State-specific detection of trapped HfF$^+$ by photodissociation

•We use REMPD as a rotational-state specific readout technique for an eEDM measurement.•We observe transitions from both the ground and first excited electronic states.•We achieve a single shot detection efficiency of 18%.•We use this method to observe rotational-state heating of trapped molecular ions.•We determine the lifetime of the 3Δ1 state to be 2.1(2)s.