Observing the birth of a spectral line

For the first time, physicists managed to observe in real time how an atomic spectral line emerges within the incredibly short time span of a few femtoseconds, verifying a theoretical prediction. This has been possible by applying a very fast temporal switch:

An intense laser pulse cuts off the natural decay shortly after excitation by a preceding laser pulse. The build-up of the asymmetric Fano line shape of two quantum-mechanically interfering electrons in the Helium atom is measured by varying the time delay between the two laser pulses.

In a classical picture, the electrons in an atom are allowed to revolve only on certain orbits around its nucleus – or in terms of quantum mechanics occupy certain orbitals or energy levels. Light may lift (excite) an electron into a higher orbit, if its energy (color) matches the energy difference of the orbitals.

That is why the atom only absorbs light of certain colors, called its absorption spectrum. In most cases, the single spectral lines are symmetrically shaped, but under some conditions asymmetric line shapes appear, which are termed Fano profiles.

The decay of doubly excited helium stands for such a case: One of the two excited electrons falls back to the lowest-energy (ground) state after a collision with the other electron, thereby kicking the latter out of the atom. As the free electron is no longer limited to discrete energy levels, physicists are speaking about the coupling of a discrete state to a continuum, a general phenomenon at work in many different processes in nature, and fundamentally at the interface of quantum (discrete energies) and classical (continuous energies) mechanics.

Theoretical calculations predicted that the corresponding Fano profile does not emerge instantaneously but takes some time to develop: In helium, the unfolding of the line shape takes a few femtoseconds – some millionths of a billionth of a second.

Recently, experimental physicists from the MPI for Nuclear Physics (MPIK, Heidelberg), together with theoretical physicists at the Vienna University of Technology and the Kansas State University succeeded to take a movie of an emerging Fano spectral line. To gain access to the short time scales, they used two ultrashort laser-controlled flashes of light. The first one in the extreme ultraviolet excites both electrons of the helium atom. Some femtoseconds later, the second, intense, near-infrared laser flash triggers ionization ahead of time, i.e., it cuts off the natural autoionization decay process.

Alexander Blättermann, postdoc in the group of Thomas Pfeifer at the MPIK, explains: “You may think of the excited helium atom as an oscillating dipole that produces the optical absorption line. The subsequent strong infrared pulse acts as a temporal switch stopping this oscillation before the line has fully built up.” By varying the time delay between the two laser pulses – this has been done with a precision of less than a femtosecond – the researchers tracked the evolution of the line shape in real time.

“The experimental results nicely show how the Fano profile builds up with increasing time delay”, says Andreas Kaldun who has recently moved from the MPIK to SLAC in Stanford. At very short time delays, the spectral line is completely smeared out to a broad and flat band. With increasing time delay, the dipole is granted more and more time to oscillate, thus the line narrows and steepens step by step and finally approaches the original Fano profile, in very good agreement with the theory prediction.

“Our results thus not only verify the prediction, but at the same time demonstrate the power of the applied time-gating method for the exploration of the origin and evolution of many different fundamental quantum processes that could thus far only be studied by interpreting their static absorption spectra”, concludes Thomas Pfeifer.

The study of such atomic processes by different experimental methods has always fueled the evolution of physics (e.g. the discovery of quantum mechanics in the past) and remains an active topic of international contemporary research. In the same issue of Science magazine, a completely independent team of scientists from France and Spain have used the complementary method of time-resolved photoelectron spectroscopy to obtain a view on the Fano resonance “from the outside” of the atom.

This was achieved by reconstructing the time-dependent outgoing quantum-mechanical electron wavepacket (DOI: 10.1126/science.aah5188). Together with the time-resolved view “from the inside” by the time-gated dipole approach as described above (DOI: 10.1126/science.aah6972), atomic physics continues to transform our perception of the building blocks of nature. On the long run, this understanding eventually leads to technological discoveries. Physics revolutions of the past brought the laser and x-ray sources, the future could bring laser-controlled chemistry and molecule-sized ultrafast computers and devices.

Original publication:
Observing the ultrafast build-up of a Fano resonance in the time domain, A. Kaldun, A. Blättermann, V. Stooß, S. Donsa, H. Wei, R. Pazourek, S. Nagele, C. Ott, C. D. Lin, J. Burgdörfer, T. Pfeifer, Science, 11 November 2016, DOI: 10.1126/science.aah6972

Contact:
Prof. Dr. Thomas Pfeifer, MPI für Kernphysik
phone: +496221 516380
e-mail: thomas.pfeifer@mpi-hd.mpg.de

Prof. Dr. Joachim Burgdörfer, Technische Universität Wien
phone:+ 43 1 58801 136 10
e-mail: joachim.burgdoerfer@tuwien.ac.at

Prof. Dr. Chii-Dong Lin, Kansas State University
phone: +1-785-532-1617
e-mail: cdlin@phys.ksu.edu

http://www.mpi-hd.mpg.de/mpi/en/pfeifer/pfeifer-division-portal/ Division Pfeifer at MPIK