Molecular oxygen, although homonuclear, hence with zero electric dipole
moment, has a triplet electronic ground state, with two electrons
paired with parallel spins. The resulting electronic spin couples efficiently with
the magnetic fields caused by the end-over-end rotation of the molecule,
yielding a ``large'' magnetic dipole moment, = 10-20 emu.
The magnetic dipole transitions of O2 have intrinsic strengths
times weaker than the water transitions. O2, however, is
102-3 times more abundant than H2O, so that the atmospheric lines of
the two species have comparable intensities.
The spin of 1 makes of the ground electronic state of O2 a triplet state
(). N, the rotational angular momentum couples with S, the
electronic spin, to give J the total angular momentum: N +S = J. The N
S interaction (and the electronic angular
momentum-electronic spin interaction L
S) split each rotational level
of rotational quantum number
into three sublevels with total quantum
numbers
The magnetic dipole transitions obey the rules
and
. Transitions within the fine structure sublevels of a rotational level
(i.e.
) are thus allowed. The first such transition is the
transition, which has a frequency of 118.75 GHz. The second, the
transition, has a frequency of 56.26 GHz. It is surrounded by a
forest of other fine structure transitions with frequencies ranging from 53 GHz to
66 GHz. The first "true" rotational transition, the
transitions,
have frequencies above 368 GHz (368.5, 424.8, and 487.3 GHz).
The rare isotopomer 18O16O is not homonuclear, hence has odd
N levels and a non-zero electric dipole moment. This latter, however,
is vanishingly small (10-5D). 18O16O, moreover, has a
very low abundance (few hundred times smaller than the main
isotopomer), so that its magnetic dipole transitions (even the
, which have stronger intrinsic strengths), can be
neglected.
The line opacity and absorption coefficients of 16O2 are given by relations similar to Eq.8.24 and Eq.8.25.