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Highlights 2010
 
Highlights from 2010
 

Holography with electrons

16th December 2010

Experimental mesurements of electrons, wich was ionised from 7 Mikrometer FELICE Laser from Xenonatoms: velocity distribution along of (horizontal) and vertical to polarisation direction The principle of holography was discovered in 1947 by the Hungarian scientist Dennis Gábor, in connection with attempts to improve the resolution of electron microscopes. The experimental realization of the concept of holography had to wait, however, until the mid-60s. Holograms were then made using newly-discovered laser light sources, rather than with electrons. Physicists from the Max Born Institute in Berlin have now returned to the use of electrons in holography. A special element in their approach is that the electrons that image the object are made from the object itself using a strong laser. A report is published in this week´s issue of Science.
Holography, as it is encountered in everyday life, uses coherent light, that is, a source of light where all the emitted light waves march in step. This light wave is divided into two parts, a reference wave and an object wave. The reference wave directly falls onto a two-dimensional detector, for example a photographic plate. The object wave interacts with and scatters off the object, and is then also detected. The superposition of both waves on the detector creates interference patterns, in which the shape of the object is encoded.
What Gábor couldn´t do, to construct a source of coherent electrons, is commonplace in experiments with intense laser fields. With intense, ultra-short laser fields, coherent electrons can readily be extracted from atoms and molecules. These electrons are the basis for the new holography experiment, which was carried out using Xe atoms. Marc Vrakking describes what happens: ´In our experiment, the strong laser field rips electrons from the Xe atoms and accelerates them, before turning them around. It is then as if one takes a catapult and shoots an electron at the ion that was left behind. The laser creates the perfect electron source for a holographic experiment´.
Some of the electrons re-combine with the ion, and produce extreme ultra-violet (XUV) light, thereby producing the attosecond pulses that are the basis for the new attosecond science program that is under development at MBI. Most electrons pass the ion and form the reference wave in the holographic experiment. Yet other electrons scatter off the ion, and form the object wave. On a two-dimensional detector the scientists could observe holographic interference patterns caused by the interaction of the object wave with the Coulomb potential of the ion.
In order to successfully carry out the experiments, certain conditions had to be met. In order to create the conditions for holography, the electron source had to be put as far away as possible from the ion, ensuring that the reference wave was only minimally influenced by the ion. The experiments were therefore carried out in the Netherlands, making use of the mid-infrared free electron laser FELICE, in a collaboration that encompassed – among others – the FOM Institutes AMOLF and Rijnhuizen. At FELICE, the Xe atoms where ionized using laser light with a 7 ?m wavelength, creating ideal conditions for the observation of a hologram.
The ionization process produces the electrons over a finite time interval of a few femtoseconds. Theoretical calculations under the guidance of MBI Junior Groupleader Olga Smirnova show that the time dependence of the ionization process is encoded in the holograms, as well as possible changes in the ion between the time that the ionization occurs and the time that the object wave interacts with the ion. This suggests a big future promise for the new technique. As Vrakking states: "So far, we have demonstrated that holograms can be produced in experiments with intense lasers. In the future we have to learn how to extract all the information that is contained in the holograms. This may lead to novel methods to study attosecond time-scale electron dynamics, as well as novel methods to study time-dependent structural changes in molecules".
The following highlight (also from 16th December 2010) stands in thematical connection with this.

More information:

DOI:10.1126/science.1198450
Original article on demand

Contact: Prof. Marc Vrakking, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Tel.: 030-6392 1200, Mobil: 0151-57153446, further e-mail

 
     
 

Understanding the "Ionization Surprise"

