Archiv: Highlights at the MBI
Highlights 2014

Filming chemistry with a high speed x-ray camera

26th November 2014

Chemistry happens all around us. A chemical reaction is a rearrangement of atoms in and between molecules, the breaking of old and the formation of new bonds. The glue that binds atoms in molecules and creates the bonds between them is made out of valence electrons.

While the motion of valence electrons is at the very heart of chemical reactions, only a small fraction among them participates actively. The valence electron charge transferred between the atoms is often just a fraction of the charge of an electron. And those that do participate, do it very quickly: the duration of many very important chemical processes, such as first steps in vision and light harvesting, is measured in only tens to a hundred of femtoseconds - a femtosecond is a millionth of a billionth of a second. Making a movie of the chemically active electrons is therefore very challenging. First, one needs a camera with exquisite temporal and spatial resolution. Second, one needs a very sensitive camera. Indeed, one would really like to see not just how the atoms move, but also how the new bonds are formed as the old ones are broken - and this means filming the few active valence electrons in the sea of all electrons attached to the many atoms in the molecule.

An X-ray camera easily fits the first requirement. X-ray scattering has been indispensable in studying the structure of matter with atomic-scale spatial resolution since the discovery of x-rays. Thanks to enormous technological progress, it is now becoming possible to generate ultrashort flashes of x-rays, adding femtosecond temporal resolution to structural sensitivity. These flashes of x-rays promise to provide stroboscopic snapshots of chemical and biological processes in individual molecules.

However, fitting the second requirement - the sensitivity to active valence electrons - has never been the strength of an x-ray camera. X-ray scattering is always dominated by core and inert valence electrons. The small fraction of valence electrons actively participating in a chemical reaction is generally presumed lost in the scattering signal, seemingly placing ultrafast x-ray imaging of these electron densities out of the realm of possibility

Our work, published in Nature Communications, suggests a way to resolve this challenge. In this work, we theoretically demonstrate a robust and effective method to extract the contributions of chemically active valence electrons from the total x-ray scattering by a single molecule - a critical step in the endeavor to film bond making and bond breaking as it happens, in space and time. Our paper shows how, by combining the standard analysis of the full x-ray scattering pattern with an additional analysis of the part of the scattering pattern, which is limited to relatively small momentum transfer, one nearly effortlessly brings to the fore the motion of chemically active valence electrons.

The work not only showed how to film chemically active valence electrons with x-rays, it has also provided an experimental access to the long-standing problem: Are the new bonds made at the same time as old bonds are broken, or is there a time-delay between these two processes?  The x-ray camera confirms that the answer depends on whether the atoms have enough energy to climb over the energy barrier, which separates reactants from products, or if they have to resort to the quantum trick of tunneling through the energy barrier – the only option available when their energy is not sufficient to overcome it. In the first case we confirm a time-delay between the breaking of old and the formation of new bonds. In the second case, we see no delay: the new bonds are built in concert with the destruction of the old ones. We hope our work will bring new insights into ways to initiate and control complex chemical and biological reactions.

Original publication:
Timm Bredtmann, Misha Ivanov, Gopal Dixit
X-ray imaging of chemically active valence electrons during a pericyclic reaction
Nature Communication doi:10.1038/ncomms6589



Fig. : Filming bond making and bond breaking during a pericyclic reaction: We show theoretically that the ultrafast x-ray camera is not only sensitive to inert core electrons but may also visualize the motion of chemically active valence electrons.  

Abb. (click to enlarge) credit: Gopal Dixit MBI

Abb.: A combination of the standard analysis of the full x-ray scattering pattern (A, B) with an additional analysis of the part of the scattering pattern, which is limited to relatively small momentum transfer, one nearly effortlessly brings to the fore the motion of chemically active valence electrons during a pericyclic reaction (C, D). The breaking and making of chemical bonds along different reaction paths may thus be filmed and analyzed directly

Abb. (click to enlarge)  

Dr. Timm Bredtmann Tel: 030 6392 1239
Prof. Misha Ivanov Tel: 030 6392 1210
Dr. Gopal Dixit Tel: 030 6392 1239


The longer the better: Optical long-wavelength pulses generate brilliant ultrashort hard x-ray flashes

10th November 2014

Researchers from the Max-Born-Institut and the Technical University of Vienna present a novel table-top source of ultrashort hard x-ray pulses with an unprecedented photon flux.

X-rays are a key tool for imaging materials and analyzing their composition - at the doctor, in the chemistry lab and in materials research. Shining so-called hard x-rays of a wavelength comparable to the distance between atoms on the material, one can determine the atomic arrangement in space by analyzing the pattern of scattered x-rays. This method has unraveled equilibrium time-averaged structures of increasing complexity, from simple inorganic crystals to highly complex biomolecules such as DNA or proteins.

