Metastable Helium: A New Determination of the Longest Atomic Excited-State Lifetime

2 August 2009

A finding of Australian National University researchers could hold the secret to more efficient lighting. A team from the Research School of Physics and Engineering have accurately measured the lifespan of the longest-lived excitable atom for the first time. Physicist Professor Ken Baldtvin said the find, published in the journal Physical Review Letters, was a culmination of more than a year of work by the team.

They determined that when helium atoms are excited by a high velocity electron collision, they remain excited for more than 8000 seconds, or over two hours. "Excited atoms, which are also known as metastable atoms, are an important source of stored energy in ionised gases that occur in the Earth and planetary atmospheres, as well as in lighting and laser technologies," he said. "It's this kind of ionised gas, or plasma, that's inside the compact fluorescent light bulbs and other fluorescent lights around your house. It's of fundamental interest to know exactly how long metastable atoms remain excited, but it also has implications for how we design better lighting systems in future."

The ANU team used lasers to control and isolate a cloud of metastable helium atoms from their surrounding environment. This allowed them to measure the rate at which the atoms emitted ultraviolet photons as they relax back to their normal, stable state. "[Our discovery] came about because we developed a way of isolating these helium atoms in a vacuum so that they don't interact or collide with any other particles," he said. "This means we can study them in great detail for a reasonably long period of time - a minute or so - which is unusual, because normally in the atmosphere molecules and atoms collide with other molecules and atoms on time scales of microseconds or nanoseconds. It is a very long time to have an atom isolated on its own. "The work we have done, which enables us to measure the lifetime of these atoms, is going to be useful for establishing more efficient light sources because in light bulbs or laser lights the atoms collide a lot in the discharge of electrons and get excited and you need to know how long the energy is retained by the atom in order to work out how efficient the lighting process is." Professor Baldwin said the finding was an example of how fundamental physics, which is the understanding of how an atom keeps energy, "has a practical use in designing more efficient things for everyday life". He said the team's finding was also in "excellent agreement" with the predictions of the theory of quantum electrodynamics, established in the 1940s and "arguably the besttested theory in modern physics". The research was funded by the Australian Research Council Centre of Excellence for Quantum-Atom Optics.

Taken from
"Find may lead to better lights"
BY EMILY SHERLOCK
Canberra Times
11/07/2009 10:13:00 AM