Linear and Circular Dichroism of Core Excitons in LiF

Recent attosecond transient absorption results in He [1] show that NIR-induced couplings between bright np and dark ns Rydberg states that are allowed when using parallel XUV and NIR polarizations are eliminated when using orthogonal beams, while np to nd couplings are unaffected by this change. To determine if similar degrees of sensitivity are possible in the condensed phase, we investigate the polarization dependence of core exciton dynamics in LiF at the Li K edge using attosecond transient absorption spectroscopy and theoretical analysis. Linear XUV pump pulses excite electrons from the Li 1s core level into 2p-like conduction bands, forming electron-core hole pairs (core excitons). A NIR probe pulse arriving at variable time delays then perturbs the XUV-excited states, allowing measurement of exciton coherence lifetimes of 2.4 ± 0.4 fs, attributed to phonon-mediated dephasing [2] as in other time-resolved core exciton studies [3-5]. The transient absorption features are observed to be highly dependent on the polarization of the NIR pulse relative to the XUV. Parallel field polarizations lead to couplings between the bright p-like exciton and previously unidentified resonant s-like dark excitons. When the pump and probe polarizations are orthogonal, this coupling is suppressed by up to 90%. This interpretation, in particular the identification of the s-like dark state, is supported by simulation of a few-level model system and DFT calculations of the band structure.

[1] M Reduzzi et al., Phys Rev A 92,3 (2015)

[2] G D Mahan, Phys Rev B 15,10 (1977)

[3] A Moulet et al., Science 357, 6356 (2017)

[4] R Géneaux et al., Phys Rev Lett 124, 20 (2020)

[5] R Quintero-Bermudez and S R Leone, Phys Rev B 109, 2 (2024)