= Solution
The two extra progressions are \b[rotational transitions within thermally populated $v=1$ and $v=2$ vibrational levels]. Isotopic purity rules out attributing them to different isotope masses. Their <rotational constants> are
$$
\boxed{B_1=10.9115\ \mathrm{GHz},\qquad B_2=10.8235\ \mathrm{GHz}.}
$$
The <Boltzmann distribution> makes each higher vibrational manifold less populated, so its rotational spectrum is weaker. A <Morse oscillator> explores larger separations as it is excited: the bond's vibrationally averaged moment of inertia increases and the rotational spacing decreases. More precisely, $B_v$ involves an average of the inverse square separation; it is not simply the inverse square of the mean separation.
The observed decrement is constant to the given precision:
$$
B_0-B_1=B_1-B_2=\boxed{\alpha_e=0.0880\ \mathrm{GHz}}.
$$
This fits the leading <vibration-rotation interaction> law $B_v=B_e-\alpha_e(v+\tfrac12)$.
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