Today, there is a strong quest for mapping atoms ‘on the fly’, that is, for following atomic motions in space, during a vibration, a chemical reaction, or a change of the material’s structure. Atomic  motions typically occur in the time range of femtoseconds (1 femtosecond = 10-15s), requiring an exposure by extremely short x-ray flashes to take snapshots. There are essentially two complementary approaches to generate ultrashort hard x-ray pulses, large scale facilities based on electron accelerators such as the free electron lasers in Stanford (LCLS at SLAC) or at SACLA in Japan, or highly compact table-top sources driven by intense ultrashort optical pulses. While the overall x-ray flux from accelerator sources is much higher than from table-top sources, the latter are versatile tools for making femtosecond x-ray ‘movies’ with a quality that is eventually set by the number of x-ray photons scattered from the sample. A joint research team from the Max-Born-Institut (MBI) in Berlin and the Technical University in Vienna has now accomplished a breakthrough in table-top x-ray generation, allowing for an enhancement of the generated hard x-ray flux by a factor of 25. As they report in the current issue of Nature Photonics. the combination of a novel optical driver providing femtosecond mid-infrared pulses around a 4000 nm (4µm) wavelength with a metallic tape target allows for generating hard x-ray pulses at a wavelength of 0.154 nm with unprecedented efficiency.

The x-ray generation process consists of 3 steps (Fig. 1), (i) electron extraction from the metal target induced by the electric field of the driving pulse, (ii) electron acceleration in vacuum by the strong optical field and return into the target with an increased kinetic energy, and (iii) generation of x-rays in the target by inelastic collisions of electrons with atoms. Longer optical wavelengths are equivalent to a longer oscillation period of the optical field and, thus, to a longer period of electron acceleration in vacuum. As a result, the accelerated electrons acquire a higher kinetic energy before they re-enter the target and generate x-rays with a higher efficiency. A simple analogy of the acceleration process is the mechanical acceleration when jumping from platforms at different height into water (Fig. 2). Here, the time interval Δt between leaving the platform and reaching the water surface increases with height and the kinetic energy at the water surface is proportional to Δt2. The electron pathways in vacuum were analyzed in detail by theoretical calculations and are shown in the movie attached (Fig. 3).

The experiments were performed at the TU Vienna combining a novel driver system based on Optical Parametric Chirped Pulse Amplification (OPCPA) with an x-ray target chamber from MBI. Pulses of 80 fs duration and up to 18 mJ energy at a center wavelength of 3900 nm (3.9 µm) were focused down onto a 20 µm thick copper tape. This scheme allows for generating an unprecedented number of 109 hard x-ray photons at a 0.154 nm wavelength per driving pulse. A comparison with previous experiments performed with 800 nm driver pulses shows that the enhancement of the x-ray flux in the new scheme scales with the square of the wavelength ratio, i.e., (3900 nm/800 nm)2 ≈ 25. This behavior is in quantitative agreement with a theoretical analysis of the 3-step generation scheme of Fig. 1. The results pave the way for a new generation of table-top hard x-ray sources, providing up to 1010 x-ray photons per pulse at elevated, e.g., kilohertz repetition rates.

Original publication:
Jannick Weisshaupt, Vincent Juvé, Marcel Holtz, ShinAn Ku, Michael Woerner, Thomas Elsaesser, Skirmantas Ališauskas, Audrius Pugzlys and Andrius Baltuška
High-brightness table-top hard X-ray source driven by sub-100 femtosecond mid-infrared pulses
Nature Photonics doi:10.1038/nphoton.2014.256.


Abb 1 Röntgen

Fig. 1: Left: x-ray generation in a conventional x-ray tube. Electrons which were emitted from the heated cathode (-) are accelerated by a constant electric field towards the anode (+). Within the metal target (e.g. copper) inelastic collisions of the accelerated electrons with atoms lead to the generation of both characteristic line emission of x-rays (sharp lines in the spectrum at the bottom) and Bremsstrahlung. Right: Femtosecond mid-infrared pulses (λ = 3900 nm) from a OPCPA system are focused onto a copper band target. Electrons are extracted from the surface, accelerated into the vacuum and smashed back into the target by the strong electric field of the light. During deceleration in the metal target the energetic electrons produce characteristic line emission and Bremsstrahlung which can be measured by an x-ray detector.


Fig. 1 (click to enlarge)
Abb 2 Kabul

Fig. 2: Analogy of electron acceleration in the vacuum to the acceleration by gravity when jumping from platforms at different height into water. Longer optical wavelengths are equivalent to a longer oscillation period of the optical field and, thus, to a longer period of electron acceleration in vacuum. The time interval Δt between leaving the platform and reaching the water surface increases with height and the kinetic energy at the water surface is proportional to Δt2. As a result, electrons accelerated in longer time interval Δt acquire a higher kinetic energy before they re-enter the target and generate x-rays with a higher efficiency.


Fig. 2 (click to enlarge)  
Movie Movie of the acceleration of electrons (blue balls) in the vacuum above a metal surface when applying the strong oscillating electric field of the laser pulse (black line).

Jannick Weisshaupt Tel: 030 6392 1471
Vincent Juvé Tel: 030 6392 1472
Michael Wörner Tel: 030 6392 1470
Thomas Elsaesser Tel: 030 6392 1400
Andrius Baltuška Tel: +43 1 58801 38749


Now you see me, now you don’t ...

4th September 2014

New experiments in the helium atom allow to turn electron correlation on and off at will

After the hydrogen atom, which consists of one proton and one electron, the helium atom is nature’s simplest atom. The helium atom consists of a doubly-charged nucleus surrounded by two electrons. The presence of two electrons in the atom introduces a novel aspect with profound consequences, namely the concept of electron correlation. In a paper published in Physical Review Letters this week [1], experiments are reported where the onset of electron correlation in the helium atom is observed. Photoionization of helium is studied under conditions where the electron correlation can be turned on or off at will. When turned off, the helium atom behaves just like a hydrogen atom; when turned on, the helium dynamics is strongly affected by the interaction between the two electrons.

