Recent Highlights
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Laser-driven electron recollision remembers molecular orbital structure

4 May 2018

Scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin combined state-of-the-art experiments and numerical simulations to test a fundamental assumption underlying strong-field physics. Their results refine our understanding of strong-field processes such as high harmonic generation (HHG) and laser-induced electron diffraction (LIED). The results have been published in "Science Advances".

Strong infrared laser pulses can extract an electron from a molecule (ionization), accelerate it away into free space, then turn it around (propagation), and finally collide it with the molecule (recollision). This is the widely used three-step model of strong-field physics. In the recollision step, the electron may, for example, recombine with the parent ion, giving rise to high harmonic generation, or scatter elastically, giving rise to laser-induced electron diffraction.

One of the commonly used assumptions underlying attosecond physics is that, in the propagation step, the initial structure of the ionized electron is "washed out", thus losing the information on the originating orbital. So far, this assumption was not experimentally verified in molecular systems.

A combined experimental and theoretical study at the Max Born Institute Berlin investigated the strong-field driven electron recollision dynamics in the 1,3-trans-butadiene molecule. In this molecule, the interaction with the strong laser field leads mainly to the ionization of two outermost electrons exhibiting quite different densities, see Figure 1. The state-of-the-art experiments and simulations then allowed the scientists to measure and calculate the high-angle rescattering probability for each electron separately. These probabilities turned out to be quite different both in the measurements and in the simulations. These observations clearly demonstrate that the returning electrons do retain structural information on their initial molecular orbital.

Schell Electron
Fig. 1 (click to enlarge)

Fig. 1: Continuum electronic wavepackets for strong-field ionization channel 1 and 2 in 1,3-trans-butadiene shortly after ionization. | Fig. MBI | Abb. MBI

Original publication:
"Molecular orbital imprint in laser-driven electron recollision"
Felix Schell, Timm Bredtmann, Claus Peter Schulz, Serguei Patchkovskii, Marc J. J. Vrakking and Jochen Mikosch
Science Advances, Vol. 4, no. 5, eaap8148 (2018), DOI: 10.1126/sciadv.aap8148

Dr. Jochen Mikosch, Tel. 030 / 6392 1295
Dr. Timm Bredtmann, Tel. 030 / 6392 1251


Freeing electrons to better trap them

16 April 2018

For the first time, researchers from UNIGE and MBI in Berlin have placed an electron in a dual state - neither freed nor bound - thus confirming a hypothesis from the 1970s.
Atoms are composed of electrons moving around a central nucleus they are bound to. The electrons can also be torn away, overcoming the confining force of their nucleus, using the powerful electric field of a laser. Half a century ago, the theorist Walter Henneberger wondered if it was possible to free an electron from its atom with the laser field, but still make it stay around the nucleus. Many scientists considered this hypothesis to be impossible. However, it was recently successfully confirmed by physicists from the University of Geneva (UNIGE), Switzerland, and the Max Born Institute (MBI) in Berlin, Germany. For the first time, they managed to control the shape of the laser pulse to keep an electron both free and bound to its nucleus, and were at the same time able to regulate the electronic structure of this atom dressed by the laser. What's more, they also made these unusual states amplify laser light. They also identified a no-go area. In this area nicknamed "Death Valley", physicists lose all their power over the electron. These results shatter the usual concepts related to the ionisation of matter. The results have been published in the journal Nature Physics.

Since the 1980s, many experiments have tried to confirm the hypothesis advanced by the theorist Walter Henneberger: an electron can be placed in a dual state that is neither free nor bound. Trapped in the laser, the electron would be forced to pass back and forth in front of its nucleus, and would thus be exposed to the electric field of both the laser and the nucleus. This dual state would make it possible to control the motion of the electrons exposed to the electric field of both the nucleus and the laser, and would let the physicists to create atoms with "new", tunable by light, electronic structure. But is this really possible?

Leveraging the natural oscillations of the electron
The more intense a laser is, the easier should it be to ionise the atom - in other words, to tear the electrons away from the attracting electric field of their nucleus and free them into space. "But once the atom is ionised, the electrons don't just leave their atom like a train leaves a station, they still feel the electric field of the laser", explains Jean-Pierre Wolf, a professor at the applied physics department of the UNIGE Faculty of Sciences. "We thus wanted to know if, after the electrons are freed from their atoms, it is still possible to trap them in the laser and force them to stay near the nucleus, as the hypothesis of Walter Henneberger suggests", he adds.