16th December 2010

In 2009 the Journal Nature Physics called it the "Ionization Surprise". Where it had been commonly thought that the ionization of atoms by strong laser fields was meanwhile well-understood, novel experiments where rare gas atoms were ionized using relatively long (few-µm) wavelength laser light suddenly revealed an unexpected and universal low-energy feature that defied explanation. In this week´s issue of Physical Review Letters, scientists from the University of Rostock, the Max-Planck Institut für Kernphysik in Heidelberg and the Max-Born Institute provide an explanation.
Ionization of atoms by strong laser fields plays an important role in today´s ultrafast laser laboratories. It is at the basis of important techniques such as high-harmonic generation, which allows the generation of attosecond (1 as = 10-18 s) laser pulses, and furthermore allows the development of tomographic methods that make it possible to observe ultrafast electronic and atomic movements on the attosecond to few femtosecond (1 fs = 10-15 s) timescale. Theoretical methods for describing strong laser field ionization were already developed a few decades ago. They are commonly based on the so-called “strong-field approximation” (SFA), which argues that after ionization the motion of the ionized electrons is largely determined by the electric field of the ionizing laser, and hardly by the Coulomb force that the electron and the ion left behind exert on each other.
For several decades the strong-field approximation has served scientists well and has allowed to understand many observations that were experimentally made in connection of with the strong field laser ionization. That is to say, until now. In a remarkable paper last year, scientists from the US and Germany reported the observation of a new phenomenon in strong-field laser ionization, namely a very pronounced peak at low energies in the photoelectron kinetic energy distribution, that contained as many as 50% of the emitted photoelectrons. Remarkably, its physical origin could not be identified.
In the new paper, the Rostock, Heidelberg and MBI scientists argue that it is the failure to include the Coulomb attraction between the departing electron and the ion left behind that is at the root of the low energy feature. They developed a novel theoretical description of strong-field ionization process, which in its initial stages mimics the traditional SFA approach, but then switches to calculating trajectories that the electrons follow in the combined Coulomb + laser field. This approach convincingly reproduces the low energy feature, and shows that it is caused by electrons that are pushed back-and-forth by the oscillatory laser field. In this process the electrons are brought into close proximity to the ion, which strongly disturbs the electron orbit, leading to a situation where the electron can just barely escape the attraction of the ion.
The 'Coulomb-corrected' SFA formalism based on interfering quantum orbits described above, not only solves the mystery of the “Ionization Surprise” but was also instrumental in related work on the appearance of holographic structures in strong-field ionization, which appears in Science Express this week.

More information:

10.1103/PhysRevLett.105.253002

Contact: Prof. Marc Vrakking, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Tel.: 030-6392 1200, Mobil: 0151-57153446, further e-mail
Prof. Dieter Bauer, Universität Rostock, Institut für Physik, Tel.: 0381 498- 6940

 
     
 

Innovation Prize Berlin Brandenburg for femtosecond x-ray plasma source mapping molecular structures in real-time

10th December 2010

The company IfG – Institute for Scientific Instruments GmbH - and the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI) jointly received the Innovation Prize 2010 for their development of a laser-driven hard x-ray plasma source. The compact new device generates – driven by an amplified femtosecond laser – extremely short x-ray pulses of 100 fs duration (1 fs = 1010-15s = 1 billionth of a millionth of a second) at kilohertz repetition rates. The x-ray pulses are synchronized with optical pulses from the laser system and allow for x-ray diffraction experiments with very high time resolution. During the award ceremony, Prof. Norbert Langhoff, IfG, and Prof. Thomas Elsaesser emphasized that 'this success is the result of an intense exchange and a close cooperation of scientists and engineers on the campus Berlin-Adlershof'. video
Basic properties and function of systems in molecular biology, nano- and/or sensor technology are connected with motions of atoms and molecules. Such motions occur on extremely short length and time scales. Understanding such processes requires their visualization in a time-resolved way by taking a sequence of 'snapshots' with an ultrafast 'camera'. This dream becomes reality with the new femtosecond x-ray plasma source enabling the direct imaging of ultrafast atomic and molecular motions with a 100 fs time resolution.
In the x-ray source, a hot plasma is generated by exciting a thin metal tape, e.g., made from copper, with intense femtosecond pulses from a commercial laser system. Electrons in the plasma are strongly accelerated by the high electric field of the laser pulses. They penetrate into the metal target and generate characteristic hard x-rays, similar to electrons in a conventional x-ray tube. The acceleration of electrons and, thus, the x-ray generation are limited in time to the 50 fs duration of the driving laser pulse, resulting in an extremely short x-ray flash. X-ray snapshots are produced by initiating the structure changing process in the sample with an optical pulse and diffracting the x-ray pulse from the excited sample. Recording such images for different delays of the x-ray pulse after the optical pulse gives a sequence of snapshots, the 'x-ray movie'.
The new x-ray source is unrivaled as a commercial device and represents a key prerequisite for the leading role of MBI in time-resolved structure research. Recently, MBI demonstrated the first x-ray powder diffraction study with femtosecond time resolution, a result that has been recognized internationally as a major breakthrough in the field. The present femtosecond x-ray research at MBI is supported through an Advanced Grant for Pioneering Frontier Research of the European Research Council. New areas of application of this technology will be materials research, nanotechnology, chemistry and pharmacy. IfG has built and sold 4 x-ray sources and is in negotiations with another 4 customers. The generated turnover has a net amount of 2 Million Euros.

