  We use (1+1$'$) resonance-enhanced multiphoton photodissociation (REMPD) to
detect the population in individual rovibronic states of trapped HfF$^+$ with a
single-shot absolute efficiency of 18%, which is over 200 times better than
that obtained with fluorescence detection. The first photon excites a specific
rotational level to an intermediate vibronic band at 35,000-36,500 cm$^{-1}$,
and the second photon, at 37,594 cm$^{-1}$ (266 nm), dissociates HfF$^+$ into
Hf$^+$ and F. Mass-resolved time-of-flight ion detection then yields the number
of state-selectively dissociated ions. Using this method, we observe
rotational-state heating of trapped HfF$^+$ ions from collisions with neutral
Ar atoms. Furthermore, we measure the lifetime of the $^3\Delta_1$ $v=0,\, J=1$
state to be 2.1(2) s. This state will be used for a search for a permanent
electric dipole moment of the electron.