The only way to do this is to find the right shape for the laser pulse to be applied, to impose oscillations on the electron that are exactly identical, so that its energy and state remain stable. "The electron does naturally oscillate in the field of the laser, but if the laser intensity changes these oscillations also change, and this forces the electron to constantly change its energy level and thus its state, even leaving the atom. This is what makes seeing such unusual states so difficult", adds Misha Ivanov, a professor at the theoretical department of MBI in Berlin.

Modulating laser intensity to avoid Death Valley
The physicists tested different laser intensities so that the electron freed from the atom would have steady oscillations. They made a surprising discovery. "Contrary to natural expectations that suggest that the more intense a laser is, the easier it frees the electron, we discovered that there is a limit to the intensity, at which we can no longer ionise the atom", observes Misha Ivanov. "Beyond this threshold, we can control the electron again". The researchers dubbed this limit "Death Valley", following the suggestion of Professor Joe Eberly from the University of Rochester.

Confirming an old hypothesis to revolutionise physics theory
By placing the electron in a dual state which is neither free nor bound, the researchers found a way to manipulate these oscillations as they like. This enables them to directly work on the electronic structure of the atom. After several adjustments, for the first time, physicists from UNIGE and MBI were able to free the electron from its nucleus, and then trap it in the electric field of the laser, as Walter Henneberger suggested. "By applying an intensity of 100 trillion watts per cm2, we were able to go beyond the Death Valley threshold and trap the electron near its parent atom in a cycle of regular oscillations within the electric field of the laser", Jean-Pierre Wolf says enthusiastically. As a comparison, the intensity of the sun on the earth is approximately 100 watts per m2.

"This gives us the option of creating new atoms dressed by the field of the laser, with new electron energy levels", explains Jean-Pierre Wolf. "We previously thought that this dual state was impossible to create, and we've just proved the contrary. Moreover, we discovered that electrons placed in such states can amplify light. This will play a fundamental role in the theories and predictions on the propagation of intense lasers in gases, such as air", he concludes.

Abb. (click to enlarge)

Schematic illustration of the Kramers Henneberger potential formed by a mixture of the atomic potential and a strong laser field. Source: UNIGE - Xavier Ravinet

Original publication:
"Amplification of intense light fields by nearly free electrons"
Mary Matthews, Felipe Morales, Alexander Patas, Albrecht Lindinger, Julien Gateau, Nicolas Berti, Sylvain Hermelin, Jérôme Kasparian, Maria Richter, Timm Bredtmann, Olga Smirnova, Jean-Pierre Wolf and Misha Ivanov
Nature Physics (2018), DOI:10.1038/s41567-018-0105-0

Prof. Dr. Misha Ivanov, Tel. 030 / 6392 1210


From insulator to conductor in a flash

16 April 2018

A clever combination of novel technologies enables us to study promising materials for the electronics of tomorrow. Over the past decades, computers have become faster and faster and hard disks and storage chips have reached enormous capacities. But this trend cannot continue forever: we are already running up against physical limits that will prevent silicon-based computer technology from attaining any impressive speed gains from this point on. Researchers are particularly optimistic that the next era of technological advancements will start with the development of novel information-processing materials and technologies that combine electrical circuits with optical ones. Using short laser pulses, a research team led by Misha Ivanov of the Max Born Institute in Berlin together with scientists from the Russian Quantum Center in Moscow have now shed light on the extremely rapid processes taking place within these novel materials. Their results have appeared in the prestigious journal "Nature Photonics".

Of particular interest for modern material research in solid state physics are "strongly correlated systems", so called for the strong interactions between the electrons in these materials. Magnets are a good example of this: the electrons in magnets align themselves in a preferred direction of spin inside the material, and it is this that produces the magnetic field. But there are other, entirely different structural orders that deserve attention. In so-called Mott insulators for example, a class of materials now being intensively researched, the electrons ought to flow freely and the materials should therefore be able to conduct electricity as well as metals. But the mutual interaction between electrons in these strongly correlated materials impedes their flow and so the materials behave as insulators instead.

By disrupting this order with a strong laser pulse, the physical properties can be made to change dramatically. This can be likened to a phase transition from solid to liquid: as ice melts, for example, rigid ice crystals transform into free-flowing water molecules. Very similarly, the electrons in a strongly correlated material become free to flow when an external laser pulse forces a phase transition in their structural order. Such phase transitions should allow us to develop entirely new switching elements for next-generation electronics that are faster and potentially more energy efficient than present-day transistors. In theory, computers could be made around a thousand times faster by "turbo-charging" their electrical components with light pulses.

