Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Light waves allow preferred bond breaking in symmetric molecules


An international team of scientists discovered a new quantum control mechanism to selectively shake and break C-H bonds in symmetric hydrocarbon molecules with the waveform of femtosecond laser pulses

Chemical bonds between carbon and hydrogen atoms are amongst the strongest in nature and their selective breaking, in particular in symmetric molecules, is of interest to chemical synthesis and the development of new biologically active molecules.

Figure 1: Illustration of the directional proton emission in acetylene with a specific laser waveform. The superposition of vibrational modes, which are responsible for the selective bond breaking, results from a combination of laser excitation of the anti-symmetric CH stretching mode and excitation of the symmetric CH stretching mode through ionization steps. The ionization steps are indicated by a change in colour from green (neutral) over yellow (cation) to orange (dication). (Graphic: Christian Hackenberger, MPQ, Attosecond Physics division)

An international team of scientists has now demonstrated that ultrashort light pulses with perfectly controlled waveforms can selectively break C-H bonds in acetylene ions. The researchers demonstrated that a suitable choice of the laser-pulse waveform leads to breaking of the C-H bond on the left (or right) side of the symmetric H-C≡C-H molecule. The scientists propose that their results can be understood by a new quantum control mechanism based on light induced vibration (Nature Communications, DOI:10.1038/ncomms4800).

Hydrocarbons play an important role in organic chemistry, combustion, and catalysis. Selective breaking of C-H-bonds, can further enable novel synthesis of molecular species with new functionalities and applications in medicine. Until now a method for breaking C-H bonds selectively in symmetric hydrocarbons did not exist.

Prof. Ali Alnaser (American University of Sharjah, UAE), who spent his sabbatical in the division of Prof. Ferenc Krausz at the Max Planck Institute of Quantum Optics (MPQ) as part of the collaboration between MPQ, the King Saud University (KSU), and the Ludwig-Maximilians-Universität Munich (LMU), and a team of physicists led by Prof. Matthias Kling (LMU) used ultrashort laser pulses to solve this problem.

An important ingredient in making the experiments successful was the use of a high repetition rate laser system with ten thousand pulses per second in the group of Prof. Ulf Kleineberg (LMU), whereby the measuring times could be reduced compared to so far available systems. Mechanistic insight into how the laser light interacts with the molecules is provided by a theoretical model developed in the group of Prof. Regina de Vivie-Riedle (LMU).

For their experimental studies, the researchers used acetylene (C2H2): In this molecule the two carbon atoms are strongly bound by three electron pairs, while the hydrogen atoms symmetrically terminate the linear molecule on both ends. The scientists exposed a supersonic jet of C2H2 molecules inside a so called reaction microscope to ultrashort laser pulses with duration of only 4 fs (1 fs = 10 to the minus 15 seconds). These pulses, generated in the Laboratory for Attosecond Physics of Prof. Ferenc Krausz (MPQ, LMU), have infrared wavelengths and consist of only a few cycles.

The waveform of the light waves was precisely measured (see figure) for each laser shot interacting with the molecules. “As a result of the interaction with the light wave, the molecule fragments after its double ionization into a positively charged C2H+ ion and a proton, which are both detected with the reaction microscope.”, says Prof. Ali Alnaser. Since acetylene is a symmetric molecule, the C-H bonds on both sides of the molecule typically break with equal probability. In their experiment however, the scientists found that the laser waveform provides a means to increase the probability that the left versus the right C-H bond breaks and vice versa (see figure).

Quantum dynamical simulations show the nature of the laser-molecule interaction. “The already known scheme, where molecular reactions are controlled by electron dynamics prepared with the light waveform via laser-induced coupling of electronic states, does not work in this case. We discovered a new quantum control pathway.”, Prof. de Vivie-Riedle explains. According to her new model, the few cycle laser pulse initially excites a subset of vibrations of the molecule that are laser-active.

One of these vibrations is the anti-symmetric stretching mode, where one CH bond is elongated while the other is shortened. When the laser pulse reaches its peak electric field, it removes an electron from the triple-bond of the CC group (the molecule ionizes). By this process additionally laser-inactive vibrational modes are populated. Among those modes is the symmetric CH stretching mode, where both H atoms move synchronously towards or away from the CC group.

In the remainder of the laser pulse, the freed electron is accelerated back onto the molecular cation, removes a second electron and creates the acetylene dication, which rapidly dissociates into the proton and the C2H+ ion that are observed in the experiment.

“Independent excitation of vibrations of the molecule is insufficient to explain the experimental results. A prerequisite for the observed control is a quantum effect: the superposition of the symmetric and anti-symmetric stretching modes. As a consequence of that interference, a situation can be created where only one CH bond vibrates and the other one remains frozen.”, explains Prof de Vivie-Riedle.

“This type of shaking of the molecule leads to breaking of a particular CH bond. The laser waveform controls the direction into which the vibrational wave packet, which results from the superposition of the vibrational modes, moves once it is created on the acetylene dication.”, adds Prof. Matthias Kling.

