Molecular beam study of the collisions of state-monitored, metastable noble gas atoms with O2(X 3Σg−)

Abstract

We describe a new molecular beam-luminescence method for measuring state-resolved cross sections for the quenching of metastable noble gas atoms, and report values for “dark” collisions of Ng*(3P2,3P0) with O2(X 3Σg−), where Ng=Ar, Kr, and Xe. Cross sections for quenching σQ and, in some cases, cross sections for excited products σ* are also given for a number of state-specific, luminescent monitor reactions. The elastic reaction of Ng* with He or Ne is employed to correct the total disappearance cross section QT for viewing losses caused by nonquenching processes. The velocity-averaged, quenching cross section QQ is obtained by subtracting the nonquenching cross section QN from QT. Values of QQ measured at average relative velocity ḡ (average relative kinetic energy Ē) are deconvoluted to yield σQ(ḡ). For Ar* collisions with O2, we find σQ[3P2] values of 35.6±1.8 Å2 for ḡ (Ē) between 690 and 2000 m/s (50 and 350 meV) that gradually decrease above 2000 m/s, and σQ[3P0] values of 46±4 Å2 between 690 and 830 m/s (50 and 70 meV). For Kr* collisions, we report σQ[3P2] values of 38.1±2.5 Å2 between 575 and 810 m/s (46 and 87 meV) with no apparent velocity dependence, and a σQ[3P0] value of 56±7 Å2 at 576 m/s (46 meV). For Xe* collisions, we find σQ[3P2] values of 48±3 Å2 at 535 m/s (44 meV) and 38±2 Å2 at 697 m/s (73 meV), and a σQ[3P0] value of ∼125 Å2 at 535 m/s (44 meV). Comparisons with σQ values obtained with other techniques that do not require a viewing loss correction are excellent. We also use the Ionic-Intermediate-Curve-Crossing Model (IICCM) to calculate cross sections for the Ar*(3P2)+O2→Ar+O*(1D)+O(3P) reaction. In our application of the model, the product state dissociative continuum is coupled to Ar+O2− through the predissociating O2*(E 3Σu−) state that is valence Rydberg in character. Values of σQ derived from the model are in good agreement with our experiment.