Response of matter in gas phase to VUV- and X-FEL light

Ulf Saalmann, Ionut Georgescu, Christian Gnodtke, Jan M Rost

A new generation of light sources, X-ray Free Electron Lasers (X-FEL) will create light with properties not available before to mankind. How does matter respond when intracting with this light?

High intensity, high frequency light pulses of 100 fs length will create a novel dynamical response of matter which we will study in the next years working ahead of experiments planned at the X-FEL under construction at DESY in Hamburg. As a first step, we have investigated the response of Argon clusters to X-FEL photons of 500 eV energy. Surprisingly, the absorption of energy in the cluster is less per atom than in an isolated atom [1]. This is in sharp contrast to clusters interacting with photons of a few eV energy, see below.

On the other hand, soft VUV-radiation couples very efficiently via inverse Bremstrahlung to plasma electrons, generated in the cluster. This mechanism explains the large amount of energy absorbed by rare gas clusters in the first VUV-FEL experiments on clusters at DESY in Hamburg [2]. For an overview of the mechanism of laser-matter coupling at different frequencies, see [3]. Photoionization into a plasma of already exisiting electrons is in principle a many-body process and needs to be formulated carefully [4]. We propose to monitor it experimentally with attosecond pulses, see below. Present efforts include efficient ways to image time-resolved cluster dynamics with short VUV and X-ray laser pulses. To this end we study rare gas clusters embedded in helium droplets.

Key publication:
[1] Ulf Saalmann and Jan M Rost, Phys. Rev. Lett. 89, 143401 (2002).
[2] Christian Siedschlag and Jan M Rost, Phys. Rev. Lett. 93, 043402 (2004).
[3] U Saalmann, Ch Siedschlag and J M Rost, TopicalReview, J. Phys. B 39, R39 (2006).
[4] I Georgescu, U Saalmann and J M Rost, submitted (2007).

Read more  -  contact: Ulf Saalmann us[at]mpipks-dresden.mpg(dot)de

Dynamics of clusters in time-dependent fields

Ulf Saalmann, Ranaul Islam, Alexey Mikaberidze,
Jan M Rost

Experiments by several groups have demonstrated that surprisingly much energy per electron can be absorbed from the laser pulse into an atomic or metal cluster. This leads to effects (coherent X-ray radiation, fast and/or highly charged fragments) which are very interesting for applications as well as for the general understanding of microscopic dynamics.

We have developed a combined classical/quantum mechanical approach which describes the motion of the atoms and the ionization dynamics of individual electron levels in real time for more than 10,000 atoms and up to 100,000 electrons.

- We could show that the mechanism of energy absorption is similar to that in small molecules known as "enhanced ionization" [1].

- For clusters of intermediate size, energy absorption proceeds via a delocalized electron cloud which behaves like a driven damped harmonic oscillator [2]. For a review of this work, see [3]. Present efforts concentrate on working out ways to access the intricate time-dependent and dissipative dynamics in a cluster before energy resolved observables can be measured. To this end we have proposed two different pump-probe scenarios, using VUV-radiation as a probe and conventional IR-pulses [4] or (future) attosecond pulses [5] as a probe. With the latter it should become possible for the first time to observe the time-dependent charging of a cluster during the VUV pump pulse.

Key publications:
[1] Christian Siedschlag and Jan M Rost, Phys. Rev. Lett. 89, 143401 (2002).
[2] Ulf Saalmann Jan M Rost, Phys. Rev. Lett. 91, 223401 (2003).
[3] U Saalmann, Ch Siedschlag and J M Rost, TopicalReview, J. Phys. B 39, R39 (2006).
[4] Ch Siedschlag and J M Rost, TopicalReview, Phys. Rev. A 71 031401(R) (2005).
[5] I Georgescu, U Saalmann and J M Rost, submitted (2007).

Read more  -  contact: Ulf Saalmann

Fullerenes interacting with photons

Paula Riviere, Olaf Uhdem, Jan M Rost

external collaboration:
Himadri Chakraborty (Maryville, USA),
Lamine Madjet (FU Berlin)
Uwe Becker & group (FHI Berlin)
A. Müller & group (Giessen), R. Phaneuf (Reno, USA)
N Berrah (Kalamazou, USA)

"Free electrons cannot absorb light". This well known fact makes it possible to image delocalized electron clouds through single photon absorption in clusters and big molecules.

We have investigated the possibility to determine size and shape of the delocalized electron cloud in metal clusters and C60, as well as endohydrals [1]. In another collaboration with experimental colleagues we have identified a collective excitation in fullerenes beyond the giant dipole resonance. It has the character of a volume plasmon and its excitation by linearly polarized light is only possible through the special geometry of C60 [2]. Present projects include (i) double photo detachment of C60-, (ii) manipulation of caged species in fullerenes by laser light, and (iii) attosecond processes in fullerenes.

Key publications:
[1] A. Rüdel, R. Hentges, U. Becker, H. S. Chakraborty, M. E. Madjet and J. M. Rost, Phys. Rev. Lett. 89, 125503 (2002).
[2] S W Scully, E D Emmons, M F Gharaibeh, R A Phaneuf, A L D Kilcoyne, A S Schlachter, S Schippers, A Müller, H S Chakraborty, M E Madjet, and J M Rost, Phys. Rev. Lett. 94, 065503 (2005).

