Ultrafast Spectroscopy of Excited Molecular States

Ultrafast spectroscopy harnesses pulses of light lasting only femtoseconds (10–15 seconds) or shorter to capture the fleeting dynamics of molecules in electronically excited states. By initiating a rapid perturbation and then probing the ensuing evolution, researchers can observe fundamental processes such as internal conversion, intersystem crossing, intramolecular charge transfer and bond cleavage in real time. Techniques span pump–probe absorption and emission measurements, time-resolved photoelectron imaging and X-ray absorption spectroscopy, each offering a distinct window on electronic redistribution and nuclear motion. Applications touch fields as diverse as photochemistry, solar energy conversion, materials science and biophysics. Recent advances in pulse generation and detection have extended temporal resolution to the attosecond domain, enabling unprecedented insight into the coupling between electronic and nuclear degrees of freedom. Understanding these ultrafast events is key to controlling light-driven chemistry, improving photovoltaic efficiency and unraveling the mechanisms of photobiological damage and repair.

Research from Nature Portfolio

Recent studies have employed femtosecond X-ray absorption spectroscopy at the sulphur K-edge to elucidate the photochemistry of disulfide bonds in aqueous amino acids. Investigations reveal that ultraviolet excitation triggers homolytic cleavage of the disulfide bridge, yielding thiyl radicals that rapidly undergo geminate recombination within tens of picoseconds. Concurrent detection of perthiyl radical formation implicates vibrationally excited intermediates that fragment asymmetrically, offering a refined mechanism for bond stability in biological environments. These findings illuminate the dynamic photostability of proteinaceous disulfide motifs and demonstrate the power of element-specific ultrafast X-ray probes to resolve transient radical pathways in solution.

Ultrafast Spectroscopy of Excited Molecular States publication trend

The graph below shows the total number of articles in ultrafast spectroscopy of excited molecular states across all publications each year (not limited to Nature Index journals).

Technical terms

Femtosecond spectroscopy: Ultrafast optical measurement using pulses of ∼10–15 s to resolve rapid molecular processes.

Pump–probe spectroscopy: Technique where one pulse excites a system and a delayed pulse probes transient states.

X-ray absorption spectroscopy: Method that monitors changes in X-ray absorption near an element’s absorption edge to track electronic and structural dynamics.

Time-resolved photoelectron imaging: Approach that records kinetic energy and angular distributions of emitted electrons to characterise excited-state evolution.

Rydberg state: High-lying excited electronic state with an electron in a diffuse orbital far from the ionic core.

Non-adiabatic dynamics: Processes where transitions between electronic states occur faster than nuclear motion can adapt to changes in electronic configuration.

References

1. UV photochemistry of the L-cystine disulfide bridge in aqueous solution investigated by femtosecond X-ray absorption spectroscopy. Nature Communications (2024).
2. Spectroscopic application of few-femtosecond deep-ultraviolet laser pulses from resonant dispersive wave emission in a hollow capillary fibre. Chemical Science (2022).
3. Electronic and vibrational relaxation dynamics of NH3 Rydberg states probed by vacuum-ultraviolet time-resolved photoelectron imaging. The Journal of Chemical Physics (2019).
4. The role of novel Rydberg-valence behaviour in the non-adiabatic dynamics of tertiary aliphatic amines. Chemical Science (2016).