The excitation function refers to the translational energy dependence of the integral cross-section of a biomolecular collision process. Exemplified by a number of elementary chemical reactions, the information content of excitation functions is critically surveyed. Particular emphasis is placed on the close comparison with the available thermal kinetics data. The reactivity for an activated reaction was found to depend sensitively on the rotational state of the reagent, indicative of stereodynamical effects. The intramolecular isotope branching ratio, for the reaction A + HD, exhibits a strong dependence on the collision energy. Its isotopic propensity reverses between a non-rotating and a rotating reagent. By way of contrast, the reactive behaviour of a barrierless reaction shows little dependence on the initial rotational state, and the intramolecular isotope branching ratio instead becomes nearly independent of the collision energy. In this case, the excitation function obtained from crossed-beam experiments then provides a direct and reliable route to compare with available thermal rate constants or to extrapolate kinetics to a wider temperature range.
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Kopin Liu (2001) studied this question.