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

Abstract

Low-temperature inelastic collisions between hydrogen molecules and helium atomsRotranslational state-to-state rates and spectral representation of inelastic collisions in low-temperature molecular hydrogen 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 → m in O 2 :O 2 collisions at low temperature involving the rotational states i, j, , m of the vibrational ground state of 16 O 2 ( 3 − 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 (≈ 2 × 10 −4 mm 3 ) of O 2 travelling along the axis of a supersonic jet at a velocity of ≈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 ≈6% for the dominant rates at 34 K, up 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 .

Rotational relaxation in molecular hydrogen and deuterium: Theory versus acoustic experiments

“…Proofs in favor of this factor have been provided recently in supersonic jet experiments. 68 The rate coefficients for collisions between indistinguishable molecules are then obtained by means of the…”

Molecular description of steady supersonic free jets

“…The k ij→ m sts-rates can be calculated today in the frame of quantum scattering methods by essentially the exact solution of a set of coupled Schrödinger equations. 65 Experimental assessment for k ij→ m rates has been possible in the low-temperature range for H 2 :H 2 , N 2 :N 2 , O 2 :O 2 , H 2 :He and H 2 O:He systems by means of supersonic jet experiments [66][67][68][69][70][71] combined with well-established quantum-scattering computational codes. 72,73 Finally, since the calculated k ij→ m collision sts-rates derive from the Schrödinger equation, high-quality experimental data together with Eqs.…”

Effect of anisotropy on the glory structure of molecule–molecule scattering cross sections

“…The theory for the scattering of two identical linear rigid rotors has been reviewed in Ref. [23] and applied to O 2 +O 2 elsewhere [22,23], so here we focus in some aspects specific to this study. Using diatom-diatom Jacobi vectors R, r 1 , and r 2 in a space-fixed frame, the Hamiltonian is written as (in atomic units) [28,29],…”

“…Statistical weights appropriate to 16 O 2 + 16 O 2 are taken as explained in detail in Refs. [22,23]. Besides, for the present simulation of the number of particles lost from a molecular beam due to scattering by a target, the computed integral cross section must be corrected when the final states of the colliding partners are identical, in order to avoid a double counting [23].…”

“…In order to study the performance of this PES, we have carried out some dynamical calculations at different energy regimes including ultracold [21], cold [22], and preliminary calculations in the thermal regime [19,23]. For cold translational temperatures, 10 ≤ T t (K) ≤ 34, the quality of the ab initio PES was tested against measurements of rotational populations of O 2 molecules traveling along a supersonic jet [22]. Close-coupling calculations of rotationally inelastic state-to-state rate coefficients were found in good agreement with the experimental data, suggesting that the anisotropy of the ab initio PES is realistic.…”