Knowledge Bank

The rotational, fine, and hyperfine structure in the spectra of the Ar$\cdot$OH/D A $bands1$ has been well studied and $reported2$. In contrast, the spectra of the vibronic bands (U bands) have not been analyzed in detail. These U bands are $assigned1$to transitions with one quantum of excitation in the bending mode in the $A~$ state, and we first $reported3$ the high resolution electronic spectrum two years ago. Due to the bending excitation in the A state, these U bands have a completely different vibronic symmetry from that of the A bands. As we reported, the spectrum of U bands shows a very different rotational contour. In addition, the fine and hyperfine structure is also very complicated and different from that of the A bands. Here, we present a complete spectral fit with a standard deviation of about 50 MHz for the U band spectrum including rotational, fine, and hyperfine structure. The details of the analysis and the physical interpretation will be discussed.