The dynamics of chemical reactions in solution are of paramount importance in fields ranging from biology to materials science. Because the hydrogen-bond network and proton dynamics govern the behavior of aqueous solutions, they have been the subject of numerous studies over the years. Here, we report the observation of a previously unknown associative state in the hydroxide ion that forms when a proton from a neighboring water molecule approaches the hydroxide ion, utilizing resonant inelastic soft X-ray scattering (RIXS) and quantum dynamical simulations. State-of-the-art theoretical analysis reveals state mixing in the electronically excited states between aqueous hydroxide ions and the solvent. Our results give new insights into chemical bonding and excited-state dynamics in the aqueous environment. This investigation of associative states opens up new pathways for spectroscopic studies of chemical reaction dynamics and lays the foundation for directly accessing dynamic proton exchange in solution.

Photodissociation is one of the most important photoinduced chemical reactions. It occurs when the potential energy curve along a chemical bond is repulsive in an excited state. Typically, "ballistic" ultrafast dissociation leads to the broadening of absorption resonances and the smearing out of vibrational fine-structure. We report on the photodissociation of H2O in the (B) over tilde2(1)A(1) electronic state, characterized by a |3a(1)(-1)4a(1)(1)> configuration, which can be reached via resonant inelastic x-ray scattering or direct ultraviolet absorption. In both cases the spectra show narrow vibrational resonances, in spite of the dissociative character of the state. We find that "delayed" dissociation pathways, caused by reflection of the nuclear wave packet, are responsible for this effect. In spite of the analogous topology of the potential energy surfaces of the core- and valence-excited states, the reflection of the wave packet takes place only in the latter. The two-dimensional wave packet of the O-H stretching coordinates becomes trapped in a "cavity" near the Franck-Condon region, resulting from a mismatch between the OH vibrational frequency in the cavity and the one at the dissociation limit.

Different types of Young's double-slit experiments contain a significant amount of both particle and wave information running from full-particle to full-wave knowledge depending on the experimental conditions. We study the Young's double-slit interference in resonant Auger scattering from homonuclear diatomic molecules where opposite Doppler shifts for the dissociating atomic slits provide path information. Different quantitative formulation of Bohr's complementarity principle - path information vs interference - is applied to two types of resonant Auger scattering experiments, with fixed-in-space and randomly oriented molecules. Special attention is paid to the orientational dephasing in conventional Auger experiments with randomly oriented molecules. Our quantitative formulation of the complementarity is compared with the formulation made earlier by Greenberger and Yasin [D. M. Greenberger and A. Yasin, Phys. Lett. A 128, 391 (1988)0375-960110.1016/0375-9601(88)90114-4].

In this study, we integrate experimental observations and theoretical models to elucidate the complex phenomena observed in the resonant S K-edge KLL Auger scattering spectra of the SF6 molecule. A two-dimensional spectral map, constructed of incident photon energy and kinetic energy of the emitted Auger electron, is shown to be a versatile tool for understanding a character of the core-excited potential energy surface and change of the molecular geometry. Our findings reveal how the distinct dispersion behavior of multiple spectral lines enables mapping of ultrafast dynamics within the short-lived core-excited states. Our results confirm the presence of nuclear dynamics in the S1s-16a1g1 and S1s-16t1u1 core-excited states, while dynamics is absent in the S1s-17t1u1 state. Using a combination of ab initio analysis, simulations with Coulomb model potentials, and a simple analytical approximation, we qualitatively demonstrate how the varying characteristics of spectral dispersion - classified as Raman, non-Raman, and anti-Raman - mirror the relative gradients of the intermediate and final states in the resonant x-ray scattering process. This insight allows for the effective mapping of molecular potential energy curves, offering a prospective tool on the underlying mechanisms of resonant Auger scattering and its potential for probing molecular dynamics.

Resonant inelastic x-ray scattering (RIXS) is a major method for investigation of electronic structure and dynamics, with applications ranging from basic atomic physics to materials science. In RIXS applied to inversion-symmetric systems, it has generally been accepted that strict parity selectivity applies in the sub-kilo-electron volt region. In contrast, we show that the parity selection rule is violated in the RIXS spectra of the free homonuclear diatomic O2 molecule. By analyzing the spectral dependence on scattering angle, we demonstrate that the violation is due to the phase difference in coherent scattering at the two atomic sites, in analogy with Young's double-slit experiment. The result also implies that the interpretation of x-ray absorption spectra for inversion symmetric molecules in this energy range must be revised.

