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Photoemission Delay Time

August 23, 2010

Photoelectric effect has been known to us for a long time. Heinrich Hertz first observed this phenomena in 1887 where a material absorbs electromagnetic radiation and emits electrons. While most of details are clearly understood, one aspect which has been missing is the delay time in emission of electrons after the matter absorbs photon. If delay exists, does the delay also depend on the energy level from which electron is being emitted? Until now, it was assumed that the photo-emission process is instantaneous. Recent study by researchers at Max Planck Institute of Quantum Optics in Munich, Germany and group of other collaborators , has been able to precisely measure the delay time in photoemission process using ultra-short time measurement technology. They used Neon atoms to study the phenomena. Neon  is more complex than Helium, but it was chosen because Neon is simpler to model theoretically and it also has higher photo-ionization cross-section resulting higher Signal-to-Noise ratio. They found out that that electrons leave 2P level subshell   Formula attoseconds after the electrons have left 2S subshell.  The delaytime of 21 attoseconds between electron emission from 2S and 2P seem to be very small, but it’s an important result as it shows that the process is not instantaneous, as it was assumed until now, and there is certain delay involved possibly due to electron-electron interactions. Scientists also performed complex theoretical computations to calculate the delay time and they came with a delay time 0f 5 attoseconds. The discrepancy can be attributed to multi-electron Neon atom system which makes it very difficult to make accurate theoretical computations. For experiments, two ultrafast laser pulses were used: extreme UV pulse (<200 attosecond duration) was used to eject electrons from 2S and 2P subshells while near infra-red pulse was used for time resolved measurements.

“These to-date poorly understood interactions have a fundamental influence on electron movements in tiniest dimensions, which determine the course of all biological and chemical processes, not to mention the speed of microprocessors, which lie at the heart of computers”, explains Ferenc Krausz, co-author of the study from MPQ. “Our investigations shed light on the electrons’ interactions with one another on atomic scale“. [ Press Release]

The results of the study was published in Science (June 2010, Vol 328, pp 1658) and abstract is as follows:

Delay in Photoemission

Schultze et al.

Vol. 328. no. 5986, pp. 1658 – 1662
DOI: 10.1126/science.1189401

Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of Formula attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100–electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.

Photocredit: Press Release/ MPQ/ LMU/ T. Naeser/ C. Hackenberger

Filed under: Research,Science

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