Photo: PUBLICIS Berlin GmbH More information:

Contact: Prof. Dr. Thomas Elsässer, Tel +4930 6392 1401
Prof. Dr. Norbert Langhoff, IfG, Tel +4930 6392 6500

 
     
 

Even higher performance for high-power lasers

14th October 2010

Multi-mirror set-up of a laser amplifier at MBI Either with enormous pulse energy or with high repetition rate – this might describe the present state of the art of laser technology. Scientists of the Max Born Institute (MBI) in Berlin, in close cooperation with diode laser specialists of the Ferdinand-Braun-Institute (FBH), plan to combine the two properties. The goal are lasers which reach high single-pulse power at 100 shots per second or more. For the development of such novel lasers the scientists have obtained a grant of 3 million Euros from the European Union (European Fund for Regional Development - EFRD), the total volume of the project being 6 million Euros.
High-intensity lasers are a relatively young product of physics research. They are able to emit single light pulses of inconceivable power – much more than the worldwide capacity of all power plants together. For this purpose, the energy of a single pulse is concentrated in a period much shorter than a millionth of a millionth of a second. Due to their high single-pulse power these lasers are revolutionizing many fields of science, technology and medicine. They are being used for producing new states of matter, for ultra-precise materials processing or for the generation of particle or photon radiation with unprecedented properties. Within the next year even nuclear fusion by high-intensity lasers is expected to be demonstrated – maybe one step towards a relatively clean and virtually inexhaustible source of energy.
A technological void, however, exists in almost all of these lasers: the repetition rate of their light pulses is limited to ten shots per second (10 Hertz), often even considerably less. "Even though the single pulses are enormously powerful, the total power, i.e. the average power of conventional high-intensity lasers is little more than 10 watts. This corresponds to an energy-saving lamp", says MBI Director Professor Wolfgang Sandner, whose division is leading the new project.
The new MBI laser development leaps into this gap. For several years now, MBI has acquired a world leading position in the field of picosecond lasers of high pulse energy and repetition rate, i.e. high average power. Due to an innovative highly efficient laser cooling, these systems provide more than 100 shots per second. "In order to further boost the average power of these lasers we intend to increase the energy of the single pulses, first to a few joules, then probably to significantly more", announces MBI project coordinator Dr. Ingo Will. This kind of lasers would have an average power in the kilowatt range with a picosecond pulse duration and very high single-pulse energy which has not yet been achieved by any other laser.
High-power lasers of this kind are urgently needed as a technological basis for European large-scale projects such as ELI. The acronym stands for Extreme Light Infrastructure, the future most powerful laser for basic science in the world. First studies for the front-end of a 10 petawatt laser as demonstrator for the ELI project are already completed. The pump laser for this front-end is now being built at MBI and will be delivered to the Institut d'Optique in Palaiseau, France shortly.
The underlying concept of the new laser is a diode-pumped solid-state laser. The so-called thin disk laser architecture is one of the most promising laser architectures which has been adopted and further developed by MBI’s cooperating partners IfSW Stuttgart, DLR Stuttgart und TRUMPF Lasertechnik GmbH with financial support from the Berlin technology promotion action PROFIT and from the Leibniz programme SAW. The project partner FBH will provide novel pump diodes for the disk laser. Funding agency for the EFRD project is the Senatsverwaltung für Bildung, Wissenschaft und Forschung of the state of Berlin.
As a first milestone, the MBI researchers plan to pump a worldwide unique compact X-ray laser in their own institute. With a wavelength of 13 nanometres and a repetition rate of 100 Hertz this laser will generate coherent X-radiation in the laboratory which so far is possible only with large free electron lasers such as FLASH in Hamburg. In the long run, thin disk lasers can be used as an energy source for the next generation of high-intensity lasers. In addition MBI plans to set up a high-power attosecond source in cooperation with Prof. Marc Vrakking, the recently appointed director of MBI.

