Laser-irradiated heteronuclear clusters: Resonant ignition through nanoplasma elongation

Summary form only given. By doping a helium nanodroplet with a handful of heavier xenon atoms it can be turned into a highly reactive object which absorbs infrared light very effectively. This finding is unexpected since the pristine droplet cannot be ionized at all with laser light of 780 nm wavelength at an intensity of around 10¹⁴ W/cm² which we will apply. Yet, with a few xenon atoms inside, all electrons from the helium atoms are removed so that the entire droplet containing as many as 10⁵ helium atoms turns into a nanoplasma. Microscopic calculations reveal a two-step mechanism to be responsible for the dramatic change: avalanche-like ionization of the helium atoms on a femtosecond time scale, driven by field ionization due to the quickly charged xenon core is followed by resonant absorption enabled by an unusual cigar-shaped nanoplasma which elongates along the laser polarization direction inside the droplet. In contrast to the well known resonant absorption during the Coulomb explosion of both homogeneous and composite clusters, the resonance here occurs on an electronic time scale. For the above mechanism to work, it is important that the doped xenon atoms form a small cluster close to the droplet center. The effect takes place also for circularly polarized laser pulses, but in this case an oblate, pancake-like nanoplasma is formed. We find similar effects in argon clusters doped with water. This finding is promising in view of the experimentally observed strong enhancement in X-ray yields from water-doped argon clusters as compared to pure argon clusters.