Knowledge Bank

Molecular hydrogen is the main constituent of the atmospheres of the giant planets, Jupiter, Saturn, Uranus, and Neptune. The relative abundances of ortho- and parahydrogen are inferred from infrared emission spectra of the $S(1)$ and $S(0)$ quadrupole transitions near $17$ and $28\mu$, sampling conditions in the stratosphere and upper troposphere [1,2]. At most altitudes and latitudes the ortho/para ratio is not in statistical equilibrium at the local temperature, interpreted as resulting from vertical 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 H atoms or 4-center 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-$H2$ and para-$H2$. The only estimate in the literature [1] is based on scaling the magnetic moments of $H2(J=1)$ and $O2$, which suggests a ortho-para conversion probability of about $4\times 10-19$ and a rate coefficient of about $1\times 10-28cm3s-1$. We will discuss the expected hamiltonian operators involving nuclear spin, procedures for evaluation of matrix elements, and dynamics approaches. Especially interesting are ideas about virtual excitation of the $b3\Sigma u+$ state resulting in ortho-para coupling through nuclear-spin/electron-spin hyperfine interaction [3,4].