Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Light waves allow preferred bond breaking in symmetric molecules

08.05.2014

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

Contacts:

Prof. Ali Alnaser
Physics Department,
American University of Sharjah
PO Box 26666, Sharjah, United Arab Emirates
Phone: +97 / 165 152 340
E-mail: aalnaser@aus.edu

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
E-mail: Regina.de_Vivie@cup.uni-muenchen.de

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
E-mail: matthias.kling@mpq.mpg.de

Dr. Olivia Meyer-Streng
MPQ, Press & Public Relations
Phone: +49 (0)89 / 32 905 -213
E-mail: olivia.meyer-streng@mpq.mpg.de

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:
http://www.mpq.mpg.de

More articles from Life Sciences:

nachricht New risk factors for anxiety disorders
24.02.2017 | Julius-Maximilians-Universität Würzburg

nachricht Stingless bees have their nests protected by soldiers
24.02.2017 | Johannes Gutenberg-Universität Mainz

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

Im Focus: Three Magnetic States for Each Hole

Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

Stingless bees have their nests protected by soldiers

24.02.2017 | Life Sciences

New risk factors for anxiety disorders

24.02.2017 | Life Sciences

MWC 2017: 5G Capital Berlin

24.02.2017 | Trade Fair News

VideoLinks
B2B-VideoLinks
More VideoLinks >>>