The problem with studying these phase transitions is that they are extremely fast and it is therefore very difficult to "catch them in the act". So far, scientists have had to content themselves with characterising the state of a material before and after a phase transition of this kind. Researchers Rui E. F. Silva, Olga Smirnova, and Misha Ivanov of the Berlin Max Born Institute, however, have now devised a method that will, in the truest sense, shed light on the process. Their theory involves firing extremely short, tailored laser pulses at a material - pulses that can only recently be produced in the appropriate quality given the latest developments in lasers. One then observes the material's reaction to these pulses to see how the electrons in the material are excited into motion and, like a bell, emit resonant vibrations at specific frequencies, as harmonics of the incident light.

"By analysing this high harmonic spectrum, we can observe the change in the structural order in these strongly correlated materials 'live' for the first time," says first author of the paper Rui Silva of the Max Born Institute. Laser sources capable of targetedly triggering these transitions have only been available since very recently. The laser pulses namely have to be amply strong and extremely short - on the order of femtoseconds in duration (millionths of a billionth of a second).

In some cases, it takes only a single oscillation of light to disrupt the electronic order of a material and turn an insulator into a metal-like conductor. The scientists at the Berlin Max Born Institute are among the world's leading experts in the field of ultrashort laser pulses.

"If we want to use light to control the properties of electrons in a material, then we need to know exactly how the electrons will react to light pulses," Ivanov explains. With the latest-generation laser sources, which allow full control over the electromagnetic field even down to a single oscillation, the newly published method will allow deep insights into the materials of the future.

Fig. (click to enlarge)

High harmonic spectroscopy of light-induced phase transition. The vertical red line shows when the laser electric field (yellow oscillating curve) crosses the threshold field, destroying the insulating phase of the material. The top panel shows the average number of doublon-hole pairs per site (blue) and the decay of the insulating field-free ground state (red). (Source: MBI)

Original publication:
"High harmonic spectroscopy of ultrafast many-body dynamics in strongly correlated systems"

R. E. F. Silva, Igor V. Blinov, Alexey N. Rubtsov, O. Smirnova & M. Ivanov
Nature Photonics, (2018) (online), DOI: 10.1038/s41566-018-0129-0


Prof. Dr. Misha Ivanov, Tel. 030 / 6392 1210
Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340
Dr. R.E.F. Silva, Tel.: 030 6392 1239


Wiggling atoms switch the electric polarization of crystals

12 April 2018

Ferroelectric crystals display a macroscopic electric polarization, a superposition of many dipoles at the atomic scale which originate from spatially separated electrons and atomic nuclei. The macroscopic polarization is expected to change when the atoms are set in motion but the connection between polarization and atomic motions has remained unknown. A time-resolved x-ray experiment now elucidates that tiny atomic vibrations shift negative charges over a 1000 times larger distance between atoms and switch the macroscopic polarization on a time scale of a millionth of a millionth of a second.

Ferroelectric materials have received strong interest for applications in electronic sensors, memories, and switching devices. In this context, fast and controlled changes of their electric properties are essential for implementing specific functions efficiently. This calls for understanding the connection between atomic structure and macroscopic electric properties, including the physical mechanisms governing the fastest possible dynamics of macrosopic electric polarizations.

Researchers from the Max-Born-Institute in Berlin have now demonstrated how atomic vibrations modulate the macroscopic electric polarization of the prototype ferroelectric ammonium sulphate [Fig. 1] on a time scale of a few picoseconds (1 picosecond (ps) = 1 millionth of a millionth of a second). In the current issue of the journal Structural Dynamics [5, 024501 (2018)], they report an ultrafast x-ray experiment which allows for mapping the motion of charges over distances on the order of the diameter of an atom (10-10m = 100 picometers) in a quantitative way. In the measurements, an ultrashort excitation pulse sets the atoms of the material, a powder of small crystallites, into vibration. A time-delayed hard x-ray pulse is diffracted from the excited sample and measures the momentary atomic arrangement in form of an x-ray powder diffraction pattern. The sequence of such snapshots represents a movie of the so-called electron-density map from which the spatial distribution of electrons and atomic vibrations are derived for each instant in time ([Fig. 2], [Movie]).

The electron density maps show that electrons move over distances of 10-10m between atoms which are more than a thousand times larger than their displacements during the vibrations [Fig. 3]. This behavior is due to the complex interplay of local electric fields with the polarizable electron clouds around the atoms and determines the momentary electric dipole at the atomic scale. Applying a novel theoretical concept, the time-dependent charge distribution in the atomic world is linked to the macroscopic electric polarization [Fig. 3]. The latter is strongly modulated by the tiny atomic vibrations and fully reverses its sign in time with the atomic motions. The modulation frequency of 300 GHz is set by the frequency of the atomic vibrations and corresponds to a full reversal of the microscopic polarization within 1.5 ps, much faster than any existing ferroelectric switching device. At the surface of a crystallite, the maximum electric polarization generates an electric field of approximately 700 million volts per meter.