Time-resolved X-ray photoelectron spectroscopy (TXPS) is a well-established technique to probe coherent nuclear wavepacket dynamics using both table-top and free-electron-based ultrafast X-ray lasers. Energy resolution, however, becomes compromised for a very short pulse duration in the sub-femtosecond range. By resonantly tuning the X-ray pulse to core-excited states undergoing Auger decay, this drawback of TXPS can be mitigated. While resonant Auger-electron spectroscopy (RAS) can recover the vibrational structures not hidden by broadband excitation, the full reconstruction of the wavepacket is a standing challenge. Here, we theoretically demonstrate how the complete information of a nuclear wavepacket, i.e., the populations and relative phases of the vibrational states constituting the wavepacket, can be retrieved from time-resolved RAS (TRAS) measurements. Thus, TRAS offers key insights into coupled nuclear and electronic dynamics in complex systems on ultrashort timescales, providing an alternative to leverage femtosecond and attosecond X-ray probe pulses.

In our work, we demonstrate that X-ray photons can initiate a "molecular catapult" effect, leading to the dissociation of chemical bonds and the formation of heavy fragments within just a few femtoseconds. We reconstruct the momenta of fragments from a three-body dissociation in bromochloromethane using the ion pair average (IPA) reference frame, demonstrating how light atomic groups, such as alkylene and alkanylene, can govern nuclear dynamics during the dissociation process, akin to projectiles released by a catapult. Supported by ab initio calculations, this work highlights the crucial role of low-reduced-mass vibrational modes in driving ultrafast chemical processes.

A vibrational wavepacket in SF₆ is created by impulsive stimulated Raman scattering with a few-cycle infrared pulse and mapped simultaneously onto five sulfur core-excited states using table-top soft x-ray transient absorption spectroscopy between 170 to 200 eV. The femtosecond vibrations induce real-time energy shifts of the x-ray absorption, whose amplitude depend strongly on the nature of the core-excited state. The pump laser intensity is used to control the number of vibrational states in the superposition, thereby accessing core-excited levels for various extensions of the S-F stretching motion. This enables the determination of the relative core-level potential energy gradients for the symmetric stretching mode, in good agreement with TDDFT calculations. This experiment demonstrates a new means of characterizing core-excited potential energy curves.

In the description of resonant inelastic x-ray scattering (RIXS) from inversion-symmetric molecules the small core-level splitting is typically neglected. However, the spacing Δ between gerade and ungerade core levels in homonuclear diatomic molecules can be comparable with the lifetime broadening of the intermediate core-excited state Γ. We show that when Δ∼Γ the scattering becomes nonlocal in the sense that x-ray absorption at one atomic site is followed by emission at the other one. This is manifested in an unusual dependence of the RIXS cross section on the sum of the momenta of incoming and outgoing x-ray photons k+k′, contrary to the normal k-k′ dependence in the conventional local RIXS theory. The nonlocality of the scattering influences strongly the scattering angle and excitation energy dependence of the intensity ratio between parity forbidden and allowed RIXS channels. Numerical simulations for N2 show that this effect can readily be measured at present-day x-ray radiation facilities.

Intense x-ray and extreme ultraviolet (XUV) light sources have been available for decades, however, due to weak nonlinear interaction in the XUV photon energy range, observation of Rabi oscillation induced by XUV pulse remains a very challenging experimental task. Here we suggest a scheme where photoionization of a He medium by an intense XUV pump pulse is followed by a strong population inversion and Rabi oscillation at the He+(1s-3p) transition and is accompanied by superfluorescence (SF) of the 7.56 eV pulse at the He+(3p-2s) transition. Our numerical simulations show that the Rabi oscillation at the He+(1s-3p) transition induced by an XUV pulse with photon energy 48.36 eV results in significant signatures in the SF spectra, allowing us to identify and characterize the XUV induced Rabi-oscillatory regime. The proposed scheme provides a sensitive tool to monitor and control ultrafast nonlinear dynamics in atoms and molecules triggered by intense XUV.