In the experiment, helium atoms were ionized by the absorption of a single ultra-violet photon. This was possible because, prior to the experiment, the helium atoms were excited into a long-living excited state, by means of a collision with an energetic electron in a discharge source. The energy of the ultra-violet light was tuned in such a manner that it was only just sufficient for ionization of the atom, with 99.9% of the photon energy being used to overcome the ionization potential of the atom, and just 0.1% of the photon energy being converted into photoelectron kinetic energy. The very slow photoelectrons were accelerated towards a two-dimensional detector, where their position was measured. This position is then a measure of the velocity of the electron in the plane of the detector.

As clearly shown in the famous double-slit experiment on interference of single electrons (voted ”the most beautiful physics experiment”, in a poll conducted by Physicsworld about a decade ago) electrons exhibit both particle- and wave-like behavior. The wave-like behavior derives from the de Broglie wavelength that quantum mechanics associates with any moving particle. The lower the kinetic energy of the electron, the larger the de Broglie wavelength is. Correspondingly, for low enough kinetic energies, the de Broglie wavelength becomes observable on macroscopic length scales. In the helium photoionization experiment the wave-like nature of the slow electrons leads to the observation of a series of interference rings, with constructive and destructive interferences alternating on the detector (see Figure 1).

All this was already known from experiments performed by our team in the last decade. In fact, these experiments had revealed the existence of two distinct origins for the observed interferences. In experiments on hydrogen atoms published last year [2], it was shown that the interferences could be connected to the nodal patterns of the wavefunctions excited in the atom upon absorption of a photon. In other atoms, such as the extensively studied xenon atom, it was shown that the interferences could arise as a result of differences in the lengths of possible paths of the electron on the way to the detector. Crudely speaking, two paths differing by an integral number of de Broglie wavelengths will interfere constructively, whereas two paths differing by a half-integer number of de Broglie wavelengths interfere destructively.

In helium, both situations have now been observed to coexist. Moreover, it was observed that the helium dynamics can be controlled by tiny changes << 1%) in the strength of the external electric field. This suffices to convert an atom that reveals the nodal pattern of its wavefunction in a hydrogen-like manner, into an atom where electron correlation removes the observability of this nodal pattern, and where the observed interference patterns are completely determined by pathlength differences between the atom and the detector.

In this manner, the helium atom constitutes a wonderful nano-scale laboratory for the onset of electron correlation.

1. Stodolna, A.S., et al., Visualizing the coupling between red and blue Stark states using photoionization microscopy. Physical Revier Letters, 2014. 113. 103002.

2. Stodolna, A.S., et al., Hydrogen Atoms under Magnification: Direct Observation of the Nodal Structure of Stark States. Physical Review Letters, 2013. 110(21): p. 213001.

Original publication:
A.S. Stodolna, F. Lépine, T. Bergeman, F. Robicheaux, A. Gijsbertsen, J.H. Jungmann, C. Bordas, and M.J.J. Vrakking
Visualizing the Coupling between Red and Blue Stark States Using Photoionization Microscopy
Physical Review Letters 113.103002, (2014)



Figure: Sample images recorded for ionization of helium atoms. Interference patterns are measured that reveal the nodal structure of the electronic wavefunction that is excited, or that result from path length differences. In the former case helium behaves like a hydrogen atom, and electron correlation does not play a role, whereas in the latter case the ionization is strongly influenced by electron correlation.

Fig. (click to enlarge)

Prof. Marc J.J. Vrakking Tel: 030 6392 1200


Freedom of electrons is short-lived

25th June 2014

During the interaction of an intense extreme-ultraviolet (XUV) laser pulse with a cluster, many ions and free electrons are created, leading to the formation of a nanoscale plasma. In experiments using XUV/X-ray free electron lasers (FELs) it was previously demonstrated that only a small fraction of these electrons can leave the cluster, while the majority of the electrons remain trapped within the cluster and may therefore recombine with ions. In a novel approach using a laboratory-scale XUV source, we have now measured the time scale of these electron-ion recombination processes leading to a strong formation of excited atoms, which is in the picosecond range. The results show that it is even possible to follow the laser-induced cluster expansion process up to nanosecond times.

The formation of a large number of charges in a cluster by an intense laser pulse can lead to the generation of a transient nanoplasma consisting of free electrons and ions. In the past, fascinating processes could already be observed in nanoplasmas, including nuclear fusion or the creation of neutral atoms with very high kinetic energies. While nanoplasmas are routinely generated during the interaction of clusters with intense XUV pulses from free-electron lasers, a detailed understanding of the processes inside the plasma is challenging. Theoretical models have predicted that the majority of electrons remains trapped in the cluster and may eventually recombine with ions such that both transient species cannot be observed in usual experiments. However, an experimental investigation of these dynamics is crucial, since processes in clusters are complex and manifold, and their detailed prediction is difficult. A promising route towards a better understanding of the different mechanisms in nanoplasmas is the development of time-resolved experiments. In this context, intense high-order harmonic generation (HHG) sources that can deliver light pulses down to the attosecond regime are very interesting. This laboratory-scale XUV sources provide a straightforward way to carry out pump-probe experiments on clusters and can significantly improve the possibilities for the understanding of cluster dynamics.

