Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O <--> HO2-H2O was determined to be K2 = (5.2 +/- 3.2) x 10(-19) cm3 molecule(-1) at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm3 molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2-H2O]/[HO2] was estimated from the value of K2 to be 0.19 +/- 0.11.

Rate constants for the reactions of Si (3P, 1D, and 1S) atoms with SiH4 have been determined using a combined laser photolysis/laser induced fluorescence technique. The reaction of Si(3P) with SiH4 is shown to proceed with a rate constant k(Si(3P J ) + SiH4) = (2.1±0.2) × 10-10 cm3 molecule-1 s-1, with no discernible pressure dependence over a pressure range of 5−20 Torr of N2 diluent. The rate constants of each triplet component of Si(3P J =0,1,2) atoms exhibit no significant spin-orbit dependence within the experimental uncertainties. The results of the density functional theory calculations show that the reaction of Si(3P) atoms with SiH4 proceeds via a loose transition state. The Si(1D) + SiH4 reaction rate constant is determined to be (5.2 ± 0.3) × 10-10 cm3 molecule-1 s-1. The Si(1S) atom is less reactive with SiH4 than the Si(1D) atom, with an upper limit of k(Si(1S) + SiH4) = 4.7 × 10-13 cm3 molecule-1 s-1.

The rate constant for the reaction S + SO2 → SO + SO (1) has been investigated by measuring time profiles of S atoms behind reflected shock waves using two experimental systems:  S atoms were provided by the thermal decomposition of COS in the high-temperature range (2020−2800 K) and by excimer laser photolysis of COS in the low-temperature range (T = 1120−1540 K). The results of these experiments yield the rate constant with a non-Arrhenus temperature dependence, k 1 = 10-39.73 T 8.21 exp(4828.5/T) cm3 molecules-1 s-1, over the extended temperature range (1120−2800 K). By comparing the rate constants with that derived from a conventional transition-state theory based on the potential energy surface calculated by the G2M(CC1) methodology, a reaction mechanism including a contribution of the singlet state of the reaction intermediate S2O2 is discussed.