= Solution
A regularly repeated set of sharp features is characteristic of the unresolved \b[Q subbranches of a <perpendicular rovibrational band>]. The transition dipole is perpendicular to the <symmetry> axis, giving $\Delta K=\pm1$; for the <Q branch> $\Delta J=0$. Holding the <rotational constants> fixed between vibrational states cancels the $BJ(J+1)$ term, leaving
$$
\widetilde\nu_Q=\widetilde\nu_v+(\widetilde A-\widetilde B)\big[(K\pm1)^2-K^2\big]
=\widetilde\nu_v+(\widetilde A-\widetilde B)(\pm2K+1).
$$
Many J transitions therefore coincide within each K-changing Q subbranch in this approximation. The different K subbranches are separated by
$$
\boxed{\Delta\widetilde\nu_Q=2(\widetilde A-\widetilde B).}
$$
The P and <R branches> have $\Delta J=-1$ and $+1$, respectively. Their extra J-dependent terms spread many transitions across a broad background at modest resolution. A parallel band, with $\Delta K=0$, would not generate this repeated Q-subbranch spacing under the stated assumptions.
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