Controlled Experiments and Optimized Theory of Absorption Spectra of Li Metal and Salts

<b><u>Abstract:</u></b> Investigations of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li <i>K</i>-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li<i>-K</i> sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of <i>in-situ</i> prepared samples, we report the intrinsic Li<i>-K</i> sXAS of Li-metal that displays a prominent leading peak, which has never been revealed before. Furthermore, theoretical simulations show that the Li<i>-K</i> sXAS is strongly affected by the response of the valence electrons to the core-hole due to the low number of valence electrons in Li. We successfully reproduce the Li<i>-K</i> sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, resolution, and especially contributions from transitions which are forbidden in the so-called single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Our work provides the first reliable Li<i>-K</i> sXAS of Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li surface chemistry in both metallic and ionic states.

Controlled Experiments and Optimized Theory of Absorption Spectra of Li Metal and Salts

<b><u>Abstract:</u></b> Investigations of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li <i>K</i>-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li<i>-K</i> sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of <i>in-situ</i> prepared samples, we report the intrinsic Li<i>-K</i> sXAS of Li-metal that displays a prominent leading peak, which has never been revealed before. Furthermore, theoretical simulations show that the Li<i>-K</i> sXAS is strongly affected by the response of the valence electrons to the core-hole due to the low number of valence electrons in Li. We successfully reproduce the Li<i>-K</i> sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, resolution, and especially contributions from transitions which are forbidden in the so-called single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Our work provides the first reliable Li<i>-K</i> sXAS of Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li surface chemistry in both metallic and ionic states.

Towards theoretical spectroscopy with error bars: systematic quantification of the structural sensitivity of calculated spectra

Uncertainty quantification is applied in theoretical spectroscopy to obtain error bars accounting for the structural sensitivity of calculated spectra.

Towards Theoretical Spectroscopy with Error Bars: Systematic Quantification of the Structural Sensitivity of Calculated Spectra

Molecular spectra calculated with quantum-chemical methods are subject to a number of uncertainties (e.g., errors introduced by the computational methodology) that hamper the direct comparison of experiment and computation. Judging these uncertainties is crucial for drawing reliable conclusions from the interplay of experimental and theoretical spectroscopy, but largely relies on subjective judgment. Here, we explore the application of methods from uncertainty quantification to theoretical spectroscopy, with the ultimate goal of providing systematic error bars for calculated spectra. As a first target, we consider distortions of the underlying molecular structure as one important source of uncertainty. We show that by performing a principal component analysis, the most influential collective distortions can be identified, which allows for the construction of surrogate models that are amenable to a statistical analysis of the propagation of uncertainties in the molecular structure to uncertainties in the calculated spectrum. This is applied to the calculation of X-ray emission spectra of iron carbonyl complexes, of the electronic excitation spectrum of a coumarin dye, and of the infrared spectrum of alanine. We show that with our approach it becomes possible to obtain error bars for calculated spectra that account for uncertainties in the molecular structure. This is an important first step towards systematically quantifying other relevant sources of uncertainty in theoretical spectroscopy.

Towards Theoretical Spectroscopy with Error Bars: Systematic Quantification of the Structural Sensitivity of Calculated Spectra

Molecular spectra calculated with quantum-chemical methods are subject to a number of uncertainties (e.g., errors introduced by the computational methodology) that hamper the direct comparison of experiment and computation. Judging these uncertainties is crucial for drawing reliable conclusions from the interplay of experimental and theoretical spectroscopy, but largely relies on subjective judgment. Here, we explore the application of methods from uncertainty quantification to theoretical spectroscopy, with the ultimate goal of providing systematic error bars for calculated spectra. As a first target, we consider distortions of the underlying molecular structure as one important source of uncertainty. We show that by performing a principal component analysis, the most influential collective distortions can be identified, which allows for the construction of surrogate models that are amenable to a statistical analysis of the propagation of uncertainties in the molecular structure to uncertainties in the calculated spectrum. This is applied to the calculation of X-ray emission spectra of iron carbonyl complexes, of the electronic excitation spectrum of a coumarin dye, and of the infrared spectrum of alanine. We show that with our approach it becomes possible to obtain error bars for calculated spectra that account for uncertainties in the molecular structure. This is an important first step towards systematically quantifying other relevant sources of uncertainty in theoretical spectroscopy.