In an international collaboration led by researchers from the Max-Born-Institut, the first pump-probe experiment on clusters using an intense HHG source was now performed. In the current issue of Physical Review Letters [112, 253401 (2014)] Bernd Schütte, Marc Vrakking and Arnaud Rouzée and their colleagues Filippo Campi from the University of Lund and Mathias Arbeiter and Thomas Fennel from the University of Rostock present the results of these investigations. The development of a technique allowing the Reionization of Excited Atoms from Recombination (REAR) makes it possible for the first time to infer information on ion charge states prior to recombination. By using near-infrared (NIR) probe pulses, a surprisingly extensive formation of excited atoms was observed and could be shown to originate from recombination between electrons and ions. It was demonstrated that in the nanoplasma electrons released by means of photo-ionization only remain quasi-free for a short time up to 10 picoseconds before they undergo a recombination process with the surrounding ions. More information about these processes was obtained by generating special clusters that consist of a xenon core and an argon shell. These investigations showed that recombination preferentially takes place in the xenon core of the cluster. The wavelength of the ionizing pulse interacting with the cluster was shown not to be important: excited atom formation attributed to recombination processes was also observed when using NIR or blue pump pulses instead of XUV pulses. This demonstrates the general implications of the current findings for the explanation of previous experiments carried out in different wavelength regimes. Moreover, the cluster expansion dynamics could be traced up to the nanosecond range by using the REAR technique.

Our results show the remarkable versatility of intense HHG pulses for the study of dynamic processes in clusters. In the future, the investigation of other extended systems like biomolecules will benefit from the availability of these laboratory-scale XUV light sources.

Original publication:
Bernd Schütte, Filippo Campi, Mathias Arbeiter, Thomas Fennel, Marc J. J. Vrakking and Arnaud Rouzée:
"Tracing electron-ion recombination in nanoplasmas produced by extreme-ultraviolet irradiation of rare-gas clusters", Physical Review Letters 112.253401,(2014)


Figure 1 (click to enlarge)

Fig. 1: (a) Two dimensional electron momentum map, showing the momentum distribution of the ejected electrons along (vertical) and perpendicular (horizontal) to the XUV/NIR laser polarization, after XUV ionization and NIR probing of argon clusters with an average size of 3500 atoms. The ring structure corresponds to the ionization of excited atoms by the NIR pulse. (b) The corresponding kinetic energy spectrum shows a peak at an energy of 0.6 eV that results from single-photon NIR ionization of the 4d and 5p excited states of argon.



Fig. 2: Time-dependent Xe+ ion yield after XUV ionization of mixed clusters consisting of a xenon core and an argon shell. An NIR pulse at two different intensities is used for probing. At an intensity of 2x1013 W/cm2, the Xe+ ion yield has a maximum at a delay of appr. 3 picoseconds, due to a well-known plasma resonance effect. At the lower intensity of 2x1012 W/cm2, the signal monotonically increases during the first 10 picoseconds, which is identified as the time scale of electron-ion recombination.


Figure 2 (click to enlarge)

Dr. Bernd Schütte Tel: 030 6392 1248
Prof. Marc J.J. Vrakking Tel: 030 6392 1200
Dr. Arnaud Rouzée Tel: 030 6392 1240


AC/DC for terahertz waves - rectification with picosecond clock rates

09th April 2014

Researchers at the Max-Born-Institute in Berlin, Germany discover an ultrafast rectifier for terahertz radiation. In the unit cells of a lithium niobate crystal alternating currents (AC) with a frequency 1000 times higher than that of modern computer systems are transformed into a direct current (DC), thereby generating simultaneously a series of overtones of the terahertz radiation.

When the guitarist Angus Young of the Australian hard rock band AC/DC touches the strings of his electric guitar, a strongly distorted sound rings out from the loudspeaker. The origin of the electronically generated overtones is the rectifying effect in the electronic tubes of the guitar amplifier. In the simplest case an (A)lternating (C)urrent generates a (D)irect (C)urrent, an effect which finds its application in telecommunications at much higher radio or mobile phone frequencies. From a physics point of view the highly interesting question arises: up to which cut-off frequencies can one generate directed currents (DC) and which microscopic mechanisms underlie them?

For the generation of a direct current out of alternating currents the material used must feature a preferred direction. This condition is fulfilled by ferroelectric crystals, in which the spatial separation of positively and negatively charged ions is connected to a static electric polarization. Most ferroelectrics are electric insulators, i.e., low electric fields cannot cause any detectable electric currents in the material. A drastic change of this behavior is observed if one applies for a short period an extremely high electric field in the range of several 100.000 volts per centimeter. At such field strengths, bound electrons, the so called valence electrons can be freed for a short period by means of the quantum mechanical tunneling process leading in turn to a current through the crystal.

