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Olga Smirnova receives the Karl Scheel Prize 20105th. February 2010 |
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Molecules in real-time – how hydrogen bonds determine structure and function.5th January 2010
Hydrogen bonds are weak chemical bonds and represent a fundamental interaction
in Nature. They determine the structure of biomolecules such as deoxyribonucleic
acid (DNA), the basic carrier of genetic information in cells. On the other
hand, they undergo fluctuations due to their weak binding forces. In water,
this leads to extremely fast changes in the arrangement of molecules including
the breaking and reformation of hydrogen bonds. Although hydrogen bonds have
been studied for a long time, their structural dynamics which occur in the femtosecond
time domain (1 femtosecond = 10-15 s = one millionth of a billionth of a second),
are understood only in part. Biographical information on Thomas Elsaesser is available at http://staff.mbi-berlin.de/elsasser/ |
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| Highlights from the year 2009 | ||
Light pressure – the route to efficient laser ion acceleration9th December 2009
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Carbonic acid now measured in liquid water
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Acceleration of neutral atoms in strong short-pulse laser fields
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Miniature particle accelerator: Micro-water droplets as a source for laser driven ion accelerationThe cover of Physical Review Letters (Vol. 103 Issue 13) shows a result of a recent MBI publication. In the underlying work (T.Sokollik et al., PRL 103, 135003 (2009)) ion acceleration from isolated spherical targets was investigated by proton imaging for the first time. Already in a previous work (S. Ter Avetisyan et al. PRL 2006), scientists from the Max-Born-Institute in Berlin found that laser irradiated water (or heavy water) droplets can generate a quasi-monoenergetic proton (or deuteron) beam. On the base of simulations they could argue that this, besides additional premises might be connected to a spatially asymmetric field structure which favours a directional emission. Using proton imaging now, the evidence for an advantageous field structure was found which leads to a directional ion beam emission using a micro-sphere target which is a versatile target system. The great advantage of such a system is the MHz repetition rate of droplet generation with a liquid jet. On the other hand, the use of evaporating liquids seems to have a drawback. In case of such targets which evaporate in vacuum, the presence of an ambient plasma counteracts the energy transfer between laser and ion beam. Current investigations aim to avoid these disadvantages of liquids and to explore further fundamental processes of isolated targets. |
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High harmonic interferometry of multi-electron dynamics in moleculesHigh harmonic emission occurs when an electron, liberated from a molecule by an incident intense laser field, gains energy from the field and
recombines with the parent molecular ion. The emission provides a snapshot of the structure and dynamics of the recombining system, encoded in the
amplitudes, phases and polarization of the harmonic light. Here we show with CO2 molecules that high harmonic interferometry can retrieve
this structural and dynamic information: by measuring the phases and amplitudes of the harmonic emission, we reveal 'fingerprints' of
multiple molecular orbitals participating in the process and decode the underlying attosecond multi-electron dynamics, including the
dynamics of electron rearrangement upon ionization. These findings establish high harmonic interferometry as an effective approach to
resolving multi-electron dynamics with sub-Ångström spatial resolution arising from the de Broglie wavelength of the recombining electron,
and attosecond temporal resolution arising from the timescale of the recombination event. |
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Multispot writing in fused glassNature photonics selected recent work from the ongoing collabortion between MBI and Université Jean Monnet at Saint Etienne, France, as one of their Research Highlights. We quote: "Multispot writing in fused glass, Opt. Express 17, 3531–3542 (2009). Owing to its highly deterministic and nonlinear absorption process, infrared femtosecond laser writing offers the means to create buried, localized structural modifications in transparent materials. By moving the sample with respect to the laser's focal point, three-dimensional structures can be inscribed. However, the fabrication of complex structures often involves long processing times. Cyril Mauclair and co-workers from France and Germany have now demonstrated that the problem of speed can be solved by parallel photoinscription that uses multiple laser spots with reconfigurable patterns. The trick is to use a periodical binary phase mask to spatially modulate the wavefront of the laser beam. By varying the period (cycling frequency) of the binary phase, the team show that a simple grating phase mask and therefore dynamic double-spot operation can be achieved. The team use a liquid-crystal spatial light modulator, addressed optically, to create the binary phase mask. A 800-nm Ti:sapphire laser emitting 150-fs pulses at a repetition rate of 10 kHz and with a power of 30 mW is used for the process. By controlling the motion of the sample, the team succeeded in manufacturing three-dimensional light dividers and fabricating wavelength-division demultiplexing devices in fused silica. They are confident that with sufficient energy, more machining foci can be used." |
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Hot Electrons in Carbon – Graphite behaves like a semiconductorMarkus Breusing, Claus Ropers und Thomas Elsaesser, three scientists from the Max-Born-Institute in Berlin, have now investigated
the behavior of electrons in thin graphite films in real time. As they now report in Physical Review Letters (Volume 102, 086809/1-4 (2009)),
they recorded the dynamics of electrons with an unprecedented temporal resolution of only 10 femtoseconds (one femtosecond is a millionth of
a billionth of a second). Electrons were excited to high energy states with ultrashort laser pulses, and their return to equilibrium
was observed. The individual steps of this process are temporally resolved, and the momentary distribution of electrons in the material
is identified. Within 30 femtoseconds, electrons form a hot gas with temperatures of 2500 °C, which cools down to about 200 °C in only 500
femtoseconds. The energy dissipated in this process is transferred to the crystal lattice. After this process, the electrons slowly return
to their initial states. For the first time, the study shows conclusively that, on ultrashort time scales, graphite behaves like
a semiconductor, such as silicon or gallium arsenide, and not like a metal. |
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Ionisation dynamics in the light of elliptically polarised femtosecond laser pulses
Scientists from the MBI (I.V. Hertel, I. Shchatsinin, T. Laarmann, N. Zhavoronkov, H.-H. Ritze, and C. P. Schulz) have shown, that elliptically polarized, ultrashort light pulses allow a particularly clear view into the dynamics of ionisation processes in intense laser fields. They found e.g. convincing evidence for a so called „doorway state“ in the football molecule C60 (Buckminsterfullerene), which is populated in a first step prior to ejecting an electron from the molecule. Subsequently the molecule is so strongly deformed, that many other electrons can participate in the process and several of them can finally leave the system – on a time scale of a few femtoseconds. The work, recently published in the renowned Journal Physical Review Letters (Phys. Rev. Lett. 102, 023003 (2009)) has also been included in the Virtual Journals on "Ultrafast Science" and "Nanoscale Science & Technology". |
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Dr. Olga Smirnova, head of MBI's 
The force experienced by a charged particle in an oscillating electric field is proportional to the cycle-averaged intensity gradient.
Extremely strong kinematic forces are now observed on neutral atoms in short-pulse laser fields; the ponderomotive force on electrons
is identified as the driving mechanism, leading to probably the highest observed acceleration on neutral atoms in an external field to date.