The results establish time-resolved ultrafast x-ray diffraction as a method for linking atomic-scale charge dynamics to macroscopic electric properties. This novel strategy allows for testing quantum-mechanical calculations of electric properties and for characterizing a large class of polar and/or ionic materials in view of their potential for high-speed electronics.

Fig. 1 (click to enlarge)

Fig. 1: Crystal lattice of ferroelectric ammonium sulfate [(NH4)2SO4] with tilted ammonium (NH4+) tetrahedra (nitrogen: blue, hydrogen: white) and sulfate (SO42-) tetrahedra (sulfur: yellow, oxygen: red). The green arrow shows the direction of macroscopic polarization P. Blue arrows: local dipoles between sulphur and oxygen atoms. The electron density maps shown in the bottom left panel, in Fig. 2, and the movie are taken in the plane shown in grey. Bottom left: Stationary electron density of sulfur and oxygen atoms, displaying high values on the sulfur (red) and smaller values on the oxygens (yellow). Bottom right: Change of local dipoles at a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate crystallites. An anisotropic shift of charge reduces the dipole pointing to the right and increases the other 3 dipoles.

Fig. 2 (click to enlarge)
Fig 2: (a) Stationary electron density in the grey plane shown in Fig. 1. (b) Change of electron density at a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate crystallites. The circles mark the atomic positions, the black arrows indicate the transfer of electronic charge between one of the oxygen atom and the SO3 group of a single sulfate ion. The vibrational displacements of the atoms are smaller than the line thickness of the circles and, thus, invisible on this length scale. (c) The reverse charge transfer occurs at a delay time of 3.9 ps.
Movie (click to enlarge)
Movie: The movie shows the entire temporal evolution of the electron density map.
Fig. 3 (click to enlarge)
Fig 3: Upper panel: Change of the S-O bond length as a function of the delay time. The maximum change of 0.1 pm is 1000 times smaller than the bond length itself, i.e., the atomic motions cannot be observed in Fig. 2. Middle panel: Charge transfer from one oxygen atom to the SO3 group of the sulfate ion (left black arrows in Fig. 2) as a function of delay time. Lower panel: Change of the macroscopic polarization P along the c axis which is the sum of all microscopic dipole changes of the local S-O dipoles within the sulfhate ions (red and blue arrows in Fig. 1 bottom right).

Original article:
Christoph Hauf, Antonio-Andres Hernandez Salvador, Marcel Holtz, Michael Woerner, and Thomas Elsaesser, Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction, Struct. Dyn. 5, 024501 (2018).

Further information:
Dr. Michael Wörner, Tel.: 030 6392 1470
Dr. Christoph Hauf, Tel.: 030 6392 1473
Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400




X-ray snapshots of reacting acids and bases - Erik T. J. Nibbering receives an ERC Advanced Grant for groundbreaking basic research

9 April 2018

Dr. Erik T. J. Nibbering of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin receives an Advanced Grant from the European Research Council (ERC). Goal with this prestigious award is to investigate and elucidate the elementary steps of aqueous proton transfer dynamics between acids and bases. The ERC Advanced Grant is endowed with 2.5 million euro and awarded to well-established top researchers in Europe pursuing scientifically excelling projects.

Dr. Erik T. J. Nibbering, head of the department "Femtosecond Spectroscopy of Molecular Systems" at MBI, has a major track record in time-resolved spectroscopy of ultrafast chemical reactions, in particular proton transfer between acids and bases, electron transfer in donor-acceptor complexes, and trans/cis isomerization. In recent years his activities have focused on the dynamics of the hydrogen bond structure of photoacid-base complexes and of hydrated protons.

How acids and bases react in water is a question raised since the pioneering days of modern chemistry. Recent decades have witnessed an increased effort in elucidating the microscopic mechanisms of proton exchange between acids and bases and the important mediating role of water in this. With ultrafast spectroscopy it has been shown that the elementary steps in aqueous proton transfer occur on femtosecond to picosecond time scales (1 femtosecond = 10-15 s = 1 millionth of a billionth of a second). Aqueous acid-base neutralization predominantly proceeds in a sequential way via water bridging acid and base molecules. These ultrafast experiments probing molecular transitions in the ultraviolet, visible and mid-infrared spectral ranges, though, only provide limited insight into the electronic structure of acids, bases and the water molecules accommodating the transfer of protons in the condensed phase. Soft-x-ray absorption spectroscopy (XAS), probing transitions from inner-shell levels to unoccupied molecular orbitals, is a tool to monitor electronic structure with chemical element specificity.