More information:

Contact: Dr. Ingo Will, Tel +4930 6392 1320
see also Pressemitteilung Forschungsverbund

 
     
 

Elementary particles starring a dance movie

16th August 2010

Crystal structure of ammonium sulfate Scientists of the Max-Born-Institute in Berlin (Germany) measure directly the spatial positions of electrons and protons during a chemical reaction using ultrashort x-ray flashes
A chemical reaction generates new compounds out of one or more initial species. On a molecular level, the spatial arrangement of electrons and nuclei changes. While the structure of the initial and the product molecules can be measured routinely, the transient structures and molecular motions during a reaction have remained unknown in most cases. This knowledge, however, is a key element for the exact understanding of the reaction.
The ultimate dream is a "reaction microscope" which allows for an in situ imaging of the molecules during a reaction. The technological challenges of such an ultrafast "cinema" were mastered only recently. Using ultrashort x-ray pulses, researchers at the Max-Born-Institute in Berlin, Germany, now made a "molecular movie" of a chemical reaction with a resolution on atomic length (10-10 meters) and time scales (10-13 seconds).
Michael Woerner, Flavio Zamponi, Zunaira Ansari, Jens Dreyer, Benjamin Freyer, Mirabelle Premont-Schwarz und Thomas Elsaesser report on a direct time-resolved observation of a chemical reaction in ammonium sulfate crystals [(NH4)2SO4] in the most recent issue of The Journal of Chemical Physics. Using an advanced femtosecond laser system, they generate a blue pulse of 50 femtosecond duration (1 fs = 10-15 seconds) which initiated the chemical reaction. After a very short period they probe the structure of the excited material with high spatial resolution using a synchronized x-ray flash with only 100 fs duration. The x-ray pulse is diffracted off a powder made of small crystallites (the so called Debye-Scherrer method. Measuring simultaneously many different x-ray reflections the physicists could reconstruct the transient distances of atomic lattice planes and in turn the three-dimensional distribution of electronic charge within the crystal. Taking x-ray snap shots at various delay times after triggering the reaction, they created a molecular movie according to the well known stroboscope effect.
Surprisingly, they observed a reversible chemical reaction which is fundamentally different from the already known, slow thermal phase transitions of ammonium sulfate. First, the blue flash caused a release of both a proton (positive charge) from the ammonium ion (NH4)+ and an electron (negative charge) from the sulfate ion (SO4)-. The two elementary particles merged to a hydrogen atom which jumps back and forth between two distant spatial positions. The corresponding motion is illustrated in the attached movie which was derived from the x-ray snapshots. The circles indicate the initial positions of the protons. The red blots show the motion of the hydrogen atoms after the chemical reaction.
Femtosecond x-ray powder diffraction demonstrated here for the first time can be applied to many other systems, for instance for investigating molecular magnets or for monitoring electron motions in (bio)molecular light harvesting complexes used in solar cells.

More information:

Search and Discovery article of Johanna Miller in Physics Today
Journal of Chemical Physics: Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction
J. Chem. Phys. 133, 064509 (2010); doi:10.1063/1.3469779
Contact: Dr. Michael Wörner, Tel +4930 63921470
s. auch Pressemitteilung Forschungsverbund

 
     
 

