  We demonstrate a simple technique for molecular ion spectroscopy. BaCl$^+$
molecular ions are trapped in a linear Paul trap in the presence of a
room-temperature He buffer gas and photodissociated by driving an electronic
transition from the ground X$^1\Sigma^+$ state to the repulsive wall of the
A$^1\Pi$ state. The photodissociation spectrum is recorded by monitoring the
induced trap loss of BaCl$^+$ ions as a function of excitation wavelength.
Accurate molecular potentials and spectroscopic constants are determined.
Comparison of the theoretical photodissociation cross-sections with the
measurement shows excellent agreement. This study represents the first
spectroscopic data for BaCl$^+$ and an important step towards the production of
ultracold ground-state molecular ions.