Now, researchers at the Max-Born-Institute in Berlin, Germany investigated the properties of such a current for the first time and report their results in the current issue of the journal Physical Review Letters 112.146602 (2014)). Using ultrashort, intense terahertz pulses (1 Terahertz = 1012 Hz, period of a field oscillation 1 picosecond=10-12 seconds) they applied an AC field to a thin lithium niobate (LiNbO3) crystal which causes an electric current in the material. The properties of this current were studied in detail by measuring and analyzing the electric field radiated by the accelerated electrons. Besides an oscillating current with the frequency of the applied terahertz field (2 THz) and several overtones of the latter, the researchers observed the signature of a directed current (DC) along the c-axis the preferred direction of the ferroelectric LiNbO3 crystal.

The rectified current along the ferroelectric c-axis has its origin in the interplay of quantum mechanical tunneling of electrons between the valence and several conduction bands of the LiNbO3 crystal and the deceleration of electrons by friction processes. The tunneling process generates free electrons which in absence of friction would spatially oscillate in time with the applied terahertz field. The friction destroys this oscillatory motion, a mechanism called decoherence. Due to the asymmetry of the tunneling barrier along the ferroelectric c-axis decoherence results in a spatially asymmetric transport, i.e., the tunneling barrier lets pass more electrons from right to left than from left to right. This mechanism is operative within each unit cell of the crystal, i.e., on a sub-nanometer length scale, and causes the rectification of the terahertz field. The effect can be exploited at even higher frequencies, offering novel interesting applications in high frequency electronics.

Original article:
C. Somma, K. Reimann, C. Flytzanis, T. Elsaesser, und M. Woerner:
High-Field Terahertz Bulk Photovoltaic Effect in Lithium Niobate
Physical Review Letters 112.146602 (2014)

LiNbO3 Figure 1 (click to enlarge)

Experiment: The high electric field of the intense terahertz pulse accelerates electrons in a lithium niobate LiNbO3 crystal. The hexagonal unit cell contains lithium atoms (green spheres), niobium atoms (blue spheres), and oxygen atoms (red spheres) the latter being arranged on the corners of a unit cell. The crystal lacks inversion symmetry and, thus, shows a ferroelectric polarization along the c-axis.


LiNbO3ACDC During transport along the c-axis, electrons see alternating different distances between lithium and niobium atoms. Moreover, the niobium atoms are not in the center of the oxygen octahedrons. Such geometry leads to asymmetric barriers the electrons have to pass by quantum mechanical tunneling when moving along the c-axis. The electrons are driven through the barriers by the high terahertz AC field. The barrier asymmetry together with decoherence/friction processes result in a spatially asymmetric transport, i.e., the rectification to a DC current.
Figure 2 (click to enlarge)  
Movie Asymmetric tunneling probability through an asymmetric barrier under the action of decoherence mechanisms. The barrier (black curve) transmits more electrons shown as red wavepacket from left to right than in the opposite direction. The main part of the wavepacket is reflected from the barrier and a small part marked by the diamond is transmitted. The amplitude of the transmitted part depends on the propagation direction. The blue curves show the wavepacket motions in absence of any barrier.

Dr. Michael Woerner Tel: 030 6392 1470
Carmine Somma Tel: 030 6392 1474
Prof. Dr. Thomas Elsaesser Tel: 030 6392 1400


Acceleration of atoms in an intense standing light wave

21st March 2014

Laser induced strong-field phenomena in atoms and molecules on the femtosecond (fs) time scale have been almost exclusively investigated with traveling wave fields. In almost all cases, approximation of the strong electromagnetic field by an electric field purely oscillating in time suffices to describe experimental observations. In this approximation, momentum transfer from the light field to the center of mass of the atom, which results in a deflection, is not possible. Scientists at MBI have succeeded in deflecting Helium atoms in an intense standing wave of extremely short duration. Those He atoms that survive the interaction with the intense standing wave are accelerated, as a consequence of the strong stationary intensity gradient. The experimental results can be explained only, if one takes into account the intensity gradient as well as the generated magnetic fields.

The fundamental process of diffraction or deflection of atomic particles in a standing light wave was formulated for electrons as early as 1933 by the famous physicists Kapitza and Dirac. Electrons interact only weakly with a standing electromagnetic wave. Thus, it required intense lasers to observe the Kapitza-Dirac effect for electrons experimentally only 15 years ago. For the diffraction and deflection of atoms, on the other hand, much lower intensities are required, since resonant enhancement as well as the use of ultracold slow atoms reinforce the interaction strength. The process is of eminent importance in atom and quantum optics.

In the March 21 issue of Physical Review Letters, S. Eilzer, H. Zimmermann, and U. Eichmann published their scientific work on deflection of atoms in an intense standing light wave, which was generated by two counter propagating short laser pulses of 55 fs duration, see Fig. 1. Thereby, the atomic Kapitza Dirac effect was demonstrated in a laser intensity regime, in which the field amplitude as well as the field gradient were strong enough, to ionize atoms with a high probability. The phenomenon that ionization does not necessarily occur at these high intensities, and instead, the whole undestroyed atom is accelerated, has its origin in the frustrated tunneling process recently investigated at MBI. Although the electron quivers heavily in the laser field, it does not gain enough drift energy to finally overcome the attractive Coulomb potential of the ionic core. During the interaction with the laser pulse, the electron feels the ponderomotive force, which acts in the standing wave along the laser direction. Since the electron remains bound, the ponderomotive force accelerates the whole atom. In contrast to earlier investigation, where one observed deflection in the intensity gradient of a focused laser field, the intensity gradient in the standing wave is important on the atomic length scale rather than on the length scale of the focused laser beam and might influence the atomic dynamics profoundly, see Figure 2. Consequently, a thorough theoretical understanding requires to consider the electron dynamics as well as the center of mass motion, which represents a challenge to current strong-field theory. It is planned in further experiments to investigate more closely this new regime of strong field physics.

