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A: Fachverband Atomphysik

A 26: Atomic Clusters III

A 26.3: Invited Talk

Thursday, March 5, 2009, 15:00–15:30, VMP 6 HS-B

Helium-embedded clusters exposed to intense laser pulses: From “local ignition” to “global cooling” — •Ulf Saalmann — MPI-PKS, Nöthnitzer Str. 38, 01187 Dresden

As well known intense-laser interaction with matter strongly depends on the laser frequency. For atoms and molecules the famous Keldysh parameter is used in order to quantify the transition from tunneling to multi-photon ionization, which occurs for a fixed intensity by increasing the frequency.

In clusters, where a strong laser quickly creates a nano-plasma inside the cluster, we will show that the response is better characterized by the quiver amplitude x_quiv = F/ω², the amplitude of a free electron oscillating in the field of a laser with strength F and frequency ω. This amplitude has to be compared to the cluster radius r. Whereas for large amplitudes x_quiv≥r collective plasma oscillations dominate, electronic charge migration occurs when the quiver amplitude is small or neglible x_quiv≪r. Since x_quiv shrinks quadratically with ω, samples exposed to free-electron laser radiation (either UV or X-ray) will, despite the high field strengths available, show charge migration, as has recently been seen in experiments at FLASH.

We present microscopic calculations for clusters embedded in helium nano-droplets where both mechanism — collective excitations and charge migration — can be clearly identified. They have profound consequences like igniting a helium droplet with a handful of xenon atoms or slowing down the Coulomb explosion of a highly-charged sample.