  The $\mathrm{\tilde{C}}$ $^1$B$_2$ state of SO$_2$ has a double-minimum
potential in the antisymmetric stretch coordinate, such that the minimum energy
geometry has nonequivalent SO bond lengths. However, low-lying levels with odd
quanta of antisymmetric stretch (b$_2$ vibrational symmetry) have not
previously been observed because transitions into these levels from the
zero-point level of the $\mathrm{\tilde{X}}$ state are vibronically forbidden.
We use IR-UV double resonance to observe the b$_2$ vibrational levels of the
$\mathrm{\tilde{C}}$ state below 1600 cm$^{-1}$ of vibrational excitation. This
enables a direct characterization of the vibrational level staggering that
results from the double-minimum potential. In addition, it allows us to
deperturb the strong $c$-axis Coriolis interactions between levels of a$_1$ and
b$_2$ vibrational symmetry, and to determine accurately the vibrational
dependence of the rotational constants in the distorted $\mathrm{\tilde{C}}$
electronic state.