S. Eilzer, H Zimmermann und U. Eichmann
Strong-field Kapitza-Dirac scattering of neutral atoms
Phys. Rev. Lett . 112, 113001 (2014).

We show a sketch of the experimental setup. A well collimated beam of Helium atoms is exposed to a standing wave generated by two counterpropagating laser pulses for about 40 fs and with a spatial extension of about 20 µm. The deflection of the atomic beam is measured with a position dependent detector, whereby each atom hitting the detector is recorded individually. Without the accelerating forces of the standing wave, the detector signal would only be a small spot, as indicated by the dashed circle. In fact, we obtain a broad stripe, as visible on the detector image. How can one think of the acting forces? Atoms in a traveling light field, where the electromagnetic fields are periodically build up in space and time, experience no accelerating forces. This is like a floating object that see-saws on a water wave, but essentially does not move horizontally. However, if one freezes the wave crests and troughs, as it is the case in a standing light wave, where the electromagnetic fields are momentarily fixed in space, it is easy to imagine that our floating object is sliding down the hill. Here, the accelerating force is gravity, in the atomic case, it is the ponderomotive force.


Figure 1 (click to enlarge)

The red and blue curves show velocity distributions, obtained from the measured deflections, for standing waves generated by two elliptically polarized counterpropagating laser pulses with ellipticity ∈=0.85 and ∈=0.6, respectively. Investigations on the dependency of the deflection on the laser intensity have revealed for the first case that the maximum deflection is limited by ionization of the atoms above a certain intensity gradient. Due to the different polarization in the second case, atoms can be deflected twice as high without being ionized. This might stem from the dynamical processes that suppress ionization. Although a quantitative understanding of the process has been achieved, as can be seen by the fitted black curve, a quantitative confirmation by quantum mechanical strong-field calculations is still pending.
Figure 2 (click to enlarge)  

S. Eilzer, Tel: 030 6392 1341
U. Eichmann Tel: 030 6392 1371

Is there any quantum mechanics in laser-induced double ionization?

21st February 2014

The multi-electron dynamics in atoms and molecules is a great challenge in contemporary quantum physics, with implications from chemical reactions to superconductivity. A classical example, much investigated over the last two decades, has been the so-called "nonsequential" double ionization (NSDI) of atoms by an intense laser field, where "nonsequential" means that ionization does not happen one step at a time, viz. ionization of the atom later followed by ionization of the positively charged ion, but rather in one combined process. A precondition for this to happen is electron-electron correlation. Ionization appears as a paradigmatic quantum process (according to some textbooks proving the quantum nature of light). Nevertheless completely classical simulations, as they could have been carried out before the advent of quantum mechanics, have been able to describe, at least qualitatively, all experimental observations of NSDI thus far. If this were true in a broader context, it would have enormous implications, since classical simulations are so much faster and simpler compared with quantum simulations. We have now published a calculation that ascertains that the classical realm has its limits. Namely, we present a calculation of an effect -- a qualitative change in the electron-electron momentum correlation with decreasing laser intensity -- that is caused by a truly quantum-mechanical phenomenon, interference of the contributions of several different pathways.

Nonsequential double ionization (NSDI) is thought to proceed as a three-step process: first, an electron is liberated by tunneling, which may then be driven by the laser field into a recollision with its parent ion, which frees the second electron. This can happen via two different mechanisms: in a direct electron-electron collision if the energy of the recolliding electron is sufficiently high (so called "recollision impact ionization", RII) or indirectly, if the recolliding electron promotes the still bound electron to an excited state from which the latter tunnels out at a later time (so called "recollision excitation with subsequent ionization", RESI). The RESI mechanism is sketched in Fig. 1. RII and RESI lead to different correlation patterns of the momenta of the two final electrons. In diagrams such as those in Fig. 2, where the distribution of the momenta of the two electrons is plotted (more precisely of their components parallel to the laser polarization), RII tends to occupy the first and the third quadrants (the electrons are emitted side by side) while RESI populates all quadrants about equally (back-to-back emission is about as likely as side-by-side). Now, experimentally it was found that in argon with the laser intensity decreasing below the RII threshold back-to-back emission becomes dominant [Liu et al., Phys. Rev. Lett. 101, 053001 (2008)].

Theoretical investigations of NSDI have essentially followed three different routes: quantum-mechanical model calculations within the "strong-field approximation," completely classical simulations solving Newton's equations of motion, and semiclassical modeling where the first electron is liberated by tunneling and subsequently both electrons are treated classically [for a review, see, e.g., Rev. Mod. Phys. 84, 1011 (2012)]. Remarkably, the latter two approaches, containing little or no quantum mechanics at all, so far have successfully reproduced the experimental data.