The aim is now to develop steady-state and time-resolved soft-x-ray spectroscopy of acids and bases. Here novel liquid flatjet technology is utilized with soft-x-ray sources at synchrotrons as well as table-top laser-based high-order harmonic systems. Resolving the electronic structural dynamics of elementary steps of aqueous proton transport will furthermore elucidate the role of mediating water in bulk solution, and in specific conditions such as hydrogen fuel cells or trans-membrane proteins.

Further information on Dr. Erik T. J. Nibbering can be found at http://staff.mbi-berlin.de/nibberin/ and on ERC Grants at http://erc.europa.eu/.

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI)
Dr. Erik T. J. Nibbering
Phone +49 / 30 / 6392-1477




Dr. Daniela Rupp will receive the Karl Scheel Prize 2018

13 March 2018

The Physical Society of Berlin announced this year's Karl Scheel laureate. We congratulate Dr. Daniela Rupp, who will receive the award on June 22, 2018 at 5 p.m. the Magnus-Haus in Berlin Mitte.


Dr. Daniela Rupp is junior research group leader at the Max Born Institute, Berlin

For more information please consult: "Der Karl-Scheel-Preis der Physikalischen Gesellschaft zu Berlin"


Dr. Daniela Rupp, Tel.: 030 6392 1280




A spinning top of light

27 February 2018

Short, rotating pulses of light reveal a great deal about the inner structure of materials. An international team of physicists led by Prof. Misha Ivanov of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) has now developed a new method for precisely characterising such extremely short light pulses. The research results have been published in Nature Communications.

Not all light is equal: depending on how it is prepared, it can exist in very different forms. Not only can we choose different wavelengths or colours but, as an electromagnetic wave, light can also exhibit different forms of oscillation. It can occur in different polarisations, for example - either linearly polarised or circularly polarised, where the oscillations of the electromagnetic fields follow a line or go round in circles, respectively. Above all, extremely short pulses of polarised light waves are excellent for studying many different types of materials. We have methods for producing such pulses, but these methods are already pushing the limits of technical feasibility and the light pulses are not always produced with the desired properties.

A new method now allows us to characterise these short light pulses with unprecedented precision. The trouble starts with the fact that the processes of interest taking place inside matter, which we would like to study with our light pulses, are extremely short-lived. Accordingly, the light pulses have to be similarly short, in the range of around 100 attoseconds (billionths of a billionth of a second). In this unimaginably short timespan, a light wave can only undergo a few rotations. Even using the latest laser methods to produce such ultrashort pulses, it can easily happen that the light wave will not come out rotating the right way.

The concept for the new method can be described as follows: one fires an extremely short, high-energy and circularly polarised light pulse at an atom or a solid body where, upon being absorbed, the light pulse knocks an electron out of the body. This electron then carries information about the light wave itself and can furthermore reveal clues as to the properties of the sample being examined. Because the light pulses are circularly polarised, the ejected electrons also fly off with a rotating motion.

"You can compare the electrons being ejected with a one-armed sprinkler, which either continues turning in the direction you want it to, or which keeps stuttering and even changing its direction," says Misha Ivanov, Head of the Theory Department of the Max Born Institute. If the sprinkler is allowed to run for a while, then it will wet the grass in a full circle - irrespective of whether it rotates consistently or not. So, merely looking at the grass will not reveal whether the sprinkler has been turning exactly the way it was desired or not. "But if a gusty wind comes along, then we can distinguish whether the sprinkler has been turning regularly or irregularly," Ivanov says. If the wind blows alternately from the left or right each time the arm of the sprinkler faces left or right, then the patch of wet grass will not be circular, but rather elliptical in shape. A sprinkler rotating completely irregularly would magically conjure up an ellipse on the grass stretched in the wind direction, while a regularly rotating sprinkler will display a tilted ellipse.

This "wind" is added into the experiment in the form of an infrared laser pulse whose oscillations are perfectly synchronised with the ultrashort pulses. The infrared radiation accelerates the electron either to the left or right - just like the wind blows the water droplets.

"By measuring the electrons, we can then determine whether the light pulse possessed the desired consistent rotation or not," says the first author of the publication in "Nature Communications", researcher Álvaro Jiménez-Galán of the Max Born Institute. "Our method allows one to characterise the properties of the ultrashort light pulses with unprecedented precision," Jiménez-Galán adds. And the more precisely these light pulses are characterised, the more detailed information can be derived about the electron's place of origin within an exotic material.