Electrons in motion

11th August 2010

Generation of attosecond light flashes to observe the movements of electrons. The world of atoms and molecules is very different from our everyday experience. We like to think of electrons as little particles. "And that is to some extent a valid picture" explains Marc Vrakking, director at the Max Born Institute in Berlin, "but quantum mechanics also forces us sometimes to consider the wave-nature of electrons". Using this abstract representation of an electron, physicists can explain intriguing phenomena that in the end connect to our simple picture of the electron as a particle.
Because it is not possible to observe the motions of electrons directly, the European research team characterized this motion by a number of separate measurements that completely characterized the wave-nature of the electron, or, as Vrakking likes to call it "the electronic wavepacket". With this, the particle-like motion of the electron is completely defined.
In the experiment, the researchers have exploited the fact that waves can be made to interfere. In many respects their experiment resembled the famous double-slit experiment that Thomas Young performed at the start of the 19th century, where light fell onto a pair of slits and an interference pattern was observed on a screen placed downstream. The way to understand this observation is to view the incident light as a wave, and to view the interference pattern as a consequence of the fact that light passing through one slit may be in- or out-of-phase with light passing through the other. This observation cannot be explained when viewing the incident light as a collection of particles ("fotons").
In order to characterize an electronic wavepacket, the researchers have also used interference, and have exploited the interference between an (“unknown”) electronic wavepacket that they wanted to characterize and a reference ("known") wavepacket, that they produced with an attosecond laser pulse, by ionizing the atom. The properties of this reference wavepacket are fully known because they follow from the properties of the fully-known attosecond pulse. By overlapping and interfering the two wavepackets, and observing the interference pattern that resulted, they could extract all the properties of the unknown wavepacket.
Marc Vrakking explains the method: "Initially the reference wavepacket produced by the attosecond laser has a much higher energy than the unknown wavepacket In order to create the interference pattern, we therefore had to lift the unknown wavepacket to the same energy. We have done this using an infrared laser pulse. By varying the time-delay between the attosecond pulse and the infrared pulse we could acquire a whole series of interference patterns, which allowed to extract all that there is to be known about the unknown wavepacket." A full characterization of the electronic wavepacket follows from a knowledge of the energies, the populations and the relative phases of all the states that contribute to the wavepacket. Once known, the time-dependent motion of the electronic wave can be pictured, and the picture of a particle-like motion re-emerges, bringing us back to our everyday world.

More information:

Physical Review Letters: Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique
http://dx.doi.org/10.1103/PhysRevLett.105.053001
Contact: Prof. Dr. Marc Vrakking, Tel +4930 63921200
see also press releases Forschungsverbund

 
     
 

A look into the Interior of Molecule

10th Juni 2010

Molecular Landscape In order to not only observe, but also really understand a chemical reaction, scientists have to know how electrons move within molecules. Until now it was technically impossible to observe how electrons move within a molecule, because they move so incredibly fast. However, a group of European researchers including scientists from the Max Born Institute has now achieved this goal with the help of attosecond laser pulses.

An attosecond is a billionth of a billionth of a second. During an attosecond, light covers a distance of less than one millionth of a millimeter –equivalent to the distance from one end of a small molecule to the other. Physicists are therefore making great efforts to create attosecond laser pulses: using attosecond laser pulses they can “photograph” the movement of electrons within molecules, just like in a photo shoot.

In the European research team, Prof. Marc Vrakking, Director of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin worked together with groups from – among others - Milan, Amsterdam, Lund (Sweden), Garching, Lyon and Madrid. The physicists examined the hydrogen molecule (H2) – which is nature’s simplest molecule, with just two protons and two electrons. The researchers wanted to find out exactly how ionization takes place in a hydrogen molecule. In this process, one electron is removed from the molecule, and the remaining electron undergoes a rearrangement. Marc Vrakking explains, “In our experiment we were able to show for the first time that with the help of an attosecond laser we really have the ability to observe the movement of electrons in molecules. First we irradiated a hydrogen molecule with an attosecond laser pulse. This led to the removal of an electron from the molecule – the molecule was ionized. In addition, we split the molecule into two parts using an infrared laser beam, just like with a tiny pair of scissors. This allowed us to examine how the charge distributed itself between the two fragments – since one electron is missing, one fragment will be neutral and the other positively charged. We knew where the remaining electron could be found, namely in the neutral part.”

