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 o btained in one-color experiments using two- and three-photon ionization. An enhancement similar to resonance Raman yields changed spectra if the excit ation proceeds via continuous intermediate states. A molecule which after a bsorbing a photon started to dissociate is stabilized on the ionic surface by a delayed second photon, with the observed spectra reflecting geometry c hanges due to this arrested dissociation process. Using two colors, the tec hnique should be applicable to almost any molecule, and yield a detailed in sight into its dissociation dynamics.