This is of special significance when it comes to studying a whole series of novel materials. These could include superconductors, which can conduct electricity without electrical resistance, or topological materials that exhibit exotic behaviour, the research of which earned a Nobel Prize in Physics in 2016. Materials like these could be used to make a quantum computer, for example, or could allow superfast, energy-efficient processors and memory chips to be built into normal computers and smartphones.

The new sprinkler method still only exists in theory for the moment, but ought to be implementable in the near future. "Our requirements are fully within the latest state of the art, so there is nothing to preclude this from being realised soon," Ivanov asserts.


Fig. 1 (click to enlarge)


Left alone, the circular sprinkler distributes the water evenly, and the grass grows in a circular pattern regardless of whether the sprinkler rotates clock-wise, counter-clockwise, or randomly. If wind blows, the grass is watered unevenly, as seen in its growth. If the wind blows in a regular pattern, changing its strength with clock-work precision, the asymmetry in the grass growth allows us to reconstruct the properties of the sprinkler, distinguishing the precision-made, regularly rotating sprinkler from a randomly oscillating cheap version.

In our micro-world setup, the sprinkler is the short pulse (in blue), lasting only about 10-16 sec, with its electric field rotating even faster in an unknown pattern. The wind is a linearly polarized and precisely controlled infrared laser field (in red). The grass is the measured photoelectron angular distribution (in green). The asymmetry in the latter allows us to reconstruct, for the first time, the polarization properties of the ultra-short pulse that lasts about 10-16 sec. (Figure credit: Felipe Morales und Álvaro Jiménez-Galán)

Original Publication: Nature Communications
Álvaro Jiménez-Galán, Gopal Dixit, Serguei Patchkovskii, Olga Smirnova, Felipe Morales & Misha Ivanov: Attosecond recorder of the polarization state of light
Nature Communications, volume 9, Article number: 850 (2018), doi:10.1038/s41467-018-03167-2


Prof. Dr. Misha Ivanov, Tel.: 030 6392 1340

Prof. Dr. Álvaro Jiménez-Galán, Tel.: 030 6392 1340

Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340




C'mon electrons, let's do the twist! Twisting electrons can tell right-handed and left-handed molecules apart.

19 February 2018

Identifying right-handed and left-handed molecules is a crucial step for many applications in chemistry and pharmaceutics. An international research team (CELIA-CNRS/INRS/Berlin Max Born Institute/SOLEIL) has now presented a new original and very sensitive method. The researchers use laser pulses of extremely short duration to excite electrons in molecules into twisting motion, the direction of which reveals the molecules' handedness. The research results appear in Nature Physics.

Are you right handed or left handed? No, we aren't asking you, dear reader; we are asking your molecules. It goes without saying that, depending on which hand you use, your fingers will wrap either one way or the other around an object when you grip it. It so happens that this handedness, or "chirality", is also very important in the world of molecules. In fact, we can argue that a molecule's handedness is far more important than yours: some substances will be either toxic or beneficial depending on which "mirror-twin" is present. Certain medicines must therefore contain exclusively the right-handed or the left-handed twin.

The problem lies in identifying and separating right-handed from left-handed molecules, which behave exactly the same unless they interact with another chiral object. An international research team has now presented a new method that is extremely sensitive at determining the chirality of molecules.

We have known that molecules can be chiral since the 19th century. Perhaps the most famous example is DNA, whose structure resembles a right-handed corkscrew. Conventionally, chirality is determined using so-called circularly polarised light, whose electromagnetic fields rotate either clockwise or anticlockwise, forming a right or left "corkscrew", with the axis along the direction of the light ray. This chiral light is absorbed differently by molecules of opposite handedness. This effect, however, is small because the wavelength of light is much longer than the size of a molecule: the light's corkscrew is too big to sense the molecule's chiral structure efficiently.

The new method, however, greatly amplifies the chiral signal. "The trick is to fire a very short, circularly polarized laser pulse at the molecules," says Olga Smirnova from the Max Born Institute. This pulse is only some tenths of a trillionth of a second long and transfers energy to the electrons in the molecule, exciting them into helical motion. The electrons' motion naturally follows a right or left helix in time depending on the handedness of the molecular structure they reside in.

Their motion can now be probed by a second laser pulse. This pulse also has to be short to catch the direction of electron motion and have enough photon energy to knock the excited electrons out of the molecule. Depending on whether they were moving clockwise or anticlockwise, the electrons will fly out of the molecule along or opposite to the direction of the laser ray.

