Benzophenone and its derivatives are important diaryl ketone building blocks that have applications ranging from UV blockers to organic optoelectronics. This class of substances exhibits efficient intersystem crossing processes that make them a popular choice. However, the ultrafast internal conversion processes that precede the intersystem crossing are less frequently discussed in the literature. This work provides a comprehensive theoretical investigation of these nonadiabatic relaxation events in meta-methylbenzophenone utilizing spectroscopy techniques. Based on a full quantum mechanical description of the nonadiabatic dynamics, we simulate both time-dependent X-ray absorption and off-resonant-stimulated X-ray Raman spectra at the oxygen K-edge. We show that the use of time-resolved X-ray spectroscopy allows for a selective probing of the first singlet excited state by filtering out signals from other excited states with different electronic character. The element sensitivity of oxygen core level spectroscopy restricts the sensitivity to nonadiabatic processes localized at the carbonyl double bond.

Vibrational polaritons are formed by strong coupling of molecular vibrations and photon modes in an optical cavity. Experiments have demonstrated that vibrational strong coupling can change molecular properties and even affect chemical reactivity. However, the interactions in a molecular ensemble are complex, and the exact mechanisms that lead to modifications are not fully understood yet. We simulate two-dimensional infrared spectra of molecular vibrational polaritons based on the double quantum coherence technique to gain further insight into the complex many-body structure of these hybrid light–matter states. Double quantum coherence uniquely resolves the excitation of hybrid light–matter polaritons and allows one to directly probe the anharmonicities of the resulting states. By combining the cavity Born–Oppenheimer Hartree–Fock ansatz with a full quantum dynamics simulation of the corresponding eigenstates, we go beyond simplified model systems. This allows us to study the influence of self-polarization and the response of the electronic structure to the cavity interaction on the spectral features even beyond the single-molecule case.

As pioneering experiments have shown, strong coupling between molecular vibrations and light modes in an optical cavity can significantly alter molecular properties and even affect chemical reactivity. However, the current theoretical description is limited and far from complete. To explore the origin of this exciting observation, we investigate how the molecular structure changes under strong light–matter coupling using an ab initio method based on the cavity Born–Oppenheimer Hartree–Fock ansatz. By optimizing H₂O and H₂O₂ resonantly coupled to cavity modes, we study the importance of reorientation and geometric relaxation. In addition, we show that the inclusion of one or two cavity modes can change the observed results. On the basis of our findings, we derive a simple concept to estimate the effect of the cavity interaction on the molecular geometry using the molecular polarizability and the dipole moments.

Protons in low-barrier superstrong hydrogen bonds are typically delocalized between two electronegative atoms. Conventional methods to characterize such superstrong hydrogen bonds are vibrational spectroscopy and diffraction techniques. We introduce soft X-ray spectroscopy to uncover the electronic fingerprints for proton sharing in the protonated imidazole dimer, a prototypical building block enabling effective proton transport in biology and high-temperature fuel cells. Using nitrogen core excitations as a sensitive probe for the protonation status, we identify the X-ray signature of a shared proton in the solvated imidazole dimer in a combined experimental and theoretical approach. The degree of proton sharing is examined as a function of structural variations that modify the shape of the low-barrier potential in the superstrong hydrogen bond. We conclude by showing how the sensitivity to the quantum distribution of proton motion in the double-well potential is reflected in the spectral signature of the shared proton.