When light is incident on photoelectric materials, electrons are ejected from the surface. Einstein offered a Nobel Prize winning explanation of this phenomenon in 1905 in which he talked of packets of energy, "light quanta". It is usually considered to be essentially an instantaneous effect, but it does take a finite amount of time for the material to absorb the light energy and for that to be transferred to an electron, which then makes the quantum leap from one state to another.
The photoelectric effect is critical to many modern devices such as photovoltaic solar panels, fiber optic communications, to name just two. Improving our understanding of the effect could ultimately lead to improvements in a wide range of applications.
New precision measurement techniques used by a team at the Vienna University of Technology, Austria, working with colleagues in Garching, Munich, and Berlin, Germany, has determined the length of time taken for the photoelectric effect to occur at a tungsten surface. [M. Ossiander et al. Nature (2018) 561 (7723): 374; DOI: 10.1038/s41586-018-0503-6]
"With the help of ultra-short laser pulses, it has been possible in recent years to gain for the first time insight into the timing of such effects," explains team leader Joachim Burgdörfer. "We were able to determine the time interval between different quantum jumps and show that different quantum jumps take different amounts of time," he adds. Until now, it was time differences, rather than absolute duration, that were the only accessible parameter in this process. The problem is starting the figurative stopwatch at the precise moment the quantum leap is initiated. A combination of experiments, computer simulations, and theoretical calculations as at last led to just such a breakthrough.
The team took a stepwise approach. In order to have an absolute, precisely calibrated reference scale, they had to study the electrons ripped out of helium atoms by laser pulses, something that is impossible with something as complicated as a metal surface. Helium atoms became the team's reference clock. In a second experiment, photoemission from helium and iodine was compared, to calibrates an "iodine clock". Finally, the researchers used their calibrated iodine clock to study the photoelectric phenomenon in a tungsten surface by depositing iodine atoms on a tungsten surface and hitting it with ultrashort laser pulses. The ultrashort laser pulse is triggers starts the iodine clock and simultaneously initiates the photoelectric effect.
"In tungsten, the duration of [the] process can be studied particularly well because the interface of the material can be defined very precisely there," explains team member Florian Libisch. "The tungsten surface is an excellent finish line for electron-time measurement."
The researchers thus found that the photoemission process depends on the initial state of the electrons and can range from 100 attoseconds for inner shell electrons to 45 attoseconds for conduction band electrons. The actual measurements were carried out at the Max Planck Institute for Quantum Optics in Garching while Vienna's Libisch, Burgdörfer, and Christoph Lemell undertook the theoretical work and computer simulations.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.