Real-Time Mapping of Coupled Electronic and Nuclear Wavepacket Dynamics in Photoexcited Molecules Using Time-Resolved Photoelectron Spectroscopy and Time-Resolved X-ray Absorption Spectroscopy

Understanding photochemical processes in molecules remains a fundamental challenge, as they involve coupled electronic–nuclear dynamics that often extend beyond the validity of the Born–Oppenheimer approximation (BOA). In such situations, molecular potential energy surfaces can become degenerate, particularly in the vicinity of conical intersections (CIs). These intersections act as ultrafast funnels for radiationless transitions and play a central role in numerous photochemical processes in nature. Notable examples include the cis–trans isomerization of the retinal chromophore in rhodopsin, the remarkable photostability of UV-excited DNA and RNA nucleobases, and the ring-opening reaction leading to the formation of pre-vitamin D₃. Real-time observation of coupled electronic–nuclear dynamics is therefore essential for achieving predictive control over photochemical reactivity in areas such as biological systems, atmospheric chemistry, photocatalysis, and solar energy conversion. The earliest stages of photoinduced processes occur on ultrashort timescales, ranging from a few to several hundred femtoseconds, and involve vibronically coupled electronic–nuclear wavepackets. Experimentally capturing these dynamics remains highly challenging, as it requires both ultrafast temporal resolution and sensitivity to electronic coherence and nuclear rearrangement.

Advances in ultrafast laser technology and pump–probe spectroscopy—particularly following the pioneering work of Ahmed Zewail, later recognized with the Nobel Prize in Chemistry—have enabled direct observation of molecular dynamics on their intrinsic femtosecond timescales. While many time-resolved techniques primarily probe either nuclear motion (e.g., vibrational spectroscopy and diffraction) or electronic dynamics (e.g., attosecond streaking), most photochemical processes are governed by strongly coupled electronic and nuclear motion. Capturing this interplay therefore requires techniques that are simultaneously sensitive to both degrees of freedom. In this context, time-resolved photoelectron spectroscopy (TRPES) and time-resolved X-ray absorption spectroscopy (TRXAS) have emerged as powerful tools for probing coupled electronic–nuclear wavepacket dynamics.

In this talk, the speaker will focus on TRPES, in which tuneable, few-femtosecond vacuum-ultraviolet (VUV) pulses generated via resonant dispersive wave (RDW) emission in gas-filled hollow-core fibers are used to probe the early-time dynamics of photoexcited ethylene and its deuterated isotopologue. This approach enables the direct observation of a previously unrecognized mechanism governing the initial stages of ethylene’s ultrafast excited-state dynamics. In particular, a strong nonadiabatic coupling between the initially populated π π* state and the σ π* state drives rapid torsional motion in VUV-excited ethylene, resulting in substantial population transfer between these states within less than 10 fs. Also, the speaker will present their recent work using TRXAS, in which ultrashort X-ray pulses generated from a table-top high-harmonic-generation source are employed to investigate the ultrafast structural rearrangement of nitromethane following strong-field ionization. These measurements are the first of their kind, to identify the transient intermediate involved in the nitro–nitrite rearrangement.