This lets the experimentalists of CELIA to determine the chirality of the molecules very efficiently, with a signal 1000 times stronger than with the most commonly used method. What's more, it could allow one to initiate chiral chemical reactions and follow them in time. It comes down to applying very short laser pulses with just the right carrier frequency. The technology is a culmination of basic research in physics and has only been available since recently. It could prove extremely useful in other fields where chirality plays an important role, such as chemical and pharmaceutical research.

Having succeeded in identifying the chirality of molecules with their new method, the researchers are now thinking already of developing a method for laser separation of right- and left-handed molecules. Text: Dirk Eidemueller/Forschungsverbund Berlin e.V. - Translation: Peter Gregg


Fig. (click to enlarge)

Fig: Following excitation by an ultra-short circularly polarised laser pulse, electrons follow a right or left helix depending on the handedness of the molecular structure they reside in. Source: Samuel Beaulieu

Original Publication: Nature Physics
S. Beaulieu, A. Comby, D. Descamps, B. Fabre, G. A. Garcia, R. Géneaux, A. G. Harvey, F. Légaré, Z. Mašin, L. Nahon, A. F. Ordonez, S. Petit, B. Pons, Y. Mairesse, O. Smirnova and V. Blanchet: Photoexcitation Circular Dichroism in Chiral Molecules
Nature Physics (2018) online, doi:10.1038/s41567-017-0038-z.


Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340




Flexibility and arrangement - the interaction of ribonucleic acid and water

16 January 2018

Ribonucleic acid (RNA) plays a key role in biochemical processes which occur at the cellular level in a water environment. Mechanisms and dynamics of the interaction between RNA and water were now revealed by vibrational spectroscopy on ultrashort time scales and analyzed by in-depth theory

Ribonucleic acid (RNA) represents an elementary constituent of biological cells. While deoxyribonucleic acid (DNA) serves as the carrier of genetic information, RNA displays a much more complex biochemical functionality. This includes the transmission of information in the form of mRNA, RNA-mediated catalytic function in ribosomes, and the encoding of genetic information in viruses. RNA consists of a sequence of organic nucleobase molecules which are held together by a so-called backbone consisting of phosphate and sugar groups. Such a sequence can exist as a single strand or in a paired double-helix geometry. Both forms are embedded in a water shell and their phosphate and sugar groups are distinct docking points for water molecules. The structure of the water shell fluctuates on a time scale of a few tenth of a picosecond (1 ps = 10-12 s = 1 millionth of a millionth of a second). The interactions of RNA and water and their role for the formation of three-dimensional RNA structures are only understood insufficiently and difficult to access by experiment.

Scientists from the Max Born Institute have now observed the interaction of RNA with its water shell in real time. In their new experimental method, vibrations of the RNA backbone serve as sensitive noninvasive probes of the influence of neighboring water molecules on the structure and dynamics of RNA. The so-called two-dimensional infrared spectroscopy allows for mapping the time evolution of vibrational excitations and for determining molecular interactions within RNA and between RNA and water. The results show that water molecules at the RNA surface perform tipping motions, so-called librations, within a fraction of a picosecond whereas their local spatial arrangement is preserved for a time range longer than 10 ps. This behavior deviates strongly from that of neat water and is governed by the steric boundary conditions set by the RNA surface. Individual water molecules connect neighboring phosphate groups and form a partly ordered structure which is mediated by their coupling to the sugar units.

The librating water molecules generate an electrical force by which the water fluctuations are transferred to the vibrations of RNA. The different backbone vibrations display a diverse dynamical behavior which is determined by their local water environment and reflects its heterogeneity. RNA vibrations also couple mutually and exchange energy among themselves and with the water shell. The resulting ultrafast redistribution of excess energy is essential for avoiding a local overheating of the sensitive macromolecular structure. This complex scenario was analyzed by detailed theoretical calculations and simulations which, among other results, allowed for the first complete and quantitative identification of the different vibrations of the RNA backbone. Comparative experiments with DNA reveal similarities and characteristic differences between these two elementary biomolecules, showing a more structured water arrangement around RNA. The study highlights the strong potential of non-invasive time-resolved vibrational spectroscopy for unraveling the interplay of structure and dynamics in complex biomolecular systems on molecular length and time scales.


Fig. 1 (click to enlarge)

Fig. 1: Left: Structure of a RNA double helix. The blue spheres represent sodium counterions. Right: Enlarged segment of the sugar-phosphate backbone of RNA, including bridging water molecules. Vibrations of the RNA backbone serve as sensitive real time probes for mapping the influence of the neighboring water molecules on RNA's structure and dynamics.

