Knowledge Bank

The rotational spectrum of $HCIO4$ has been observed in selected regions between 50 and 435 GHz. The entire region between 413 and 434.3 GHz has been recorded. The molecule is a near spherical rotor with a moderately low barrier to internal rotation. The spectrum is characterized by strong $\mu a$ R-branch clusters which have been observed for $J\prime =5,8-11,20,28,30,40$ and 41. Although there are no E torsional states, the R-branches show two distinct groups of lines. There is a relatively tight cluster of symmetric rotor like transitions with $K=3n$ and a rather spread out group of transitions with $K=3n\pm 1$. Because of the spin statistics of the $ClO3$ group, the latter transitions arise from energy levels containing one unit of internal angular momentum $l=1$, and display a pattern quite similar to that expected from a degenerate vibration of a symmetric rotor with $l$-type doubling. These show numerous anomalous splittings and shifts due to avoided crossings within the $K$ stacks which allow the relative positions of the energy levels with $I=1$ to be reasonably well determined. Splittings of the $K=2$ and $K=3$ transitions provide a fair estimate for the $K=2-0$ and $K=3-1$ intervals. With this information it has been possible to assign numerous $\mu bP-,Q-$, and R-branch transitions and to determine the energy levels very precisely to quite high K. Excited torsional states have been assigned which correspond to $l=2$ and $l=3$ levels. The $l=3$ transitions consist of two groups of lines with $K=3n$. The excited states are near the top of the barrier and the pattern of lines arising rom them is free rotor like. The fitting of the spectrum will be described and the derived parameters will be discussed in terms of the molecular structure and the internal rotation. The CI quadrupole coupling constant $\chi aa$ has been determined.