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The Mössbauer effect allows recoilless resonant nuclear excitation through the interaction of nuclear states with the crystal lattice [1]. This enables energy exchange with vibrational modes (phonons), which can dress the nuclear transition and even allow resonant absorption at slightly detuned photon energies. This process is typically studied using nuclear inelastic scattering. While radiative decay has traditionally been used to probe these interactions, in many Mössbauer isotopes such as ⁵⁷Fe and ⁴⁵Sc, internal conversion (IC) dominates due to large IC coefficients. Several competing channels contribute to IC electrons, along with secondary Auger-Meitner emissions, each carrying specific signatures of the phonon response of the material [2-3]. Owing to their limited escape depth, IC electrons are particularly valuable for surface-sensitive studies. Traditional conversion electron Mössbauer spectroscopy, which relies on low-brilliance radioactive sources, offers limited performance. In contrast, synchrotron-based and free-electron laser sources now allow far more effective generation – and thus detection – of IC electrons, unlocking new possibilities for investigating thin films and ultrathin layers. While the phonon density of states (PDOS) is typically measured via fluorescence, it can also be explored through electron emission detection, providing a powerful tool for surface studies [4], and – more generally – investigate the energy exchange between electron and nuclear states.
In this work, we resonantly photo-excite the Mössbauer isotope ⁵⁷Fe with a 14.4 keV synchrotron source in order to investigate electron emission channels and their coupling to phonon modes. Experiments were conducted at the P01 beamline at the PETRA III synchrotron (DESY), utilizing an electron time-of-flight spectrometer to detect IC and Auger-Meitner electrons following the nuclear decay. Additionally, we employed photoelectron spectroscopy to measure the PDOS in ultrathin FeO layers. By firstly utilizing the synchrotron Mössbauer source setup, we obtained high-resolution nuclear resonant spectra of ⁵⁷Fe in both foil (see Figure 1a) and thin-film forms, clearly resolving the expected hyperfine sextet structure in both the X-ray absorption and conversion electron channels. Secondly, we used the available high-resolution monochromator to detune the photon energy off-resonance to enable the extraction of a phonon spectrum by detecting the photoelectrons (see Figure 1b). These results demonstrate the capability of combined nuclear and electronic spectroscopy techniques to probe nuclear-electron-phonon interactions in condensed matter systems. Beyond advancing our understanding of nucleus-phonon coupling and material properties, this research could also open new perspectives for the quantum control of energy exchange between electronic and nuclear degrees of freedom even in the time domain, with potential applications in various fields related to quantum technologies.
[1] R.L. Ingalls, “Application to Solid-State Physics. In: May, L. (eds) An Introduction to Mössbauer Spectroscopy”, Springer, Boston, MA, 104-119 (1971).
[2] W. Sturhahn, et al., “Electron emission from ⁵⁷Fe nuclei excited with synchrotron radiation”, Phys Rev B 53, 171 (1996).
[3] Yuri Shvyd’ko, R. Röhlsberger, O. Kocharovskaya, J. Evers, et al., “Resonant X-ray excitation of the nuclear clock isomer 45Sc”. Nature 622, 471 (2023).
[4] Taizo Kawauchi, Katsuyuki Fukutani, et al., “Observation of internal-conversion electrons induced by inelastic nuclear resonant scattering”. Applied Surface Science 256, 962-964 (2009).