Fig. 2 (click to enlarge)
Fig. 2: Two-dimensional vibrational spectra of RNA (upper panel) and DNA (lower panel) in the frequency range of the sugar-phosphate vibrations of the backbone. The RNA spectrum displays additional bands (contours) along the frequency diagonal ν13 and a more complex distribution of off-diagonal peaks. In addition to the frequency positions the line shapes of the individual bands (contours) give insight in details of the interactions with neighboring water molecules.

Original publication:
E. M. Bruening, J. Schauss, T. Siebert, B. P. Fingerhut, T. Elsaesser: Vibrational Dynamics and Couplings of the Hydrated RNA Backbone: A Two-Dimensional Infrared Study.
J. Phys. Chem. Lett. 9, 583-587 (2018). DOI: 10.1021/acs.jpclett.7b03314.


Dr. Benjamin Fingerhut, Tel.: 030 6392 1404

Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400




Instant x-ray footprints

15 January 2018

MBI scientists together with colleagues from Italy have established a way to detect the exact x-ray fluence footprint generated on a sample by a free electron laser pulse.

Free electron lasers (FELs) deliver intense and coherent x-ray pulses - a prerequisite to investigate and exploit non-linear processes in the interaction of x-rays with matter. Analogous to the development of nonlinear optics after the invention of lasers in the optical regime, the application of these processes is expected to have a widespread impact on numerous research fields. A pivotal parameter in this context is the fluence of the radiation deposited on the sample during a single ultrashort pulse - in essence, the number of photons hitting the sample per unit area for a given photon energy. Unfortunately, this fluence can vary at FELs from shot to shot both in its total amount as well as in the way it is distributed in a focal spot. Simply put, there is a footprint of x-rays on the sample which can vary in both shape and intensity. This makes quantitative experiments on nonlinear processes at x-ray wavelengths very challenging, as they are inherently sensitive to the precise fluence distribution.

Scientists from MBI and the Italian research institutes ELETTRA and IOM have now demonstrated a method, which allows to take a snapshot picture of the fluence distribution impinging on the sample while at the same time recording the scattering signal of interest generated by that very same FEL shot. The approach relies upon the fabrication of very shallow grooves of only a few nanometer depth into the membrane holding the sample. Via a tailored two-dimensional distortion, this groove pattern forms a diffractive optical element that is designed to image the footprint of the incident x-ray beam on a two-dimensional detector. In the figure below, the beam footprint on the sample is visible (in two conjugate copies) as a spot with a checkerboard of many side maxima and minima, while the magnetic scattering from this sample with ferromagnetic domains is visible as a ring on the very same detector. Using this approach, scientists can now relate a scattering signal from a specimen to the exact incident fluence footprint on this sample, as both originate from the identical x-ray pulse.

Furthermore, the use of the grating structure alone - without a sample - turned out to be extremely helpful when aligning the x-ray optics of the FEL or a sample relative to the focal position. Together with the detector, the distorted grating provides instant feedback on the beam shape when placed into the x-ray beam. The new method is already now routinely used at the FERMI free electron laser for alignment purposes.


Fig. 1 (click to enlarge)

Fig. 1: 2D Detector image downstream of the sample showing both the ring-shaped magnetic scattering as well as the fluence map ("footprint") of x-rays on the sample generating this magnetic scattering.

Original publikation:
M. Schneider, C. M. Günther, B. Pfau, F. Capotondi, M. Manfredda, M. Zangrando, N. Mahne, L. Raimondi, E. Pedersoli, D. Naumenko, and S. Eisebitt, In situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses, Nature Communications 9, 214 (2018).


Prof. Dr. Stefan Eisebitt, Tel.: 030 6392 1300



Marc Vrakking named Editor-in-Chief of the Journal of Physics B

1 January 2018

Prof. Marc Vrakking, director of Division A of the Max Born Institute, has been named Editor-in-Chief of the Journal of Physics B from January 1st 2018. In this capacity he succeeds Prof. Paul Corkum of the University of Ottawa, who served as Editor-in-Chief since 2011.

The Journal of Physics B is one of the publications of the Institute of Physics, and is a prominent journal serving the Atomic, Molecular and Optical (AMO) Physics community. In particular in the emerging fields of strong field physics, attosecond science and free electron laser science the journal has a prominent role, as is manifest by numerous important papers (including Topical Reviews, Tutorials, Special Issues and Roadmaps) that the journal continuously publishes. As such, the journal is strongly aligned with the research program in Division A of the Max Born Institute.


Prof. Marc Vrakking Tel. (030) 6392 1200


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