Inelastic collisions in molecular oxygen at low temperature (4 ≤ T ≤ 34 K). Close-coupling calculations versus experiment

Close-coupling calculations and experiment are combined in this work, which is aimed at establishing a set of state-to-state rate coefficients for elementary processes i j -> lm in O-2:O-2 collisions at low temperature involving the rotational states i, j, l, m of the vibrational ground state of O-16(2)((3)Sigma(-)(g)). First, a set of cross sections for inelastic collisions is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent ab-initio potential energy surface for O-2-O-2 [M. Bartolomei et al., J. Chem. Phys. 133, 124311 (2010)]. Then, the corresponding rates for the temperature range 4 <= T <= 34 K are derived from the cross sections. The link between theory and experiment is a Master Equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. This Master Equation provides a linear function of the rates for each rotational state and temperature. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (approximate to 2 x 10(-4) mm(3)) of O-2 travelling along the axis of a supersonic jet at a velocity of approximate to 700 m/s. The accuracy of the calculated rates is assessed experimentally for 10 <= T <= 34 K by means of the Master Equation. The rates, jointly with their confidence interval estimated by Monte Carlo simulation, account to within the experimental uncertainty for the evolution of the populations of the N = 1, 3, 5, 7 rotational triads along the supersonic jet. Confidence intervals range from approximate to 6% for the dominant rates at 34 K, up to approximate to 17% at 10 K. These results provide an experimental validation of state-to-state rates for O-2:O-2 inelastic collisions calculated in the close-coupling approach and, indirectly, of the anisotropy of the O-2-O-2 intermolecular potential employed in the calculation for energies up to 300 cm(-1). (C) 2011 American Institute of Physics. [doi:10.1063/1.3585978]