arXiv subjects

Guoyan Ge

Publications and source records attributed to Guoyan Ge.

4 recordsLinked to original sources

arXiv2026

Core-Level Spectroscopy Decodes Bond-Alternation Dynamics of Cyclo[18]Carbon

The advent of X-ray free-electron lasers and high-harmonic generation has made time-resolved X-ray spectroscopy a powerful tool for probing local atomic environments, yet whether localized core excitations can report on global collective distortions remains open. Cyclo[18]carbon (C$_{18}$), with its polyynic ground state (D$_\text{9h}$) and cumulenic transition state (D$_\text{18h}$), provides an ideal model to address this long-standing issue in bond-length alternation (BLA) dynamics. Mapping two-dimensional potential energy surfaces by first-principles simulations, we find that core ionization symmetrizes the ground-state double-well potential along the BLA coordinate. Our calculated X-ray spectra reveal remarkable sensitivity to bond-length variations: C1s ionization potentials vary by up to 2.4~eV across the BLA coordinate (1.1--1.4~\AA), with a 0.9~eV variation for minima predicted by different functionals, while NEXAFS $\pi^*$ peaks shift by up to 4~eV across the same coordinate. These predicted signatures provide a quantitative spectroscopy--structure dictionary for decoding transient structures in future ultrafast X-ray experiments and monitoring bond-alternation dynamics in real time.

physics.atm-clus↗

arXiv2025

Mapping Transient Structures of Cyclo[18]Carbon by Computational X-Ray Spectra

The structure of cyclo[18]carbon (C$_{18}$), whether in its polyynic form with bond length alternation (BLA) or its cumulenic form without BLA, has long fascinated researchers, even prior to its successful synthesis. Recent studies suggest a polyynic ground state and a cumulenic transient state; however, the dynamics remain unclear and lack experimental validation. This study presents a first-principles theoretical investigation of the bond lengths ($R_1$ and $R_2$) dependent two-dimensional potential energy surfaces (PESs) of C$_{18}$, concentrating on the ground state and carbon 1s ionized and excited states. We examine the potential of X-ray spectra for determining bond lengths and monitoring transient structures, finding that both X-ray photoelectron (XPS) and absorption (XAS) spectra are sensitive to these variations. Utilizing a library of ground-state minimum structures optimized with 14 different functionals, we observe that core binding energies predicted with the $\omega$B97XD functional can vary by 0.9 eV (290.3--291.2 eV). Unlike the ground state PES, which predicts minima at alternating bond lengths, the C1s ionized state PES predicts minima with equivalent bond lengths. In the XAS spectra, peaks 1$\pi^*$ and 2$\pi^*$ show a redshift with increasing bond lengths along the line where $R_1 = R_2$. Additionally, increasing $R_2$ (with $R_1$ fixed) results in an initial redshift followed by a blueshift, minimizing at $R_1 = R_2$. Major peaks indicate that both 1$\pi^*$ and 2$\pi^*$ arise from two channels: C1s$\rightarrow\pi^*_{z}$ (out-of-plane) and C1s$\rightarrow\pi^*_{xy}$ (in-plane) transitions at coinciding energies.

physics.chem-ph↗

arXiv2024

Predicting Accurate X-ray Absorption Spectra for CN$^+$, CN, and CN$^-$: Insights from Multiconfigurational and Density Functional Simulations

High-resolution X-ray spectroscopy is an essential tool in X-ray astronomy, enabling detailed studies of celestial objects and their physical and chemical properties. However, comprehensive mapping of high-resolution X-ray spectra for even simple interstellar and circumstellar molecules is still lacking. In this study, we conducted systematic quantum chemical simulations to predict the C1s X-ray absorption spectra of CN$^+$, CN, and CN$^-$. Our findings provide valuable references for both X-ray astronomy and laboratory studies. We assigned the first electronic peak of CN$^+$ and CN to C1s $\rightarrow \sigma^*$ transitions, while the peak for CN$^-$ corresponds to a C1s $\rightarrow \pi^*$ transition. We explained that the two-fold degeneracy ($\pi^*_{xz}$ and $\pi^*_{yz}$) of the C1s$\rightarrow\pi^*$ transitions results in CN$^-$ exhibiting a significantly stronger first absorption compared to the other two systems. We further calculated the vibronic fine structures for these transitions using the quantum wavepacket method based on multiconfigurational-level, anharmonic potential energy curves, revealing distinct energy positions for the 0-0 absorptions at 280.7 eV, 279.6 eV, and 285.8 eV. Each vibronic profile features a prominent 0-0 peak, showing overall similarity but differing intensity ratios of the 0-0 and 0-1 peaks. Notably, introducing a C1s core hole leads to shortened C-N bond lengths and increased vibrational frequencies across all species. These findings enhance our understanding of the electronic structures and X-ray spectra of carbon-nitrogen species, emphasizing the influence of charge state on X-ray absorptions.

physics.chem-ph↗

arXiv2023

Franck-Condon Simulation of Vibrationally-Resolved X-ray Spectra for Diatomic Systems: Validation of Harmonic Approximation and Density Functional Theory

Under the Franck-Condon approximation, we systematically validated the performance of density functional theory (DFT) and the effects of anharmonicity in simulating C/N/O K-edge vibrationally-resolved X-ray spectra of common diatomic molecules. To get ``transparent'' validations, vibronic fine structures of only the lowest 1s excited or ionized state in the X-ray absorption (XAS) or photoelectron (XPS) spectra were investigated. All 6 systems (N$_2$, N$_2^+$; NO, NO$^+$; CO, CO$^+$) were studied within the harmonic oscillator (HO) approximation using DFT with four functionals (BLYP, BP86, B3LYP, M06-2X) for 10 XAS and 4 XPS spectra, and excellent agreement between theoretical and experimental spectra was found in most systems, except O1s XAS of NO, CO, and NO$^+$. We analyzed and established a connection between their complex vibronic structures (many weak oscillating features within a broad peak) and the significant geometrical changes induced by the O1s hole. The three spectra were well reproduced with anharmonic (AH) calculations by using quantum wavepacket dynamics based on potential energy curves (PECs) generated by DFT methods or multiconfigurational levels, highlighting sensitivity to the anharmonic effect and the PEC quality. In other examples of XAS (CO$^+$, C1s and O1s; NO, N1s) corresponding to smaller structural changes, HO and AH approaches lead to similar fine structures, which are dominated by 0-0 and 0-1 transitions. This study highlights the use of DFT with selected functionals for such diatomic calculations due to its easy execution and generally reliable accuracy. Functional dependence in diatomic systems is generally more pronounced than in polyatomic ones. We found that BLYP, BP86, and B3LYP functionals consistently exhibited high accuracy in predicting spectral profiles, bond lengths, and vibrational frequencies, which slightly outperformed M06-2X.

physics.chem-ph↗