Read more  -  contact: Jan M Rost

Dynamics in ultracold gases

Cenap Ates, Ivan Liu, Andrei Lyubonko, Jan M Rost

external collaboration:
Thomas Pohl (ITAMP, Harvard)
Thomas Pattard (APS, Ridge, USA)

Recently, it has become possible to cool atomic or molecular gases to ultralow temperatures in the nano-Kelvin range. These advances in experimental techniques have helped to create experimentally extreme states of matter such as atomic Bose-Einstein condensates or a lattice of optically arranged atoms. The new branch of ultracold atomic physics is emerging with a wealth of experimental information which calls for interpretation and explanation. In this project, we are in particular concerned with the theoretical description of ultracold Rydberg gases.

We have developed a hybrid-molecular dynamics description which allows one to follow the dynamics of ions and electrons for a time long enough to determine the degree of correlation which develops. As a spectacular result we predict the formation of ionic crystals during the free expansion of the two-component plasma if the ions are laser cooled during the expansion [1]. Moreover, after photoionization of the atomic cloud, the plasma created exhibits interesting non-equilibrium dynamics [2].
A second topic are blockade effects in ultrcold gases. We have formulated a statistical model which can handle many-body effects classically and describes recent experiments well. Thereby, we have found an antiblockade effect in an ordererd gas [3]. Prsesent interest includes long range correlations of ultracold Rydberg atoms in BECs. Finally, we are interesting in bound structures which may form out of an ultracold gas, such as long range diatomic or polyatomic molecules [4].

Key publications:
[1] T Pohl, T Pattard and Jan M Rost, Phys. Rev. Lett. 92, 155003 (2004)
[2] T Pohl, T Pattard and Jan M Rost, Phys. Rev. Lett. 94, 205003 (2005)
[3] C Ates, T Pohl, T Pattard and J M Rost, Phys. Rev. Lett. 98, 023002 (2007)
[4] I C H Liu and J M Rost, EPJD 40, 65 (2006)

Read more - contact: Cenap Ates

Multiple excitation and fragmentation of atoms

Peije Wang, Ulrich Kleiman, Jan M Rost

external collaboration:
Agapi Emmanouilidou (Eugene, USA),
Ivan Yastremski
Horst Schmidt-Böcking, Reinhard Dörner & group (Frankfurt)

The "classic" topic in our group, tackled with timely classical and semiclassical methods.

Current projects include three- and four- electron processes such as the triple-photoionization of Lithium. We have found a classification scheme in terms of electron-electron collisions for triple ionizing events which holds promise to be generally valid also for more than three electrons. The collsisions happen on an attosecond time scale and may be experimentally accessible in the future [2]. This work started out from double ionization of helium described classically, based on separate formulations of shake-off and knock-out for double photoionization [1]. Further projects include the search for the transition to chaos with increasing double excitation in two-electron atoms [3] and the exploration of the limits of classical microscopic descriptions through comparison with exact quantum results.

Key publications:
[1] Tobias Schneider, Peter L Chocian, Jan M Rost, Phys. Rev. Lett. 89, 073002 (2002). 
[2] A Emmanouilidou and J M Rost, PRA 75, 022712, (2007).
[3] A Czasch, M Schöffler, M. Hattass, S Schöler, T Jahnke, Th Weber, A Staudte, J Titze, C Wimmer, S Kammer, M Weckenbrock, S Voss, R E Grisenti, O Jagutzki, L Ph Schmidt, H Schmidt-Böcking, R. Dörner, J M Rost, T Schneider, Chien-Nan Liu, I Bray, A S Kheifets, and K Bartschat, Phys. Rev. Lett. 95, 243003 (2005).

read more - contact: Jan M Rost

Quantum systems under the influence of noise

Anatole Kenfack, Kamal Singh, Jayendra Bandyopadhyay, Jan M Rost

external collaboration:
Frank Grossmann, TU Dresden

In many applications which use classical mechanics to approximate quantum dynamics in large systems one needs a transcription of the initial quantum state into classical phase space. On the other hand, quantum systems with many degrees of freedom may eventually behave classically. To understand this transition better, we simulate the "environment" of many degrees of freedom by noise which influences a small quantum system with few degrees of freedom.
We have investigated and compared classical and quantum vibrational dynamics of a diatomic polar molecule exposed to an external noise source by solving the stochastic Schrödinger equation for the quantum case and the Langevin equation in the classical case [1]. Present projects include the investigation of the full (electronic and nuclear) dynamics of H2 under noise, as well as atomic strong laser field phenomena under additional noise. For the latter we found strong enancement of ionization for a proper amplitude of the noise [2]. While this can be easily understood in the frequency domain, it may open new possibilities of control in systems where resonant frequencies are not known.

Key publications:
[1] A Kenfack and J M Rost, J. Chem. Phys. 123, 204322 (2005)
[2] K P Singh and J M Rost, PRL 98, 160201 (2007).

Read more - contact: Anatole Kenfack, Kamal Singh