Experiments demonstrating the role of cluster formation on multiphoton-induced x-ray emission and the scaling of this phenomenon into the kilovolt range have been performed on Kr. For the Kr M shell, augmentation of ${\mathrm{Kr}}_{\mathit{n}}$ formation leads to a large increase in ${\mathrm{Kr}}^{9+}$ (4p→3d) emission (∼100 Å) and the appearance of a strong band at ∼90 Å. The observation of L-shell transitions (∼5–7.5 Å) demonstrates the scaling of this phenomenon into the kilovolt region and leads to the conclusion that the interaction produces direct inner-shell excitation with the emission of prompt x rays.

• Received 28 December 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.1810