“…The two-sided arrow emphasizes the equal role of these levels . In the weak-field excitation limit, ,, in an optically thin sample, and in the absence of the chirp edge effects, has the following dependence ,− Here, k is the Boltzmann constant, E J ′ and E J ″ are the rotational energies of the J ′ and J ″ levels, ω is the mm-wave transition frequency, x S J ′ J ″ is the transition strength, μ x is the projection of the electric dipole moment, n is the number density, and L is the length of the reactor. Note that the M -degeneracy of the J levels is included in x S J ′ J ″ .…”