Augmented collisional ionization via excited states in XUV cluster interactions

The impact of atomic excited states is investigated via a detailed model of laser-cluster interactions, which is applied to rare gas clusters in intense femtosecond pulses in the extreme ultraviolet (XUV). We investigate a two-step ionization process via excited atomic states, which allows the creation of high charge states and rapid dissemination of laser energy. Through simulations corresponding to recent argon cluster-XUV experiments, this two-step process is shown to play a primary role; this is consistent with our hypothesis that XUV-cluster interactions provide a unique window into the role of excited states due to the relative lack of photoionization and laser field-driven phenomena. Our analysis suggests that excited states may be important for material interactions with intense radiation in a variety of wavelength regimes, including potential implications for proposed studies of single-molecule imaging with intense x-rays.