Rotationally inelastic scattering of O3–Ar: state-to-state rates with the multiconfigurational time dependent Hartree method

Abstract

The Chapman cycle, proposed in 1930, describes the various steps in the ongoing formation and destruction of stratospheric ozone. A key step in the formation process is the stabilization of metastable ozone molecules through collisions with a third body, usually an inert collider such as N₂. The “ozone isotopic anomaly” refers to the observation of larger-than-expected atmospheric concentrations for certain ozone isotopologues. Previous studies point to the formation steps as the origin of this effect. A possibly key aspect of the ozone formation dynamics is that of the relative efficiencies of the collisional cooling of different isotopologues. Although the substitution of low-abundance ¹⁸O for ¹⁶O in O₃ molecules corresponds to a relatively small net change in mass, related to this are some subtleties due to symmetry-breaking and a resulting more than doubling of the density of allowed states governed by nuclear-spin statistics for bosons. Recently, a highly accurate 3D potential energy surface (PES) describing O₃–Ar interactions has been constructed and used to benchmark the low-lying rovibrational states of the complex. Here, using this new PES, we have studied the collisional energy-transfer dynamics using the MultiConfiguration Time Dependent Hartree method. A study of the rotationally inelastic scattering was performed for the parent ¹⁶O¹⁶O¹⁶O–Ar system and compared with that of the ¹⁶O¹⁶O¹⁸O–Ar isotopologue. The state-to-state cross-sections and rates from the 0_0,0 initial state to low lying excited states are reported. Analysis of these results yields insight into the interplay between small changes in the rotational constants of O₃ and the reduced mass of the O₃–Ar collision system, combined with that of the symmetry-breaking and introduction of a new denser manifold of allowed states.