Laser-induced fluorescence, resonance-enhanced multiphoton ionization, and cavity ringdown spectroscopic techniques offer ways of detecting electronic transitions without directly measuring light absorption. An alternative approach is possible if the excitation process leads to fragmentation of the original molecule. By monitoring one of the photofragments as a function of laser wavelength, a spectrum can be recorded. This is the basic idea behind photodissociation spectroscopy.

There are limitations to this approach. If photodissociation is slow, then the absorbed energy may be dissipated by other mechanisms, making photodissociation spectroscopy ineffective. It is also possible that some rovibrational energy levels in the excited electronic state will lead to fast photofragmentation whereas others will not. In this case there will be missing or very weak lines in the spectrum which, in a conventional absorption spectrum, may have been strong. Fast photofragmentation is clearly desirable on the one hand, but it can also be a severe disadvantage if it is too fast, since it may lead to serious lifetime broadening in the spectrum (see Section 9.1).

Despite the above limitations, photodissociation spectroscopy can provide important information. This is particularly true for relatively weakly bound molecules and complexes, since these have a greater propensity for dissociating. In this and the subsequent example the capabilities of photodissociation spectroscopy are illustrated by considering weakly bound complexes formed between a metal cation, Mg+, and rare (noble) gas (group 18) atoms.