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Ultrafast science

Ultrafast science

Helium atoms get the ride of their life

29 Oct 2009 James Dacey
Ponder the force Helium atoms accelerated at unprecedented rates

To the adrenaline junkie midway through a bungee jump, gravity must feel like it can accelerate matter at a spectacular rate. At the atomic scale, however, when it comes to shifting around neutral particles, gravity is incredibly ineffective compared with other fundamental interactions such as the strong and weak nuclear forces.

Now, however, a team of physicists in Germany has shown that a little-known interaction caused by electric fields known as the “ponderomotive” force can accelerate neutral particles at up to 1014 times the Earth’s gravitational acceleration. As well as being of interest to fundamental physics, this ability to transfer large amounts of momentum to neutral particles could lead to a host of novel applications in surface science, say the researchers.

‘Electrical pressure’

All students are taught at school that when objects possessing electrical charge are exposed to an electric field, these objects experience an electric force that can lead to motion. If the electric field is oscillating, however, then the charged object is then exposed to a second force that is proportional to the field-intensity gradient. Depending on the amount of matter and the scale of this intensity gradient, the ponderomotive force can have a significant effect.

Until now, however, physicists had assumed that the ponderomotive force would have a negligible effect on matter that is neutral. But, according to Ulli Eichmann and colleagues at the Max-Born Institute and the Institute for Optical and Atomic Physics in Germany, there is no reason for this to be the case. These researchers argue that the effect is largely independent of charge and they designed an experiment to demonstrate the magnitude of the effect on neutral matter.

The physicists began by aiming a beam of helium atoms at a detector, before firing a series of laser pulses at the beam so that individual atoms were exposed to a localized electromagnetic field. Then, by analysing data from their position-sensitive detector, they were able to show that at least one per cent of the helium atoms had undergone an acceleration, and in some cases this was as much as 1014 times that of the Earth’s gravitational acceleration (9.8 m s–1.

Like ants dragging a mountain

To explain the mechanism of this acceleration, Eichmann and colleagues refer to a model that they put forward in a paper last year. When the atoms are exposed to a laser pulse, an electron can gain energy from the laser field, causing it to be briefly “liberated” from the atom. However, this surge of energy is not sufficient for the electron to break free entirely from the Coulomb forces and it is recaptured so that it sits a long way from the nucleus in what is known as a “Rydberg state”.

It is in this state that the atom is subject to the ponderomotive force and the “quivering” electron can drag the entire atom in the direction of the localized electric field. Fortunately for the researchers, this state was long-lived enough for them to locate the positions of helium atoms at the detector and thus rule out other effects that could have caused a beam of neutral particles to be deflected and spread.

Eichmann told that he can envisage applications resulting from the “instantaneous” transfer of momentum to an atom. An example of this might be the accurate and efficient deposition of atoms on surfaces for optical applications. “Atoms may be steered by manipulating the spatial geometry of the laser fields,” he says.

Robert Jones, an atomic physicist at the University of Virginia in the US is impressed by the new research. “The possibility of controlled interactions between atoms or molecules through precisely timed collisions at well-defined relative velocities is particularly intriguing,” he told

This research is published in this week’s issue of Nature.

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