ORTHO-PARA CONVERSION IN COLLISIONS OF $H_{2}$ WITH $H_{2}$
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Date
2003
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Ohio State University
Abstract
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-$H_{2}$. 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-$H_{2}$ with ortho-$H_{2}$ and para-$H_{2}$. The only estimate in the literature [1] is based on scaling the magnetic moments of $H_{2}(J=1)$ and $O_{2}$, which suggests a ortho-para conversion probability of about $4 \times 10^{-19}$ and a rate coefficient of about $1 \times 10^{-28} cm^{3} s^{-1}$. 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 $b^{3} \Sigma^{+}_{u}$ state resulting in ortho-para coupling through nuclear-spin/electron-spin hyperfine interaction [3,4].
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[1] B. J. Conrath and P. J. Gierasch, Icarus 57, 184 (1984). [2] T. Fouchet, E. Lellouch, and H. Feuchtgruber, Icarus 161, 127 (2003). [3] E. Ilisca and S. Sugano, Phys. Rev. Lett. 57, 2590 (1986). [4] B. F. Minaev and H. {\AA}gren, J. Phys. Chem. 99, 8396 (1995).
Author Institution: SRI International
Author Institution: SRI International