Fast Electrons on the Cheap

Phys. Rev. Focus 5, 15
According to computer simulations, atomic electrons can be accelerated close to the speed of light without top-of-the-line lasers if a magnetic field is applied along with a medium-energy laser.
Figure caption
R. Grobe & Q. Su/Illinois State Univ.
Ring around the proton. According to computer simulations, atomic electrons can be accelerated close to the speed of light without top-of-the-line lasers if a magnetic field is applied along with a medium-energy laser. With this scheme, the simulations predict that a hydrogen atom’s electron cloud can form a large ring.

Blast an atom with a souped-up, high-intensity laser, and you’re sure to see some bizarre effects. Laser physicists call the effects relativistic when the classical speed of an atomic electron responding to the strong laser field is near the speed of light. But in the 10 April PRL a team suggests a cheaper approach. According to their computer simulations, the common CO2 laser in combination with a strong magnet can produce the same relativistic conditions as high-end lasers. Their simulations also suggest that new effects should be observable with this scheme, including a ring-shaped electron cloud and a sawtooth-shaped resonance curve.

Thanks to recent developments in high-intensity, short-pulse laser technology, experimenters have been observing atoms reacting to extreme conditions in ways theorists hadn’t predicted. For example, an ultrashort infrared laser pulse can strip an electron from an atom and slam it back so violently that it emits a laser-like beam of x-ray photons (high harmonics) 200 times as energetic as the infrared photons–the shortest wavelength coherent radiation yet produced. Investigating strong laser-atom interactions is “a fishing expedition” for many unexplored phenomena, says Rainer Grobe of Illinois State University (ISU) in Normal, but experiments are always limited by ionization–atoms fall apart when pushed too hard.

Video courtesy of R. Grobe and Q. Su, Illinois State University.
This video shows the development of the ring shape for a hydrogen atom electron cloud under the influence of a laser and a magnetic field.

The problem is that the nucleus can’t easily hold onto an energetic electron, but Grobe and his colleagues realized that a strong magnetic field can help. The field keeps an electron from flying away no matter how far it strays from the nucleus because the field enforces a curved path for the electron. With the field tuned so that an electron’s orbital period (cyclotron frequency) matches that of the laser’s oscillating field, the team’s computer simulations showed that electrons could absorb huge amounts of energy within an atom with easily accessible laser intensities: A 1014 W/ cm2 laser along with a 10 T magnetic field pushed electrons close to the speed of light.

Using purely classical (and relativistic) mechanics–a quantum-mechanics-free approach that others have shown agrees with experiments–the ISU team simulated a hydrogen atom containing an electron cloud made of many point-particles and found some surprises. As the magnetic field is brought close to resonance with the laser, the electron’s speed increases, but the speed-vs-field curve has a saw-toothed shape, unlike the symmetrical resonance curve expected in the absence of relativity. There is even a region of the curve where the electron moves faster with relativity than it would under Newtonian mechanics–a contrast to most cases, where relativistic effects “slow down” an accelerating mass.

Another new and purely relativistic effect Grobe and his colleagues observed was a ring-shaped electron cloud that spins around the nucleus like a hula hoop around a gyrating child. The ring of charge forms near resonance and can be 500 nm across, 5000 times larger than the atom’s normal size. The laser-with-magnet scheme also appears to generate high harmonics, which are normally seen only with much more powerful lasers.

“It’s a brilliant piece of work,” says Misha Ivanov of Canada’s National Research Council in Ottawa. He is impressed that the team managed to discover and describe “very elegant physical effects”–ones that have been overlooked by others–without getting bogged down in complex calculations. Best of all, he says, “you don’t need a super, super laser” to observe them.

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Atomic and Molecular Physics

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