Ever since the 1980s, scientists have been examining molecules and atoms with the help of femtosecond lasers – a femtosecond is one millionth of one billionth of a second, thus a thousand times slower than an attosecond. Using femtosecond lasers, the movement of atoms and molecules can be tracked, but not that of electrons. In 2001, researchers were able for the first time to produce a laser flash with a length of 250 attoseconds. This was followed by considerable efforts on the development of attosecond lasers as well as the control and measurement of the pulses. More recently, scientists are beginning to use the attosecond pulses to address problems in natural science, such as the first molecular application that was published now.

Although the European team’s experiments with attosecond lasers produced some of the results they had hoped for, there was also one surprise in store for the scientists. In order to be able to interpret the measurements even better, they involved a group of theoreticians from the University of Madrid in the project. The Spanish researchers’ work brought completely new insights. Dr. Felipe Morales from Madrid, who is currently working as a postdoctoral fellow at MBI, reports, “We nearly reached the limits of our computer capacity. We spent one and a half million hours of computer time to understand the problem.” The calculations showed that the complexity of the problem was far greater than previously thought. For example, it was shown that the electron that is removed from the molecule by the attosecond laser pulse also plays an important role in the subsequent dynamics of the molecular ion that is left behind. Vrakking describes it as follows, “We have not – as we originally expected - solved the problem. On the contrary, we have merely opened a door. But in fact this makes the entire project much more important and interesting.”

Nature-Paper: Electron localization following attosecond molecular photoionization
http://dx.doi.org/10.1038/nature09084

Contact: Prof. Dr. Marc Vrakking, Tel +4930 63921200
see also press releases Forschungsverbund

 
     
 

World record for shortest controllable time

10th May 2010

ultrashort lightimpuls with stable optical phaseLasers can now generate light pulses down to 100 attoseconds (1 attosecond = 10-18s = one billionth of the billionth of a second) thereby enabling real-time measurements on ultrashort time scales that are inaccessible by any other methods. Scientist at the Max Born Institute for Nonlinear Optics and Short Time Spectroscopy (MBI) in Berlin, Germany have now demonstrated timing control with a residual uncertainty of 12 attoseconds. This constitutes a new world record for the shortest controllable time scale. They reported about in Nature Photonics.
Light is an electromagnetic wave of very high frequency. In the visible domain, a single oscillation of the electric field only takes about 1200-2500 attoseconds. Consequently, an ultrashort laser pulse is comprised of a few of these oscillations. However, pulses from conventional short-pulse laser sources exhibit strong fluctuations of the positions of the field maximum relative to the pulse center. For maximum field strength, the center of the pulse has to coincide with a maximum of the electric field, as shown in Fig. 1 as a red curve. Consequently, methods have been developed to stabilize the position of the field maximum, i.e., the phase of the pulse.
Together with Vienna based laser manufacturer Femtolasers, MBI researchers in the group of Günter Steinmeyer have now developed a new method to control the phase of the pulse outside of the laser. In contrast to previous approaches, no manipulation inside the laser is necessary, which completely eliminates fluctuations of laser power and pulse duration and guarantees a strongly improved long-term stability. Correction of the pulse phase relies on a so-called acousto-optic frequency shifter, which is directly driven by the measured signal. Dr. Steinmeyer is convinced: "This direct correction of the phase dramatically simplifies many experiments in attosecond physics and frequency metrology."
Previously, stabilization of the position of the field maxima was only possible with a precision of about 100 attoseconds (10-16 s, corresponding to 1/20 of the wavelength), which is comparable to the shortest duration of attosecond pulses demonstrated so far. The new method allowed to push this limitation down to 12 attoseconds (1.2 x 10-17 s, 1/200 of the wavelength), which surpasses the atomic unit of time (24 attoseconds) by a factor of two. As the atomic unit of time marks the fastest possible time scale of processes in the outer shells of an atom, the new stabilization method will enable significant progress in the research on the fastest processes in nature.
This success strongly relied on a close collaboration with laser manufacturer Femtolasers who provided a specifically optimized laser for the joint experiment and is currently developing products based on this new method.
Contakt: Dr. habil. Günter Steinmeyer, Tel +4930 63921440
see also press release Forschungsverbund

 
     
 

