Spectroscopy has taught us how the very precise measurement of resonance lineshapes gives insight into the structure of matter. However, as a time-integrated measurement, the spectral lines give only indirect information on the underlying electronic dynamics. The resonance width can be related to the timescale of the electronic excitation and relaxation, but, in the general case, this is not enough for accessing the details of the full dynamics that have to be recovered from advanced modeling. A typical case is the one of autoionizing resonances, where the system (atom, molecule, nanostructure) can be ionized either directly to the continuum or be trapped in a very excited state for a very short time (femtosecond) before reaching the continuum. The interference between the two channels results in an asymmetric lineshape, called Fano profile after the Italian theoretician Ugo Fano who first modeled this process. While the Fano profile has been extremely successful in analyzing the absorption lines measured in a wide variety of systems, the details on how the process unwraps in time have remained elusive, the ultrashort timescale at stake precluding direct time-domain investigations.
In the November 11 issue of Science magazine, two articles tackle the problem of watching the buildup of the helium 2s2p Fano resonance from two different perspectives: from the ‘inside’ and from the ‘outside’.
In the article entitled “Observing the ultrafast buildup of a Fano resonance in the time domain” (DOI: 10.1126/science.aah6972), 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 look at the autoionizing process from ‘inside’ the atom by measuring the time-dependent dipole response in transient absorption spectroscopy. The dipole response being determined by the electron dynamics close to the nucleus, it provides a detailed picture of what takes place ‘inside’ the atom undergoing autoionization. In this work, short bursts of XUV light around 60.15 eV trigger the dynamic buildup of the Fano resonance by inducing an oscillating dipole moment, which in turn gives rise to the optical dipole response of the transition. A time-delayed ultrashort infrared pulse is then used to strong-field ionize the system, interrupting the autoionization process. The measured time-gated dipole response shows how the absorption lineshape evolves from an initially broad distribution to the characteristically ‘narrow’ converged Fano profile.
In the article “Attosecond dynamics through a Fano resonance: Monitoring the birth of a photoelectron” (DOI: 10.1126/science.aah5188), another team composed of experimental physicists from the CEA-CNRS-Université Paris-Saclay (CEA-Saclay) and theoretical chemists and physicists at the Université Pierre et Marie Curie (UPMC-Paris) and Universidad Autónoma de Madrid look at the autoionizing process from ‘outside’ the atom by measuring the time-dependent outgoing wavepacket, i.e. by probing the photoelectron itself. Using spectrally resolved electron interferometry, they could measure the spectral amplitude and phase of the resonant wave packet. In this scheme, replicas obtained by perturbative two-photon transitions interfere with reference wave packets that are formed through smooth continua, allowing the full temporal reconstruction, purely from experimental data, of the resonant wave packet released in the continuum. In turn, this allows resolving the ultrafast buildup of the autoionizing resonance, revealing the decomposition of the process in two nearly consecutive steps governed by fairly different time scales: during the first 3 fs, the direct ionization channel dominates; then, the resonant path starts contributing as the doubly excited state decays in the continuum, resulting in interferences between the two channels that ultimately shape the celebrated Fano profile.
These two complementary studies illustrate the large potential of the diverse techniques developed in attosecond spectroscopy: detection of photons or electrons, time-domain versus frequency-domain measurements, strong-field vs. perturbative regime. They open multiple opportunities for studying ultrafast strongly correlated dynamics in a variety of systems, from molecules and nanostructures to surfaces, and controlling matter changes at a most fundamental level.
(left) Absorption spectra measured for a series of XUV-IR delays from 6 to 32 fs (from DOI: 10.1126/science.aah6972).
(right) Experimentally-retrieved photoelectron spectrum for accumulation times from -10 to 20 fs (step=1fs) (from DOI: 10.1126/science.aah5188)