Observation and Control of Laser-Enabled Auger Decay

When isolated atoms are electronically excited, they have two possible ways of releasing electronic energy: by radiation or by Auger decay. The Auger process, in which the decaying electron transfers its energy to another electron causing it to detach (ionization), has played an important part in modern physics, particularly surface science, because it is by far the strongest decay channel for core holes of light elements such as carbon, nitrogen, and oxygen. In some cases, the Auger process is energetically forbidden, because the energy being exchanged is not sufficient for ionization. In this case, new electronic mechanisms for deexcitation may be discovered that “borrow” energy from the surroundings. One of these is interatomic Coulombic decay (ICD) where the energy is “borrowed” from surrounding atoms. Another mechanism is laser enabled Auger Decay (LEAD), where the energy is “borrowed” from an ancillary laser field; up to now LEAD has been observed with low-energy photons, meaning that more than one photon must be absorbed to make the process possible. Single-photon LEAD (spLEAD) has been theoretically predicted [B. Cooper and V. Averbukh, Phys. Rev. Lett. 111, 083004 (2013)], but not previously observed. The “single photon” aspect is far from marginal: the importance of spLEAD lies in the fact that in the simplest theoretical model of an atom (treating electrons as mutually independent), spLEAD is forbidden by symmetry arguments. Thus, the observation of spLEAD opens a window on the subject of electron correlation, a central one in chemistry, for example as related to charge migration in molecules. In this work we report the use of light pulses from the Free Electron Laser FERMI to achieve the first experimental observation ever of spLEAD. The experiments were performed at the LDM beamline of FERMI. The target consisted of neon atoms, and the process is shown in the left part of the title figure. The target signal is the intensity (photoelectron yield) of the peaks labeled as ¹S, ¹D, in Fig. 1: these are doubly ionized states, the fingerprint of an Auger process. Note however that the direct process is extremely weak and would be buried in noise. 

Figure 1. Photoelectron spectrum of neon atoms irradiated by FEL bichromatic pulses (ω=26.91 eV; 2ω=53.82 eV; dashed blue curve) and 2ω only (continuous red curve). - Reprinted figure with permission from [D. Iablonskyi, et al. "Observation and Control of Laser-Enabled Auger Decay", Phys. Rev. Lett. 119, 073203 (2017). http://dx.doi.org/doi: 10.1103/PhysRevLett.119.073203]. Copyright (2017) by the American Physical Society.

The novel method which we use consists of adding to the strong FEL pulse at the base frequency (ω) a weaker, phase-locked pulse at twice the frequency (2ω) and measuring the asymmetry in the direction of the outgoing electrons as a function of the relative ω–2ω phase (Fig. 2). The concept is analogous to that of homodyne detection, which in radio technology is used to single out and amplify a weak station by tuning in to its broadcasting frequency.

Figure 2. Fig. 2: Asymmetry of the distribution of outgoing photoelectrons for the target ¹S, ¹D peaks (see Fig. 1) versus relative ω–2ω phase; the ³P peak, which is not predicted by theory to oscillate, is also reported. ). - Reprinted figure with permission from [D. Iablonskyi, et al. "Observation and Control of Laser-Enabled Auger Decay", Phys. Rev. Lett. 119, 073203 (2017). http://dx.doi.org/doi: 10.1103/PhysRevLett.119.073203]. Copyright (2017) by the American Physical Society.

Our results resolve the outstanding problem of whether this spLEAD can be detected experimentally, and the novel method which we use, namely coherent control with fully coherent Free Electron Laser radiation, not only allows measurement but also control of this new phenomenon, as we demonstrate. Since our phase resolution corresponds to a few attoseconds, this opens up new prospects for time resolved experiments on this time scale, for example in the investigation of charge dynamics in molecules.

This research was conducted by the following research team:

D. Iablonskyi¹, K. Ueda^1,*, K. L. Ishikawa^2,3, A. S. Kheifets⁴, P. Carpeggiani⁵, M. Reduzzi⁵, H. Ahmadi⁵, A. Comby⁵, G. Sansone^5,6, T. Csizmadia⁷, S. Kuehn⁷, E. Ovcharenko⁸, T. Mazza⁸, M. Meyer⁹, A. Fischer¹⁰, C. Callegari¹⁰, O. Plekan¹⁰, P. Finetti¹⁰, E. Allaria¹⁰, E. Ferrari¹⁰, E. Roussel¹⁰, D. Gauthier¹⁰, L. Giannessi^10,11, and K. C. Prince^10,12

¹ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980-8577 Sendai, Japan
² Department of Nuclear Engineering and Management, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
³ Photon Science Center, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
⁴ Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia
Theoretische Chemie, Universität Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
⁵ Dipartimento di Fisica, CNR-IFN, Politecnico di Milano, 20133 Milan, Italy
⁶ Physikalisches Institut der Albert-Ludwigs-Universität, Stefan-Meier-Strasse 19, 79104 Freiburg, Germany
⁷ ELI-ALPS, Pintér József utca, 6728 Szeged, Hungary
⁸ European XFEL, Hamburg 22761, Germany
⁹ Max Planck Institute for Nuclear Physics, Heidelberg 69117, Germany
¹⁰ Elettra-Sincrotrone Trieste, Strada Statale 14 - km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italy
¹¹ ENEA C.R. Frascati, 00044 Frascati, Rome, Italy
¹² Molecular Model Discovery Laboratory, Department of Chemistry and Biotechnology, Swinburne University of Technology, Melbourne 3122, Australia

Carlo Callegari, email: