Researchers have demonstrated the first high-performance table-top all-attosecond transient absorption spectroscopy (AATAS) system capable of directly tracking coherent electron wavepacket dynamics in noble gas ions with both attosecond temporal resolution and high spectral precision.
Study: Table-top all-attosecond transient absorption spectroscopy. Image Credit: S. Singha/Shutterstock.com
Reported in Nature Communications, the new study shows that using two attosecond extreme-ultraviolet (XUV) pulses generated through high-harmonic generation (HHG) overcomes key limitations of conventional two-color spectroscopy, enabling the direct observation of ultrafast electron motion, including the previously inaccessible 3.1-fs coherent hole dynamics in xenon ions, with exceptional sensitivity and signal quality.
All-Attosecond Spectroscopy Motivation
Attosecond transient absorption spectroscopy (ATAS) has become a powerful technique in attosecond science. It offers extremely high temporal resolution on the order of attoseconds alongside excellent spectral resolution.
Traditional implementations combine an attosecond extreme-ultraviolet (XUV) pulse with a near-infrared (NIR) femtosecond pulse, using the sub-cycle dynamics of the NIR field to achieve attosecond time resolution. However, this two-color approach suffers from two main drawbacks: the strong NIR fields present can obscure intrinsic electron dynamics of the system, and the typical duration of the NIR pulses (≥3 fs) limits the ability to resolve faster electronic processes occurring on 1-5 fs timescales.
This means that observing very rapid coherent electron motions, such as hole dynamics in xenon ions with a period around 3 fs, has been challenging.
A promising solution is all-attosecond transient absorption spectroscopy (AATAS), wherein both the pump and probe pulses are attosecond XUV pulses.
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High-Harmonic Generation Setup
Using a high-harmonic generation (HHG) setup as a source of intense, broadband XUV attosecond pulses, the authors implemented a table-top AATAS system optimized for stability and signal-to-noise ratio (SNR).
The experimental system comprised a commercial Ti:sapphire laser producing 37 fs pulses at 1 kHz with 9 mJ energy.
These pulses were compressed in a three-stage non-guided compression scheme to 3.7 fs with 4.9 mJ energy, yielding excellent stability with single-shot pulse energy fluctuations of 0.17% rms. The compressed NIR pulses were loosely focused into a 7.5-cm-long gas cell filled with xenon to generate XUV harmonics via HHG.
The system favored maximizing XUV intensity over pulse energy through careful optimization of propagation effects and focusing geometry, producing XUV pulse energies around 200 nJ at the source and pump pulse energies about 3.1 nJ in the interaction region.
The XUV pump and probe beams were focused on waists of approximately 1.3 μm and 1.6 μm, respectively, minimizing spatial chirp and ensuring tight interaction volumes. The attosecond structure of the pulses was characterized by ion cross-correlation measurements, revealing isolated pulses of ~270 as full width at half maximum (FWHM) with minor pre- and post-pulse satellite features.
The pump fluence was close to saturation levels for argon and xenon, optimizing ionization without excessive multiphoton absorption. After the interaction with noble gas targets (Xe, Kr, Ar, Ne) in an effusive gas jet, the transmitted XUV spectra of pump and probe pulses were recorded individually by an XUV spectrometer equipped with a laminar grating and microchannel plate detector.
This spectral resolution (30–70 meV) allowed us to resolve spin–orbit split states and Rydberg transitions with high precision. Systematic noise reduction methods, including beam-pointing stabilization, environmental control, and long acquisition times, significantly enhanced data quality.
Coherent Electron Dynamics Observed
Utilizing this advanced AATAS setup, the authors resolved coherent electronic wavepacket dynamics in singly ionized noble gases with unprecedented clarity. In xenon, the attosecond pump ionized neutral atoms, creating Xe+ ions in a superposition of spin–orbit states 5p53/2 and 5p51/2, separated by 1.306 eV, corresponding to an oscillation period of approximately 3.17 fs.
The probe pulse then excited these ions to Rydberg states, and the transient absorption changes revealed clear oscillations in optical density at the expected frequency, confirming coherent hole motion in Xe+ on the natural timescale of the spin–orbit splitting.
The observed 3.1 ± 0.1 fs period could not previously be resolved using two-color methods limited by the duration of NIR pulses. In argon, similar coherent oscillations with a period of 23.3 ± 0.1 fs corresponding to 0.177 eV spin–orbit splitting were observed with a high signal-to-noise ratio (~3 × 10–4 OD noise level).
Moreover, oscillations at different photon energies showed out-of-phase behavior, revealing rich electron dynamics across multiple absorption lines. In neon ions, out-of-phase oscillations with a period of 42.9 ± 0.4 fs (spin–orbit splitting of 0.097 eV) were also recorded. The high spectral and temporal resolution allowed the disentangling of complex electronic coherences and their interpretation with quantum orbital pictures.
By eliminating strong NIR fields, the technique provides a more straightforward and direct view of intrinsic electronic motions in atoms. The approach benefits from the broad XUV bandwidth generated by HHG and the excellent stability of the laser and beamline. Compared to free-electron lasers, the high repetition rate (kHz), long acquisition times, and stable output of the table-top setup enabled superior signal averaging and noise reduction.
Future Prospects of AATAS
This work establishes table-top all-attosecond transient absorption spectroscopy as a robust and highly sensitive technique for probing electron dynamics with attosecond time resolution and excellent spectral specificity.
Future improvements, including advanced referencing schemes, higher repetition rates, and longer acquisition times, will further enhance sensitivity while reducing sample damage risks, especially in solids.
Overall, this advancement opens exciting avenues for direct time-domain studies of electron dynamics, core-hole decay, and charge transfer phenomena, underpinning fundamental research in physics, chemistry, and materials science.
Journal Reference
Volkov M., Svirplys E., et al. (2026). Table-top all-attosecond transient absorption spectroscopy. Nature Communications 17, 7572. DOI: 10.1038/s41467-026-76019-z, https://www.nature.com/articles/s41467-026-76019-z