In a recently published paper, quantum-mechanical calculations are presented that take into account that for argon several different excited states of the positive argon ion may play a role in the RESI mechanism. Their contributions must be added coherently. If this is done, then the momentum-momentum correlation changes to predominant back-to-back emission when the laser intensity is reduced below the RII threshold intensity as shown in Fig. 3, in good agreement with the data. Discrete excited states do not exist in a semiclassical or completely classical framework, and interference is outside that scope as well. Hence, we may have finally discovered a genuine quantum effect in nonsequential double ionization.

Reference: "Quantum effects in double ionization of argon below the threshold intensity," XL. Hao, J. Chen, WD. Li, B. Wang, X. Wang, and W. Becker, Phys. Rev. Lett. 112, 073002 (2014).


Fig 1

Fig. 1: A sketch of the RESI (rescattering excitation with subsequent ionization) mechanism. The oblique red straight line represents the interaction potential of the electron-field interaction at a time when the electron is pulled to the right (half a period later the electron is pulled into the opposite direction). The blue line represents the "effective binding potential." It is the sum of the former and the potential that binds the second electron to the doubly positively charged ion. The recolliding first electron, which is represented by the horizontal green arrow, may excite the second electron from its ground state to one of several excited states (vertical green arrows). From these states, the electron is eventually freed by field ionization (along the horizontal dashed line).


Figure 1  
Fig. 2 Fig. 2: Schematical sketch of the electron-electron momentum distributions expected from different scenarios. The abscissa and the ordinate specify the components of the electron momentum parallel to the laser polarization, and the density represents the number of events for given momenta. The remaining components are not detected or summed over. Panel (a) shows the distribution expected when the two electrons are uncorrelated. Panels (b) and (c) are clearly betray some correlation. Panel (b) indicates side-by-side emission, as generated by the RII mechanism. The distances of the two blobs to the origin are related to the momentum imparted by the laser field to each electron after the recollision while the diameter of a blob derives from the energy of the recolliding electron. Panel (c) shows a possible distribution in the RESI case: the second electron may depart a half cycle (or an integer number of half cycles) after the first so that the two electrons move away in opposite directions.
Figure 2  
Fig. 3

Fig. 3: False-color plots of the calculated distribution of the longitudinal (parallel to the laser polarization) electron momenta in nonsequential double ionization of argon at 800 nm and different laser intensities: (a) and (b) 4 x 1013 W/cm2; (c) and (d) 7 x 1013 W/cm2; (e) and (f) 9 x 1013 W/cm2. In the panels of the upper row, the contributions of the various excited states of the Ar+ ion (two of them are represented in Fig. 2) are added incoherently, in the lower row they are added coherently with the calculated phases. Comparison of the first and the second row illustrates the dramatic effect of quantum-mechanical interference.

Figure 3  

Contact: Dr. Wilhelm Becker, Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy


secrets of exploding clusters

21st February 2014

The investigation of cluster explosion dynamics under intense extreme-ultraviolet (XUV) pulses has so far been limited to large scale facilities likefree-electron lasers. In a recent publication it was shown that the research on clusters is now also possible with intense XUV pulses obtained in a laboratory-scale environment with a newly developed light source that makes use of the high-order harmonic generation process. For the first time, the formation of high-lying Rydberg atoms by electron-ion recombination during the cluster expansion initially triggered by an intense XUV pulse was identified, giving new insight into the cluster dissociation process.

An intense light pulse interacting with a weakly bound van der Waals cluster consisting of thousands of atoms can eventually lead to the explosion of the cluster and its complete disintegration. During this process, novel ionization mechanisms occur that are not observed in atoms. With a light pulse that is intense enough, many electrons are removed from their atoms that can move within the cluster, where they form a plasma with the ions on the nanometer scale, a so called nanoplasma. Due to collisions between the electrons, some of them may eventually gain sufficient energy to leave the cluster. A large part of the electrons, however, will remain confined to the cluster. It was theoretically predicted that electrons and ions in the nanoplasma recombine to form Rydberg atoms, however, an experimental proof of this hypothesis is still missing. Previous experiments were carried out at large scale facilities like free-electron lasers that have sizes from a few hundred meters to a few kilometers showing already surprising results such as the formation of very high charge states when an intense XUV pulse interacts with the cluster. However, the accessibility to such sources is strongly limited, and the experimental conditions are extremely challenging. The availability of intense light pulses in the extreme-ultraviolet range from an alternative source is therefore important to gain a better understanding of the various processes occurring in clusters and other extended systems such as biomolecules exposed to intense XUV pulses.

