Decomposition of contributions from core-levels exhibiting spin-orbit splitting in XUV core-level spectroscopy

Abstract

Attosecond transient absorption spectroscopy (ATAS) is a versatile technique that allows observing ultrafast charge dynamics in solid-state samples. In ATAS, the transient absorption of a core-level excitation in the extreme UV (XUV) is measured following optical excitation of the sample. The lack of spin-selectivity in the XUV pulse results in overlapping XUV absorption spectra from spin-orbit split core-levels and leads to difficulty in disentangling the spectral signatures. Here, we demonstrate the successful retrieval of the contribution of a single spin-orbit level on the XUV transient absorption signal. Under the approxn. that the spin-orbit split levels yield identical absorption spectra which are energetically shifted and weighted by the degeneracy of the spin-orbit states, the contribution of one single spin-orbit state can be retrieved by the Fourier transform of the measured signal. In this case the energy shift of the spectra from the spin-orbit split states becomes a phase shift. By dividing out this phase factor and taking the inverse Fourier transform, the underlying signal referenced to a single spin-orbit level can be retrieved. We apply this method to an ATAS measurement at the germanium M_(4,5)-edge (30 eV), where the spin-orbit energy splitting (0.57 eV) is comparable to the germanium band gap (0.66 eV). The successful decompn. of the transient absorption signals yields clear, spectrally-resolved signatures of electrons and holes such that carrier dynamics can be simultaneously measured and characterized. The presented method allows decompg. contributions of spin-orbit split core-levels in a transient absorption spectrum. This allows clearer assignment of spectroscopic features and reveals weak signal contributions not visible in the exptl. raw data. In the presented case of germanium M-edge spectroscopy, a clear assignment of features assocd. with electrons, holes and the bandgap becomes possible only after applying this method.

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