Comparison of the Ca+HF(DF) and Sr+HF(DF) reaction dynamics

A comparative study of the reaction family, Ca and Sr with rovibrationally selected HF or DF, has been carried out under single-collision conditions. A thermal beam of the alkaline earth atoms, Ca or Sr, is fired into a low-pressure gas of HF or DF in which the reagent molecules have been prepared in a selected vibration-rotation state by the use of a tunable infrared light source (optical parametric oscillator). The resulting alkaline earth monofluoride reaction products, CaF or SrF, are detected in a quantum-state-specific manner with a tunable visible light source (dye laser) using laser induced fluorescence. The HF molecule is aligned with its rotational angular momentum pointing preferentially either along or perpendicular to the metal atom beam. For both Ca + HF(v = 1) and Sr + HF(v = 1) the cross section and the product state distribution are found to be independent of the approach geometry, which is consistent with reaction through a bent transition state. The state-to-state reaction dynamics for Ca + HF(DF) and Sr + HF(DF) show marked differences. The Ca + HF(DF)→CaF + H(D) reaction has attributes of a statistical internal energy distribution, whereas Sr + HF(DF) → SrF + H(D) shows some deviation from this behavior. The findings for Ca + HF(DF) are consistent with a model in which the Ca atom inserts into the HF bond to form a long-lived H-Ca-F intermediate; the findings for Sr + HF(DF) are interpreted as competition between direct reaction and reaction which samples the deep H-Sr-F potential well.