Multiphoton photoelectron spectroscopy: Watching molecules dissociate

Threshold photoionization spectroscopy [zero kinetic energy (ZEKE), pulsed field ionization (PFI), mass analysed threshold ionisation (MATI)] usually takes advantage of multiphoton excitation through bound intermediate states, but we show that excellent quality, high-resolution spectra can also be obtained in one-color experiments using two- and three-photon ionization. An enhancement similar to resonance Raman yields changed spectra if the excitation proceeds via continuous intermediate states. A molecule which after absorbing a photon started to dissociate is stabilized on the ionic surface by a delayed second photon, with the observed spectra reflecting geometry changes due to this arrested dissociation process. Using two colors, the technique should be applicable to almost any molecule, and yield a detailed insight into its dissociation dynamics.