Scientists from the Max-Born-Institut have developed a new light source that is based on the process of high-order harmonic generation. In the experiment, an intense pulse in the extreme-ultraviolet range with a duration of 15 fs (1fs=10-15s) interacted with clusters consisting of argon or xenon atoms. In the current issue of Physical Review Letters (Vol. 112-073003 publ. 20 February 2014) Bernd Schütte, Marc Vrakking and Arnaud Rouzée present the results of these studies, which are in very good agreement with previously obtained results from free-electron lasers: the formation of a nanoplasma was inferred by measuring the kinetic energy distributions of electrons formed in the cluster ionization process, showing a characteristic plateau up to a maximum kinetic energy given by the kinetic energy resulting from photoionization of an individual atom. In collaboration with the theoreticians Mathias Arbeiter and Thomas Fennel from the University of Rostock, it was possible to numerically simulate the ionization processes in the cluster and to reproduce the experimental results. In addition, by using the velocity map imaging technique, a yet undiscovered distribution of very slow electrons was observed and attributed the formation of high-lying Rydberg atoms by electron-ion recombination processes during the cluster expansion. Since the binding energies of the electrons are very small, the DC detector electric field used in the experiment was strong enough to ionize these Rydberg atoms, leading to the emission of low energy electrons. This process is also known as frustrated recombination and could now be confirmed experimentally for the first time. The current findings may also explain why in recent experiments using intense X-ray pulses, high charge states up to Xe26+ were observed in clusters, although a large number of recombination processes is expected to take place. Moreover, the opportunity to carry out this type of experiment with a high-order harmonic source makes it possible in the future to perform pump-probe experiments in clusters and other extended systems with a time resolution down to the attosecond range.


Fig. 1: Time-of-flight spectrum for xenon atoms and clusters with an average size of 36000 atoms. For clusters, larger fragments like dimers and trimers are observed. In comparison, the ratio of Xe2+/Xe+ is smaller for clusters than for atoms, which is attributed to recombination processes taking place in the cluster nanoplasma.

Figure 1 (click to enlarge)  
rogue-event Fig. 2: Left side: 2D electron momentum map of argon clusters with an average size of 3500 atoms showing a pronounced central distribution attributed to the ionization of Rydberg atoms with the detector field. Right side: The electron kinetic energy spectrum (black curve) shows a good agreement with numerical calculations, which are displayed for intensities of 5x1011 W/cm2 (red), 1x1012 W/cm2 (green) and 2x1012 W/cm2 (violet).
Figure 2 (click to enlarge))  


Original publication: Physical Review Letters

Full citation:
Bernd Schütte, Mathias Arbeiter, Thomas Fennel, Marc J. J. Vrakking and Arnaud Rouzée, "Rare-gas clusters in intense extreme-ultraviolet pulses from a high-order harmonic source", Physical Review Letters 112, (2014)


Dr. Bernd Schütte, 030 6392 1248
Prof. Marc J. J. Vrakking, 030 6392 1200
Dr. Arnaud Rouzée, 030 6392 1240


Dr. Martin Hempel receives the Adlershof Dissertation Prize 2013

14th February 2014

On February 13, the Adlershof Dissertation Award was granted to Dr. Martin Hempel of the Max-Born-Institute (MBI). In competition with two other nominees and according to the jury's unanimous opinion, he presented best his dissertation work "Defect Mechanism in Diode Lasers at High Optical Output Power: The Catastrophic Optical Damage".

As a student, Martin Hempel was fascinated by research on novel semiconductor diode lasers. As a PhD student at MBI, he investigated the ultimate performance limits of these universal devices. His research was focused on the so-called Catastrophic Optical Damage (COD), a defect mechanism that affects semiconductor diode lasers at very high optical intensities. A volume of ∼1 µm3 of the laser crystal is heated up to ∼1600°C by re-absorbed laser light. This temperature jump takes place in just 1 ns. Subsequently, the COD defect expands in the laser cavity with a velocity of about 90 km/h. The entire diode laser is degraded after a few µs. The final COD defect volume is approx. 1000 times larger than the initial COD starting point. Hempel was able to resolve the spatio-temporal dynamics of the COD process by a combination of methods. His results provide new insight into the physical mechanisms behind the COD. Based on this knowledge, it is possible to identify the root causes for device failures in an efficient and fast manner. In close cooperation with manufacturers of diode lasers this makes it possible to increase output power and reliability of the devices.


About the prize


Each year since 2002, IGAFA, Humboldt University and WISTA-MANAGEMENT GmbH jointly award the Adlershof Dissertation Prize to young scientists for PhD research which was conducted at one of the research institutions in Adlershof and got an excellent grade of the scientific quality of their work. Three selected nominees present their dissertation work in form of short talks to a general audience. The jury then makes the decision which of the finalists not only conducts excellent research but also explains complex topics enthusiastically and in simple terms.

Figure 1 (click to enlarge)  

COD defect pattern in the active layer of a diode laser (grey shaded area). The thermal signature of the COD defect front was mapped temporally resolved by a thermocamera. This was done by monitoring the side of the laser cavity (along z axis) and the front facet (along x) in parallel. The center of gravity of these signals, seen form two sides of the device, gives the position of the defect front (black dots). Thermography is demonstrated to be a tool to resolve the motion of the COD defect front temporally and spatially at once. In the upper part, thermal images are shown, record at different times while looking on the front facet. The corresponding positions in the x-z-diagram are marked. The constant signal amplitude indicates a constant temperature during the entire COD defect growth. Evaluating the temporal and spatial data gives a velocity of ∼90 km/h for the movement of the defect front along the laser cavity.

Figure 2 (click to enlarge)  
rogue-event Electron microscopy image of a device damaged by COD. The front facet is indicated by the dotted line on the left side. The material extrusion is clearly visible there. The degradation of the active laser layer starts at the front facet and is expanded up to the right edge of the image. A crystallographic interpretation of the changes of the material composition in the damaged volume indicates the presence of a temperature around 1600°C
Figure 3 (click to enlarge))  


Dr. Martin Hempel, +49 (0) 30-6392 1453