Ortho-Para Conversion in Collisions of H2 with H2

Sri International, Menlo Park CA

Investigators

Abstract

AST 0406991 Huestis Molecular hydrogen is the main constituent of the atmospheres of the gas giant planets, Jupiter, Saturn, Uranus, and Neptune. The relative abundances of ortho- and parahydrogen are inferred from the ratio of the observed strengths of pure-rotation, vibrational-overtone, and Raman-scattering transitions involving the rotational levels J = 1 (ortho) and J = 0 (para). At most altitudes and latitudes the ortho/para ratio is not in statistical equilibrium at the local temperature, which is interpreted as resulting from vertical and horizontal transport from lower hotter or colder regions. Modeling atmospheric circulation requires a quantitative understanding of the rates and mechanisms of ortho-para conversion, which appears to take roughly 30 to 100 years. The two candidate mechanisms are collisions with paramagnetic aerosols and with the weak magnetic moment of ortho-H2. The better known mechanisms involving ions or Hatoms or four-center bimolecular atom exchange are inoperative at the relevant low altitudes and low temperatures. An important constraint on atmospheric models could be provided by a quantum mechanical treatment of nuclear spin coupling in collisions of ortho-H2 with ortho- and para-H2. To provide a quantitative basis for interpretation of atmospheric observations, Dr. David Huestis will perform quantum mechanical calculations of cross sections and temperature-dependent rates for ortho-para conversion in collisions of H2 with H2, including electronic structure, matrix elements of Hamiltonian operators involving nuclear spin, and coupled-channels quantum dynamics. A broader impact of this research is that it will contribute to advancement of discovery and understanding while promoting teaching, training, and learning through participation of a postdoctoral fellow, graduate, and undergraduate students. An additional broader impact of the proposed work is its fundamental nature as an unsolved problem in quantum mechanics and chemical kinetics. ***