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Hamburg 2009 – wissenschaftliches Programm

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A: Fachverband Atomphysik

A 26: Atomic Clusters III

A 26.3: Hauptvortrag

Donnerstag, 5. März 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 xquiv = F2, 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 xquivr collective plasma oscillations dominate, electronic charge migration occurs when the quiver amplitude is small or neglible xquivr. Since xquiv 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.

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