  Surprisingly strong CO emission has been observed from more than a dozen
debris disks around nearby main-sequence stars. The origin of this CO is
unclear, in particular whether it is left over from the protoplanetary disk
phase or is second-generation material released from collisions between icy
bodies like debris dust. The primary unexplored avenue for distinguishing the
origin of the material is understanding its molecular composition. Here we
present a deep search for five molecules (CN, HCN, HCO+, SiO, and CH3OH) in the
debris disk around 49 Ceti. We take advantage of the high sensitivity of the
Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7 to integrate for
3.2 hours at modest spatial (1") and spectral (0.8 km/s) resolution. Our search
yields stringent upper limits on the flux of all surveyed molecular lines,
which imply abundances relative to CO that are orders of magnitude lower than
those observed in protoplanetary disks and Solar System comets, and also those
predicted in outgassing models of second-generation material. However, if CI
shielding is responsible for extending the lifetime of any CO produced in
second-generation collisions, as proposed by Kral et al. (2018), then the line
ratios do not reflect true ice phase chemical abundances, but rather imply that
CO is shielded by its own photodissociation product, CI, but other molecules
are rapidly photodissociated by the stellar and interstellar radiation field.
