Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy

Abstract

Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic level insight into photochemistry. Herein, we performed a time-resolved study of ultrafast molecular symmetry breaking via geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft X-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry broken cation, which were detected in the X-ray signal. These oscillations were damped within 58 ± 13 femtoseconds, as vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens new avenues for exploring complex systems.

Spin Coupling Effect on Geometry-Dependent X-Ray Absorption of Diradicals

“…Coupling these electrons, or equivalently coupling three spin-1/2 systems, results in two doublet states, and therefore two possible spectroscopically accessible transitions where the initial state is an open shell doublet. This results in spin-induced splitting of these core-to-unoccupied valence transitions, a unique feature that has been experimentally observed in a number of small radical molecules − as well as studied theoretically. ,, Further, in time-resolved XAS measurements of methane and benzene cations, insights into Jahn–Teller relaxation time scales, energetics, and related vibrational dynamics have been inferred from both time-resolved core-to-SOMO (methane cation) and spin-split core-to-LUMO (benzene cation , ) transitions. Here, we are interested in extending the understanding of XAS predictions to spectral properties of diradicals, i.e., molecules in which two electrons singly occupy two spatially distinct orbitals.…”

Generalization of One-Center Nonorthogonal Configuration Interaction Singles to Open-Shell Singlet Reference States: Theory and Application to Valence-Core Pump-Probe States in Acetylacetone

“…Time-resolved near-edge X-ray absorption (TR-NEXAS) experiments aim to track the ensuing dynamics of molecular systems after a perturbation with light by monitoring the NEXAS (equivalently termed NEXAFS, XANES, or often simply XAS) features of the species involved as a function of time. With advances in synchrotron slicing techniques and the advent of free-electron lasers, the time resolution of modern TR-NEXAS experiments is well into the femtosecond regime. , Furthermore, improvements in high-harmonic generation have brought extreme UV and soft X-ray femtosecond pulses in the water window (270–550 eV) to table-top laser equipment. , With element and site specificity, as well as strong sensitivity to the electronic environment of the species being probed, the TR-NEXAS experiments enabled by these new technologies have already provided fundamental insight into the role of dark singlet and triplet states in the electronic relaxation of organic molecules and directly tracked the nuclear motion of small molecules post strong-field ionization. − …”

Ab Initio Investigation of the Auger Spectra of Methane, Ethane, Ethylene, and Acetylene

“…While Auger spectroscopy of small organic molecules such as methane, ethane, ethylene, and acetylene has been the subject of experimental and theoretical studies for many years, − recent advances in X-ray technology and computational methods have driven renewed interest in the field. − Many previous works focused on differentiating between final states of Auger decay in terms of fragmentation processes, but the computation of total and partial Auger decay widths has remained a challenge due to the necessity to deal with the ionization continuum and a multitude of final states.…”