Strong-field ionization and dissociation studies of small early transition metal oxide clusters

We present the results of a systematic experimental approach utilizing time-of-flight mass spectrometry (TOF-MS) to investigate changes in maximum ionization states as the result of femtosecond pulse ionization. Strong-field gas-phase ionization experiments were performed on well characterized distributions of small early transition metal oxide (M_xO_y) clusters, where M = Ti, V, Cr, Nb, or Ta, created as a molecular beam via supersonic expansion. Utilizing high resolution TOF-MS enabled the observation of highly charged ions resulting from ionization and subsequent fragmentation via Coulomb explosion. Maximum charge states for each cluster distribution are reported as observed under various ionization conditions. Evidence for direct correlation with previously reported ionization energies of the component species is presented. Regardless of metal composition, it is observed that every cluster distribution gains approximately the same amount of energy from the external field. The extreme ionization of the targeted clusters produced ions of Ti¹⁰⁺, V⁹⁺, Cr⁸⁺, Nb¹¹⁺, Ta ¹¹⁺, and O⁶⁺, requiring up to 216 eV of energy to create these maximum charge states via sequential ionization. Several enhanced ionization models are discussed with relation to our experimental data. Systematic investigations on the influence of cluster size and ionization laser pulse width indicate enhanced ionization contributions from the ionization ignition and charge-resonance enhanced ionization mechanisms.