Search for: All records

Elementary events that determine photochemical outcomes and molecular functionalities happen on the femtosecond and subfemtosecond timescales. Among the most ubiquitous events are the nonadiabatic dynamics taking place at conical intersections. These facilitate ultrafast, nonradiative transitions between electronic states in molecules that can outcompete slower relaxation mechanisms such as fluorescence. The rise of ultrafast X-ray sources, which provide intense light pulses with ever-shorter durations and larger observation bandwidths, has fundamentally revolutionized our spectroscopic capabilities to detect conical intersections. Recent theoretical studies have demonstrated an entirely new signature emerging once a molecule traverses a conical intersection, giving detailed insights into the coupled nuclear and electronic motions that underlie, facilitate, and ultimately determine the ultrafast molecular dynamics. Following a summary of current sources and experiments, we survey these techniques and provide a unified overview of their capabilities. We discuss their potential to dramatically increase our understanding of ultrafast photochemistry. 

Monitoring conical intersection and aromaticity changes in photo-relaxation of cyclooctatetraene by TRUECARS and TRXD. 

Consolidation of ultrafast optics in electron spectroscopies based on free electron energy exchange with matter has matured significantly over the past two decades, offering an attractive toolbox for the exploration of elementary events with unprecedented spatial and temporal resolution. Here, we propose a technique for monitoring electronic and nuclear molecular dynamics that is based on self-heterodyne coherent beating of a broadband pulse rather than incoherent population transport by a narrowband pulse. This exploits the strong exchange of coherence between the free electron and the sample. An optical pulse initiates matter dynamics, which is followed by inelastic scattering of a delayed high-energy broadband single-electron beam. The interacting and noninteracting beams then interfere to produce a heterodyne-detected signal, which reveals snapshots of the sample charge density by scanning a variable delay T . The spectral interference of the electron probe introduces high-contrast phase information, which makes it possible to record the electronic coherence in the sample. Quantum dynamical simulations of the ultrafast nonradiative conical intersection passage in uracil reveal a strong electronic beating signal imprinted onto the zero-loss peak of the electronic probe in a background-free manner. 

Time-resolved photoelectron spectroscopy (TRPES) signals that monitor the relaxation of the RNA base uracil upon optical excitation are simulated. Distinguishable signatures of coherence dynamics at conical intersections are identified, with temporal and spectral resolutions determined by the duration of the ionizing probe pulse. The frequency resolution of the technique, either directly provided by the signal or retrieved at the data-processing stage, can magnify the contribution from molecular coherences, enabling the extraction of most valuable information about the nonadiabatic molecular dynamics. The predicted coherence signatures in TRPES could be experimentally observed with existing ultrashort pulses from high-order harmonic generation or free-electron lasers.