A research program is underway with the objective of developing techniques of high resolution multiphoton spectroscopy for selective, ultrasensitive molecular detection. Methods under study include various forms of ion dip spectroscopy and new methods of ion fragmentation spectroscopy. The studies are providing a new understanding of the fundamental spectroscopy and photophysics of large molecular ions. Dimer and cluster ions of polynuclear aromatics and related species are also being investigated, with potential detection applications.