The researchers see the results of their studies as a proof-of-principle for a new quantum control mechanism. “The laser waveform control mechanism is very general and we foresee that it may be applied to other, more complex molecular processes.”, says Prof. Ali Alnaser, who wants to continue research into this direction. He adds: “While we have excited the vibrations non-resonantly

in our study, higher degrees of control can be reached with resonant excitation using ultrashort laser pulses in the mid-infrared. Such laser systems are currently being developed and pave the way to exploit the full potential of the new control scheme.” [MK/OM]

Original publication:

A.S. Alnaser, M. Kübel, R. Siemering, B. Bergues, Nora G. Kling, K.J. Betsch, Y. Deng, J. Schmidt, Z.A. Alahmed, A.M. Azzeer, J. Ullrich, I. Ben-Itzhak, R. Moshammer, U. Kleineberg, F. Krausz, R. de Vivie-Riedle, and M.F. Kling
Sub-femtosecond Steering of Hydrocarbon Deprotonation through Superposition of Vibrational Modes
Nature Communications, DOI:10.1038/ncomms4800, 8 May 2014


Prof. Ali Alnaser
Physics Department,
American University of Sharjah
PO Box 26666, Sharjah, United Arab Emirates
Phone: +97 / 165 152 340

Prof. Dr. Regina de Vivie-Riedle
Ludwig-Maximilians-Universität München
Department Chemie
Butenandt-Str. 11, 81377 Munich, Germany
Phone: +49 (0)89 / 2180 – 77 533

Prof. Dr. Matthias Kling
Laboratory of Attosecond Physics
Ludwig-Maximilians-Universität München,
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1, Garching, Germany
Phone: +49 (0)89 / 32 905 -234

Dr. Olivia Meyer-Streng
MPQ, Press & Public Relations
Phone: +49 (0)89 / 32 905 -213

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:

More articles from Life Sciences:

nachricht New Model of T Cell Activation
27.05.2016 | Albert-Ludwigs-Universität Freiburg im Breisgau

nachricht Fungi – a promising source of chemical diversity
27.05.2016 | Leibniz-Institut für Naturstoff-Forschung und Infektionsbiologie - Hans-Knöll-Institut (HKI)

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Worldwide Success of Tyrolean Wastewater Treatment Technology

A biological and energy-efficient process, developed and patented by the University of Innsbruck, converts nitrogen compounds in wastewater treatment facilities into harmless atmospheric nitrogen gas. This innovative technology is now being refined and marketed jointly with the United States’ DC Water and Sewer Authority (DC Water). The largest DEMON®-system in a wastewater treatment plant is currently being built in Washington, DC.

The DEMON®-system was developed and patented by the University of Innsbruck 11 years ago. Today this successful technology has been implemented in about 70...

Im Focus: Computational high-throughput screening finds hard magnets containing less rare earth elements

Permanent magnets are very important for technologies of the future like electromobility and renewable energy, and rare earth elements (REE) are necessary for their manufacture. The Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Germany, has now succeeded in identifying promising approaches and materials for new permanent magnets through use of an in-house simulation process based on high-throughput screening (HTS). The team was able to improve magnetic properties this way and at the same time replaced REE with elements that are less expensive and readily available. The results were published in the online technical journal “Scientific Reports”.

The starting point for IWM researchers Wolfgang Körner, Georg Krugel, and Christian Elsässer was a neodymium-iron-nitrogen compound based on a type of...

Im Focus: Atomic precision: technologies for the next-but-one generation of microchips

In the Beyond EUV project, the Fraunhofer Institutes for Laser Technology ILT in Aachen and for Applied Optics and Precision Engineering IOF in Jena are developing key technologies for the manufacture of a new generation of microchips using EUV radiation at a wavelength of 6.7 nm. The resulting structures are barely thicker than single atoms, and they make it possible to produce extremely integrated circuits for such items as wearables or mind-controlled prosthetic limbs.

In 1965 Gordon Moore formulated the law that came to be named after him, which states that the complexity of integrated circuits doubles every one to two...

Im Focus: Researchers demonstrate size quantization of Dirac fermions in graphene

Characterization of high-quality material reveals important details relevant to next generation nanoelectronic devices

Quantum mechanics is the field of physics governing the behavior of things on atomic scales, where things work very differently from our everyday world.

Im Focus: Graphene: A quantum of current

When current comes in discrete packages: Viennese scientists unravel the quantum properties of the carbon material graphene

In 2010 the Nobel Prize in physics was awarded for the discovery of the exceptional material graphene, which consists of a single layer of carbon atoms...

All Focus news of the innovation-report >>>



Event News

Networking 4.0: International Laser Technology Congress AKL’16 Shows New Ways of Cooperations

24.05.2016 | Event News

Challenges of rural labor markets

20.05.2016 | Event News

International expert meeting “Health Business Connect” in France

19.05.2016 | Event News

Latest News

11 million Euros for research into magnetic field sensors for medical diagnostics

27.05.2016 | Awards Funding

Fungi – a promising source of chemical diversity

27.05.2016 | Life Sciences

New Model of T Cell Activation

27.05.2016 | Life Sciences

More VideoLinks >>>