Photoswitching of a charged azobenzene-stilbene dye is investigated through laser excitation in a tandem ion mobility mass spectrometer. Action spectra associated with E -> Z and Z -> E photoisomerisation of the stilbene group exhibit bands at 685 and 440 nm, corresponding to S-1 <- S-0 and S-3 <- S-0 transitions, respectively. The data suggest that isomers possessing a Z configuration of the azobenzene unit rapidly convert to E isomers and are not discernible using ion mobility spectrometry, and that photoisomerisation occurs through excited state dynamics rather than statistical isomerisation on the ground state potential energy surface.