Negative Mass and High Speed: How Electrons Go Their Own Ways

12th April 2010

Wilhelm Kuehn beim Einjustieren des ExperimentsIsaac Newton found in the 17th century that a force causes a body to accelerate. The inertial mass of the body is the ratio between force and acceleration, thus, given the same force, a light body is accelerated more strongly than a heavy body. A body's mass is positive, meaning that the acceleration is in the same direction as the force. Charged elementary particles as the free electron, which has a mass of only 10-30 = 0. ...(29 zeros !)...1 kilograms, can be accelerated in electric fields to extremely high speeds. If the electric field is small, the motion of electrons in crystals is governed by the same laws. In this regime, the mass of a crystal electron is only a small part of the mass of a free electron.
Wilhelm Kuehn, Michael Woerner, Klaus Reimann and Thomas Elsaesser from the MBI have now demonstrated that crystal electrons in extremely high electric fields exhibit a completely different behavior. Their mass even becomes negative. They report in the latest issue of Physical Review Letters 104, 146602 (2010) that the electron is accelerated within the extremely short time of 100 femtoseconds = 0.000 000 000 000 1 seconds to a speed of 4 million kilometers per hour. Afterwards the electron comes to a stop and even moves backward. This means that the acceleration is in opposite direction to the force, which can only be explained by a negative inertial mass of the electron.

In the experiments, electrons in the semiconductor crystal gallium arsenide are accelerated by an extremely short electrical pulse with a field strength of 30 million Volts per meter. At the same time the speed of the electrons is measured with high precision as a function of time. The duration of the electric pulse is only 300 femtoseconds. This extremely short duration is essential as otherwise the crystal could be damaged.

DThe new results agree with calculations of the Nobel Prize winner Felix Bloch undertook more than 80 years ago. They open up a new regime of charge transport with new possibilities for future microelectronics devices. The observed frequencies are in the terahertz range (1 THz = 1000 GHz = 1012 Hz), about 1000 times higher than the clock rate of the newest PCs.
see also press release Forschungsverbund

 
     
 

Olga Smirnova receives the Karl Scheel Prize 2010

5th. February 2010

Karl Scheel medalDr. Olga Smirnova, head of MBI's Juniorgroup in Theory on "Attosecond multielectron dynamics in molecules", receives the Kar Scheel Preis of the year 2010. The prestigeous prize of the Physikalische Gesellschaft zu Berlin (PGzB) is awarded for outstanding scientific work typically achieved after PhD. It includes a cash award of 5.000 €. More about the prize ...

 
     
 

Molecules in real-time – how hydrogen bonds determine structure and function.

5th January 2010

ercThe European Research Council (ERC) has awarded Prof. Thomas Elsaesser an 'Advanced Grant' of 2.49 Million Euros. The project aims at elucidating extremely fast processes which determine the properties of hydrogen bonds in molecular systems.
Thomas Elsaesser who is with the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin, Germany, is one of the leading researchers in ultrafast science, studying ultrafast processes in condensed matter. His project is devoted to unraveling changes of molecular structures on the length scale of a chemical bond and the ultrashort time scale of molecular motions. This work will cover aspects of physics, chemistry and biology. ERC Advanced Grants allow exceptional established research leaders to pursue frontier research of their choice.

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.
Within the project, novel methods of ultrafast optics in a wavelength range from the far-infrared to hard x-rays will be applied for investigating hydrogen bonds. A key goal consists in measuring molecular structures in real-time by initiating and reading out structure changes with ultrashort light pulses. X-ray pulses of a wavelength comparable to the length of a chemical bond allow for generating a sequence of 'snapshots' of molecular structure. Infrared pulses give insight into local motions of specific molecular groups. In the experiments, the interaction of DNA with its aqueous environment will be studied, i.e., the coupling of water molecules to different functional units of the DNA double helix, the fluctuations of the water shell around DNA, and the role of water for the redistribution and the transport of energy from DNA into the environment. Hydrogen bonds play a key role for a broad range of biochemical processes and, thus, the results are expected to be of broad relevance. In a second part of the project, structures generated by charge and/or proton transfer will be studied in hydrogen bonded molecular crystals. Such elementary chemical processes govern the electrical properties of the materials which are of interest for applications in novel ferroelectric devices.

Biographical information on Thomas Elsaesser is available at http://staff.mbi-berlin.de/elsasser/

   
   

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