Kinetic Study of the Reactions PO + O2 and PO2 + O3 and Spectroscopy of the PO Radical

Abstract

The kinetics of the reactions of PO with O₂ and PO₂ with O₃ were studied at temperatures ranging from ∼190 to 340 K, using a pulsed laser photolysis-laser induced fluorescence technique. For the reaction of PO + O₂, there is evidence of both a two- and three-body exit channel, producing PO₂ + O and PO₃, respectively. Potential energy surfaces of both the PO + O₂ and PO₂ + O₃ systems were calculated using electronic structure theory and combined with RRKM calculations to explain the observed pressure and temperature dependences. For PO + O₂, at pressures typical of a planetary upper atmosphere where meteoric ablation of P will occur, the reaction is effectively pressure independent with a yield of PO₂ + O of >99%; the rate coefficient can be expressed by log₁₀(k, 120-500 K, cm³ molecule^-1 s^-1) = -13.915 + 2.470 log₁₀(T) - 0.5020(log₁₀(T))², with an uncertainty of ±10% over the experimental temperature range (191-339 K). With increasing pressure, the yield of PO₃ increases, reaching ∼90% at a pressure of 1 atm and T = 300 K. For PO₂ + O₃, k(188-339 K) = 3.7 × 10^-11 exp^(-1131/T) cm³ molecule^-1 s^-1, with an uncertainty of ±26% over the stated temperature range. Laser-induced fluorescence spectra of PO over the wavelength range 245-248 nm were collected and simulated using pgopher to obtain new spectroscopic constants for the ground and v = 1 vibrational levels of the X²Π and A²Σ⁺ states of PO.