Attosecond laser pulses see molecules vibrate for the first time
Researchers at the Foundation for Fundamental Research on Matter Institute for Atomic and Molecular Physics (FOM Institute AMOLF), in cooperation with groups in Milan (Italy), Lund (Sweden) and Paris (France), have used laser pulses lasting only a few hundred attoseconds to observe and study vibrations in hydrogen, the fastest vibrating molecule. By using these extremely short pulses (an attosecond is a billionth of a billionth of a second, short scale) a very accurate study can be made of how a second laser field influences the vibrations. The experiments show how an intense laser can control the outcome of a chemical process, using the pulse characteristics of the laser. At the same time, this is a warning to researchers who use intense laser fields to study the structure and dynamics of molecular systems, given that the laser can affect their results. The results of this study were published in the reputable journal, Physical Review Letters, on 17 September 2009.
In the experiment, attosecond laser pulses were used to initiate vibration and the vibrating molecule was subsequently exposed to a second laser field. A vibrating molecule in a strong laser field behaves differently from usual: the vibrations are slower and have less energy. If one varies the characteristics and the deceleration of the second laser field, it becomes clear how, and how fast, the molecule adapts to the laser field. The researchers demonstrated that there are two aspects to the use of intense laser. Not only does it enable the researchers to investigate the way in which the molecule falls apart under the influence of a laser field, but it causes molecules in strong radiation fields to behave differently than they would outside a laser field in the natural situation.
Attosecond lasers
The shortest possible pulse duration of a laser field has become shorter and shorter since the first laser pulses were made. The current world record is a pulse with a duration of only 80 attoseconds (10-18 s). High harmonic generation makes it possible to produce such short pulses. This is a process in which the frequency of the laser pulses is multiplied many times, consequently shortening the related wavelength. Researchers subsequently use these pulses to look at processes that are so fast that longer pulses simply cannot register them sharply. These are often processes involving electron movements. The first prototype experiments have now been carried out in atoms and on surfaces. In this case, Kelkensberg and his colleagues have shown that attosecond pulses are also eminently suitable for studying the vibrations of the fastest molecule that exists: hydrogen.
Hydrogen
There is a rich history of research on the hydrogen molecule in strong laser fields. Much of what is known about how molecules behave in a strong laser field originated from this research. Hydrogen is the smallest and simplest molecular system there is, but with two atoms. This is a big advantage because it enables detailed and exact theoretical simulations, unlike the larger, more complex molecules for which simulations are only possible in certain approaches. In this study, the researchers used exact simulations to enable them to interpret the observations.
This experimental research is a European joint venture between the FOM Institute AMOLF, Lund University in Sweden and the Politecnico Milano in Italy, in the framework of a training programme subsidized by the EU in which young researchers are trained. The research was cofinanced by the FOM and the Netherlands Organisation for Scientific Research (NWO).
Reference
Molecular Dissociative Ionization and Wave-Packet Dynamics Studied Using Two-Color XUV and IR Pump-Probe Spectroscopy, F. Kelkensberg et. al., Phys. Rev. Lett. 103, 123005 (2009)
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000103000012123005000001&idtype=cvips&gifs=yes
Information
Marc Vrakking en Freek Kelkensberg, FOM Institute AMOLF, telephone + 31 (020) 608 12 34
Gabby Zegers, FOM Foundation, telephone + 31 (030) 600 12 08