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Ultrafast X-ray/XUV transient absorption spectroscopy is a powerful tool for real-time probing of chemical dynamics. Interpretation of the transient absorption spectra requires knowledge of core-excited potentials, which necessitates assistance from high-level electronic-structure computations. In this study, we investigate Br-3d core-excited electronic structures of hydrogen bromide (HBr) using spin-orbit general multiconfigurational quasidegenerate perturbation theory (SO-GMC-QDPT). Potential energy curves and transition dipole moments are calculated from the Franck-Condon region to the asymptotic limit and used to construct core-to-valence absorption strengths for five electronic states of HBr (Σ10+, 3Π1, 1Π1, 3Π0+, 3Σ1) and two electronic states of HBr+ (2Π3∕2, 2Σ1∕2). The results illustrate the capabilities of Br-3d edge probing to capture transitions of the electronic-state symmetry as well as nonadiabatic dissociation processes that evolve across avoided crossings. Furthermore, core-to-valence absorption spectra are simulated from the neutral Σ10+ state and the ionic Π21/2,3/2 states by numerically solving the time-dependent Schrödinger equation and exhibit excellent agreement with the experimental spectrum. The comprehensive and quantitative picture of the core-excited states obtained in this work allows for transparent analysis of the core-to-valence absorption signals, filling gaps in the theoretical understanding of the Br-3d transient absorption spectra.
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This content will become publicly available on December 5, 2026
Different Rise Times of Atomic Br M 4,5 3d 3/2,5/2 Core Level Absorptions during Br 2 C 1 Π u 1 u State Dissociation via Extreme Ultraviolet Transient Absorption Spectroscopy
The reported “dissociation times” for the Br2 C (1Πu 1u) state by various measurement methods differ widely across the literature (30 to 340 fs). We consider this issue by investigating attosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the Br M4,5 3d3/2,5/2 edges (66 to 80 eV), tracking core-to-valence (3d → 4p) and core-to-Rydberg (3d → ns, np, n ≥ 5) transitions from the molecular to atomic limit. The progress of dissociation can be ascertained by the buildup of the atomic absorption in time. Notably, the measured rise times of the 3d5/2, 3/2 → 4p transitions depend on the probed core level final state, 38 ± 1 and 20 ± 5 fs for 2D5/2 and 2D3/2 at 64.31 and 65.34 eV, respectively. Simulations by the nuclear time-dependent Schrödinger equation reproduce the rise-time difference of the 3d → 4p transitions, and the theory suggests several important factors. One is the transition dipole moments of each probe transition have different molecular and atomic values for 2D5/2 versus 2D3/2 that depend on the bond length. The other is the merger of multiple molecular absorptions into the same atomic absorption, creating multiple timescales even for a single probe transition. Unfortunately, the core-to-Rydberg absorptions did not allow accurate atomic Br buildup times to be extracted due to spectral overlaps with ground state bleaching, otherwise an even more comprehensive picture of the role of the probe state transition would be possible. This work shows that the measured probe signals accurately contain the dissociative wavepacket dynamics but also reveal how the specific probe transition affects the apparent progress toward dissociation with bond length. Such potential probe-transition-dependent effects need to be considered when interpreting measured signals and their timescales.
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- Award ID(s):
- 2243756
- PAR ID:
- 10652206
- Publisher / Repository:
- American Chemical Society, J. Phys. Chem. A
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry A
- ISSN:
- 1089-5639
- Subject(s) / Keyword(s):
- laser, attosecond, bromine, dissociation
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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