Precisely helium: Single molecule calibration

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  • Published: Jun 15, 2017
  • Author: David Bradley
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thumbnail image: Precisely helium: Single molecule calibration

Beyond the realm

Schematic illustration of the alignment, induced by a 160 picosecond laser pulse (red), of an iodine molecule (purple) inside a helium droplet (blue). The iodine molecule is aligned vertically by the polarization direction of the alignment pulse, shown by the double-headed red arrow to the left. The degree of alignment is measured by a probe pulse (black) synchronized to the peak of the alignment pulse. (Credit: Henrik Stapelfeldt, Aarhus University)

Whether you get a reaction is almost always down to how you approach the situation. Same for molecules. A chemical reaction will proceed only once the molecules coming together, and the way they interact can depend on how they are aligned relative to each other. Now, researchers from Aarhus University in Denmark and the Institute of Science and Technology in Austria have developed a new technique for aligning molecules using lasers and ultracold droplets of helium so they can precisely measure alignment.

Writing in The Journal of Chemical Physics, a new technique for aligning molecules could offer a pin-sharp perspective on chemical reactions allowing essentially isolated molecules to be studied as they interact. Embedded in a droplet of helium at 0.4 Kelvin (-272.75 degrees Celsius) the molecules are at the same low temperature as their surroundings. It is rarely possible to take gas phase substances down to such a low temperature but this approach circumvents the issue of their becoming liquid or even freezing thus opening up and entirely new realm of study.

Pulse-probe

The team used a pair of laser pulses in a so-called pump-probe method. The first pulse aligns the single molecule once it has been deposited into the helium droplet. The second laser pulse, the probe pulse, is then used to determine how it is aligned by blasting it apart into separate ions and determining the angles at which the fragments fly off watched by a detectors coupled to a computer.

"Being able to control the alignment of large molecules is no simple feat," explains team member Henrik Stapelfeldt of Aarhus University, "because as molecules grow in size it becomes increasingly difficult to get them into the gas phase and cool them."

With this technique at their disposal, the team has studied three systems: iodine molecules, which have a simple linear dumbbell shape, and two more complex molecules 1,4-diiodobenzene and 1,4-dibromobenzene. In all three cases, the team could achieve strong alignment of a single molecule embedded in the cold helium droplet with the two-pulse technique. Iodine with its simple linear shape allowed the researchers to better compare their experimental data with theoretical predictions. This demonstrated to them that the laser-induced alignment of molecules in helium droplets was essentially identical to that in the gas phase, as long as the alignment was done adiabatically, or gradually with respect to the way in which the molecule responded. To carry out an alignment in this way required them to rotate the first laser pulse more slowly than the intrinsic rotational period of the molecule itself. This allowed them to align even a freely rotating iodine molecule with the axis of the polarization of the laser.

Study expansion

The next step will be to focus on aligning larger, more complex molecules in these cold helium droplets. This will allow the researchers to observe chemical reactions as they unfold in real time. Stapelfeldt explains that one day it might be possible to use the same technique to align molecules as large as proteins. "Helium droplets offer unique possibilities," he explains, "for building tailor-made molecular complexes, thus broadening the scope of systems that can be studied."

Related Links

J Chem Phys 2017, online: "Strongly aligned molecules inside helium droplets in the near-adiabatic regime"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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