Nav: Home

Helium droplets offer new precision to single-molecule laser measurement

June 13, 2017

WASHINGTON, D.C., June 13, 2017 -- Chemical reactions necessarily involve molecules coming together, and the way they interact can depend on how they are aligned relative to each other. By knowing and controlling the alignment of molecules, a great deal can be learned about how chemical reactions occur. This week in The Journal of Chemical Physics, from AIP Publishing, scientists from Aarhus University in Denmark and the Institute of Science and Technology in Austria report a new technique for aligning molecules using lasers and very cold droplets of helium.

This new method aligns molecules more sharply than is possible for the essentially isolated molecules of those in the gas phase. This is due to the fact that a molecule embedded in a very cold droplet shares the same low temperature as the droplet itself, a mere 0.4 kelvins, or -272.75 degrees Celsius. It is only rarely possible to obtain such low temperatures for molecules in the gas phase, so this technique promises to open up a significant new regime for study.

The method utilizes a pair of laser pulses in what's called a pump-probe method. The first pulse aligns the single molecule once it has been deposited into a helium droplet. The second laser pulse, the probe pulse, is used to determine the alignment, blasting the molecule apart and separating it into ions. The ions fly off at specific angles and can be detected using a camera coupled to a computer.

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

The investigators studied three systems: iodine (I2) molecules, which have a simple linear dumbbell shape, and two more complex molecules consisting of benzene rings with either iodine or bromine atoms attached to the ring. In all three cases, they achieved strong alignment of a single molecule embedded in a cold helium droplet with the two-pulse technique.

Because I2 has a simple linear shape, the investigators were better able to compare their experimental results to theoretical predictions. This revealed 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 molecules' responses.

To carry out adiabatic alignment, the first laser pulse is turned on more slowly than the inherent rotational period of the molecule being studied. This allows a freely rotating iodine molecule, say, to strongly align with the laser's polarization axis, in much the same way that a compass needle aligns with the magnetic field of the Earth.

Future studies will focus on aligning larger, more complex molecules in these cold helium droplets, allowing scientists to watch chemical reactions unfold in real time. Stapelfeldt explained that it may be possible to align molecules as large as proteins.

"Helium droplets offer unique possibilities," he said, "for building tailor-made molecular complexes, thus broadening the scope of systems that can be studied."
The article, "Strongly aligned molecules inside helium droplets in the near-adiabatic regime," is authored by Benjamin Shepperson, Adam S. Chatterley, Anders A. Søndergaard, Lars Christiansen, Mikhail Lemeshko and Henrik Stapelfeldt. The article will appear in The Journal of Chemical Physics June 13, 2017 [DOI: 10.1063/1.4983703]. After that date, it can be accessed at


The Journal of Chemical Physics publishes concise and definitive reports of significant research in the methods and applications of chemical physics. See

American Institute of Physics

Related Laser Articles:

A laser for penetrating waves
The 'Landau-level laser' is an exciting concept for an unusual radiation source.
Laser light detects tumors
A team of researchers from Jena presents a groundbreaking new method for the rapid, gentle and reliable detection of tumors with laser light.
The first laser radio transmitter
For the first time, researchers at Harvard School of Engineering have used a laser as a radio transmitter and receiver, paving the way for towards ultra-high-speed Wi-Fi and new types of hybrid electronic-photonic devices.
The random anti-laser
Scientists at TU Wien have found a way to build the 'opposite' of a laser -- a device that absorbs a specific light wave perfectly.
Laser 'drill' sets a new world record in laser-driven electron acceleration
Combining a first laser pulse to heat up and 'drill' through a plasma, and another to accelerate electrons to incredibly high energies in just tens of centimeters, scientists have nearly doubled the previous record for laser-driven particle acceleration at Berkeley Lab's BELLA Center.
More Laser News and Laser Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...