Ionizing radiation can bring an atom to a super-excited state that survives for an infinitesimal time before emitting one electron. Using advanced attosecond interferometric techniques, several researchers from Lund, Madrid, Paris, and Stockholm have now resolved the collapse of such state in time, finally confirming a theoretical prediction formalized more than 50 years ago that has its roots at the very beginning of the quantum revolution.
When irradiated with light with sufficiently high frequency, such as extreme ultraviolet rays or x rays, matter emits electrons; this is the celebrated photoelectric effect. To explain this effect, in 1905 Einstein suggested that light was composed of photons, small packets of energy that provide individual electrons with enough momentum to escape the Coulomb attraction of atomic nuclei. Some twenty years later, quantum theory finally provided the theoretical framework necessary to formalize Einstein’s intuition: matter does exchange energy by quanta, and, in the simplest scenario, the absorption of an ionizing photon is accompanied by the instantaneous emission of an electron wave packet.
In 1933, H. Beutler [Z. Physik 86, 495], an experimental physicist, showed that things were not as simple after all. He found that the absorption spectra of rare-gas atoms above the ionization threshold exhibit wide peaks with anomalously asymmetric profiles. These peaks indicated that the atom, instead of being instantaneously ionized, could be temporarily excited to some unstable state, hence the finite width. The meaning of the peak asymmetry, however, remained obscure. The following year, Enrico Fermi, then a professor in Rome, assigned this puzzle to Ugo Fano, a 22 years old post-doctoral collaborator, who published the provisional results of his investigations in 1935 in an Italian journal. According to Fano, the asymmetry of photoelectron absorption peaks arise from the interplay between the direct-ionization wave front, predicted since the beginning, and the trailing emission from the collapsing excited atom. Due to the dispersive character of electron waves (more energetic electrons move faster than less energetic ones), the latter component catches up with the direct wave front, carving a trough where the two waves interfere destructively. It is this interference that skews the spectrum of the electrons from the decaying atom, which would otherwise be a perfectly symmetric Lorentzian peak.
Fano had to interrupt his studies of atomic photoionization due to the precipitating historical conditions that eventually lead to the outbreak of WWII. For more than 25 years, his 1935 paper, which was written in Italian, was all but forgotten. In 1961, Fano managed to resume his studies, publishing a second more refined English version of his work which soon became one of the most cited physics papers of all times. Since then, the characteristic asymmetric photoelectron energy distribution predicted by Fano’s theory has been confirmed for countless systems. One inherent aspect of the phenomenology modeled by Fano, however, that is how the direct and resonant wave packets come together in time to give rise to the final spectral asymmetry, had eluded any direct experimental confirmation for all this time.
In a paper published on February 18th in Nature Communication, researches from Lund University, XChem project/Universidad Autónoma de Madrid (UAM), Université Pierre et Marie Curie in Paris, and Stockholm University have announced that they have finally closed this long-standing gap. Using a novel attosecond interferometric laser technique developed in Lund by the experimental group of Professor Anne L’Huillier, the authors were able to compare in detail the structured wave front generated by the resonant ionization of the argon atom with the simpler direct-ionization wave obtained at energies where no metastable state is excited. Their finding closely matches the prediction of the Fano model, which researchers from XChem project have extended to the multiphoton regime entailed in the experiment. This finding, which is the first “observation” of the collapse of an autoionizing atomic state, opens the way to the detailed study of the ultrafast photoelectron dynamics, which plays a fundamental role in many processes triggered by energetic light in matter, from radiation damage of biological tissues to charge emission in photoelectric cells.
Spectral phase measurement of a Fano resonance using tunable attosecond pulses
M. Kotur, D. Guénot, A. Jiménez-Galán, D. Kroon, E.W. Larsen, M. Louisy, S. Bengtsson, M. Miranda, J. Mauritsson, C.L. Arnold, S.E. Canton, M. Gisselbrecht, T. Carette, J.M. Dahlström, E. Lindroth, A. Maquet, L. Argenti, F. Martín & A. L’Huillier
Nature Communications, 7, 10566. 18 de febrero de 2016.