Dissociative ionization at high laser intensities: importance of resonances and relaxation for fragmentation

We investigated dissociative single and double ionization of the metal carbonyls Ni(CO) 4 , Fe(CO) 5 and Cr(CO) 6 in the gas phase by means of laser pulses of different durations (30–110 fs) and wavelengths (0.8 and 1.35 µm) at intensities of 2 × 10 12 –2 × 10 14 W cm −2 . The mass spectra show striking differences: for example, Fe(CO) 5 strongly fragments at 0.8 µm but little at 1.35 µm, whereas for Ni(CO) 4 fragmentation is higher at 1.35 µm than at 0.8 µm; chromium carbonyl shows little fragmentation at both wavelengths. In other cases, fragmentation first decreases and then increases again with intensity. These and other phenomena, also published ones, can readily be understood from long-known principles, namely resonances in the parent ions, sometimes also in the neutral molecules, in particular if relaxations are also taken into account. We emphasize that fragmentation and ionization are two separate processes. We also point out that in the process of dissociative ionization in intense laser radiation, one should generally consider intermediate states, even if there is no one-photon resonance.