The third-order reaction, H + O2 + M → HO2 + M, has been measured near the low-pressure limit at room temperature for M = He, Ne, Ar, Kr, O2, N2, and H2O and over an extended range of temperatures in a shock tube for M = Ar, O2, and N2. In all cases, H atoms were produced by the laser photolysis of NH3 and detected by atomic resonance absorption spectroscopy. The measurements are consistent with the available experimental record and, in particular, confirm the exceptionally high recombination rate constant when M = H2O. The standard theoretical analysis is applied to this entire experimental record to derive the value of the average energy change per collision, −ΔE all. The resulting −ΔE all values are sensible for all M but H2O. The problem with H2O motivates a change in the standard theoretical analysis that both rationalizes the behavior of H2O and also quantitatively changes the derived −ΔE all values for the other species of M. These changes involve three modifications of the standard treatment:  (1) explicit temperature dependence in the number of active rotational degrees of freedom contributing to the HO2* state density, (2) the replacement of Lennard-Jones potential for the HO2* + M interaction with an electrostatic + dispersion potential, and (3) the calculation of the collision rate between HO2* + M by a free rotor model for “complex formation” between the M and HO2*. The optimized values of −ΔE all that are produced from this new analysis have the following characteristics:  (1) the value of −ΔE all is the same for all rare gases, and (2) −ΔE all for di- and polyatomic molecules are enhanced relative to the rare gas atoms. This work supports the conclusions of previous trajectory studies that collision rates between activated complexes and bath gases are often underestimated while −ΔE all derived from recombination kinetics measurements are often overestimated.

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The study of triggered depopulation of nuclear isomers, with accompanying gamma emission, is a rapidlychanging field that is only now attaining some degree of maturity. Because isomer decays via electromagnetic transitions are strongly inhibited, the interaction of these levels with externallyproduced photons provides an important probe of nuclear structure. Also, since some isomers may store large amounts of energy for long times, a number of applications have been proposed, including the creation of a gamma-ray laser. Early experiments conclusively demonstrated triggering of the 10 15 -year isomer of 180 Ta, but even a partial correlation of the effect with known levels could only be obtained recently. Investigations of triggering for the 31-year isomer of 178 Hf were initially guided just by systematics and experiments are characterized by considerable controversy. Against this background, the field of triggered gamma emission is entering a new phase in which improved level data allow targeting of specific potentially-useful transitions. This paper summarizes the current state-of-the-art and discusses the changing nature of the field.

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