Elementary particles starring a dance movie 16th August 2010 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 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 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 Lasers 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 Isaac 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 Dr. 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 The 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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