Strong-field ionization of small niobium and tantalum clusters

Strong-field ionization and concomitant Coulomb explosion studies are performed on small (<30 atoms) homogeneous transition metal clusters composed of niobium or tantalum. Enhanced ionization of the clusters results in the formation of charge states reaching M ¹¹⁺ [M = Nb, Ta] under irradiation with 100 fs pulses of 624 nm light at intensities of approximately 1 PW/cm ². The same maximum charge states were created upon expansion of the pulse width to 350 fs with consistent pulse energy. These results are compared to previous studies involving metal oxide and carbide clusters of similar size, yielding identical ion states. This suggests that the enhanced ionization behavior of metal-containing clusters is driven by the delocalized electrons on the metal species.