Time-resolved photoelectron spectra of CS2: dynamics at conical intersections

Ab Initio Multiple Spawning Nonadiabatic Dynamics with Different CASPT2 Flavors: A Fully Open-Source PySpawn/OpenMolcas Interface

We introduce an interface between PySpawn, a simulation package to run ab initio multiple spawning (AIMS) nonadiabatic dynamics, and OpenMolcas, a software package to perform multiconfigurational perturbations theory (CASPT2) electronic structure calculations. Our interface allows us to exploit all the functionalities of the two codes: the modular and efficient Python implementation of the AIMS algorithm and the extensive analysis tools offered by PySpawn, with the cutting-edge implementation of CASPT2 equations in OpenMolcas, including the recently introduced analytical gradients and different flavors. Both are fully open-source and free of charge, making the following implementation unique in the current plethora of software for nonadiabatic dynamics. This represents an important step toward a wider application of AIMS-based nonadiabatic dynamics combined with high-accuracy excited-state calculations. The importance and the need for such an implementation are demonstrated by application to the ultrafast relaxation of fulvene from S1 to S0, which is drastically affected by the potential energy surface on which the nuclear wavepacket is propagated. Additionally, the decay is influenced by the CASPT2 flavor adopted, posing interesting questions in the choice of one over the other and opening the door to deeper studies on the effect of CASPT2 formulations in nonadiabatic dynamics.

Time-resolved X-ray and XUV based spectroscopic methods for nonadiabatic processes in photochemistry

The photochemistry of numerous molecular systems is influenced by conical intersections (CIs). These omnipresent nonadiabatic phenomena provide ultra-fast radiationless relaxation channels by creating degeneracies between electronic states and decide over the final photoproducts. In their presence, the Born-Oppenheimer approximation breaks down, and the timescales of the electron and nuclear dynamics become comparable. Due to the ultra-fast dynamics and the complex interplay between nuclear and electronic degrees of freedom, the direct experimental observation of nonadiabatic processes close to CIs remains challenging. In this article, we give a theoretical perspective on novel spectroscopic techniques capable of observing clear signatures of CIs. We discuss methods that are based on ultra-short laser pulses in the extreme ultraviolet and X-ray regime, as their spectral and temporal resolution allow for resolving the ultra-fast dynamics near CIs.

Time-resolved photoelectron spectroscopy: the continuing evolution of a mature technique

Time-resolved photoelectron spectroscopy (TRPES) has become one of the most widespread techniques for probing nonadiabatic dynamics in the excited electronic states of molecules. Furthermore, the complementary development of ab initio approaches for the simulation of TRPES signals has enabled the interpretation of these transient spectra in terms of underlying coupled electronic-nuclear dynamics. In this perspective, we discuss the current state-of-the-art approaches, including efforts to push femtosecond pulses into vacuum ultraviolet and soft X-ray regimes as well as the utilization of novel polarizations to use time-resolved optical activity as a probe of nonadiabatic dynamics. We close this perspective with a forward-looking prospectus on the new areas of application for this technique.

Correspondence between electronic structure calculations and simulations: nonadiabatic dynamics in CS2

The choice of ab initio electronic structure method is an important factor in determining the fidelity of nonadiabatic dynamics simulations.

Mapping the Complete Reaction Path of a Complex Photochemical Reaction

We probe the dynamics of dissociating CS_{2} molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

Imaging rotations and vibrations in polyatomic molecules with X-ray scattering

An approach for calculating elastic X-ray scattering from polyatomic molecules in specific electronic, vibrational, and rotational states is presented, and is used to consider the characterization of specific states in polyatomic molecules using elastic X-ray scattering.

Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules

Observing the motion of the nuclear wave packets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wave packet in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 Å and temporal resolution of 230 fs full width at half maximum. The method is not only sensitive to the position but also the shape of the nuclear wave packet.

Exploration of minimum energy conical intersection structures of small polycyclic aromatic hydrocarbons: toward an understanding of the size dependence of fluorescence quantum yields

A correlation between the fluorescence quantum yields and the energy barrier to the conical intersection region was discovered for five small PAHs.