca(4 3PJ)+ Sr(5 1S0)⇌ Ca(4 1S0)+ Sr(5 3PJ) and established following pulsed dye-laser excitation of Ca(4 1S0) to Ca(4 3P1) at λ= 657.3 nm in mixtures of calcium and strontium vapour in the presence of an excess of helium buffer gas and added quenching gases. Ca(4 3P1), Sr(5 3P1) and Sr(6 3S1) were monitored by time-resolved emission at λ= 657.3, 689.3 [Sr(5 3P1)→ Sr(5 1S0)+hν) and 679.1 nm [Sr(6 3S1)→ Sr(5 3P0)+hν], respectively, using boxcar integration. The measured first-order decay coefficients for Ca(4 3PJ) and Sr(5 3PJ) under equilibrium-coupled conditions were found to be in quantitative agreement with previously measured values of absolute second-order rate constants for the quenching gases Kr, Xe, H2, D2, N2, CO, NO, N2O, CO2, NH3, CH4, CF4, C2H2, C2H4 and C6H6, obtained using the present system, employing signal averaging, on decays of Ca(4 3PJ) and Sr(5 3PJ) individually. Finally, energy pooling according to the processes Ca(4 3PJ)+ Sr(5 3PJ)→ Sr(6 3S1)+ Ca(4 1S0), Sr(5 3PJ)Sr(5 3PJ)→ Sr(6 3S1)+ Sr(5 1S0) was investigated quantitatively from measurements at λ= 679.1 nm in the time domain and in the presence of the quenching gases.