The mechanism of fragmentation involves the absorption by a given ion of multiple infrared photons.
In the case of powerful laser pulses, the dissociation proceeds via inner-valence ionization of electrons.
[5][6] This infrared photodissociation spectroscopy allows for the measurement of vibrational spectra of (unstable) species that can only be prepared in the gas phase.
[7] The combination of mass spectrometry and IRMPD with tunable lasers (IR ion spectroscopy) is increasingly recognized as a powerful tool for small-molecule identification.
For example, molecules of UF6 containing U-235 might be ionized completely as a result of such a laser resonance, leaving UF6 containing the heavier U-238 intact.