Attosecond Spectroscopy: Advancements in Ultrafast Molecular Dynamics

By Owais AliReviewed by Lexie CornerMay 13 2024

Ultrafast processes at the molecular and atomic levels provide insights into the fundamental mechanisms governing the behavior of surrounding matter. However, the limited temporal resolution of traditional tools has made it challenging to observe the precise motion of electrons during chemical reactions, energy transfer, and material properties.

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Attosecond spectroscopy has emerged as a solution to this limitation, providing unparalleled temporal resolution to capture real-time electron movement and reveal ultrafast molecular dynamics.

Principles of Attosecond Spectroscopy

Attosecond pulses (10^-18) are generated via high harmonic generation (HHG), where a high-intensity femtosecond laser pulse is focused on a target material, often an inert gas cell.

This process involves three main steps: first, the intense electric field of the laser pulse distorts the electronic potential, leading to tunneling ionization of the gas and releasing electron wave packets into the continuum. These electrons then undergo acceleration due to the oscillating nature of the laser field, propelling them forward and increasing their energy.

Finally, the accelerated electrons are driven back towards the parent ions. Upon recombination, the surplus kinetic energy of the electrons is discharged in the form of high-energy photons, generating attosecond pulses in the soft X-Ray or extreme ultraviolet (XUV) region of the electromagnetic spectrum.^1,2

Generating attosecond pulses via HHG requires tightly controlled laser pulses with intense peak intensities and short durations. The temporal confinement of the XUV emission determines the duration of the attosecond pulse. This can be achieved through bandpass filtering of the highest energy region of the XUV spectrum obtained by HHG or by applying a temporal gate to the HG process.

These methods enable the production of isolated attosecond pulses with temporal durations down to 80 attoseconds.³

How Does It Allow Scientists to Observe Electron Dynamics in Real-Time?

By combining attosecond XUV pulses with a synchronized, delayed infrared (IR) laser pulse in a pump-probe setup, researchers can investigate ultrafast electron dynamics in real-time.

The extremely short XUV pulses excite electrons in solids, generating photoelectrons whose escape dynamics are precisely tracked by shifting their momentum with a near-infrared pulse.

Analyzing the kinetic energy of these emitted photoelectrons enables scientists to measure electron transport and interaction mechanisms and timescales accurately, providing insights into electron behavior within various materials.²

Applications in Chemistry and Physics

Attosecond spectroscopy has a broad range of applications in chemistry, physics, and materials science. It facilitates groundbreaking discoveries and enhances our understanding of matter.

This technique helps examine the intricate dynamics of molecular systems, revealing how energy flows between light, carriers, and phonons. By observing electronic and nuclear configurations during chemical reactions, researchers can gain valuable insights into the mechanisms at play, potentially leading to more efficient catalysts and optimized energy conversion technologies.⁴

For example, in a recent study, researchers at the Institute of Photonic Sciences (ICFO) used attosecond core-level spectroscopy to monitor the real-time dynamics of furan, providing detailed insights into the complex molecular ring-opening process.⁵

In solid-state systems, attosecond spectroscopy disentangles coherent and incoherent excitation and dissipation pathways, offering insights into charge carrier transport, exciton lifetimes, and phase transitions. This knowledge is crucial for developing advanced electronics, optoelectronics, and energy-harvesting materials.⁶

Attosecond spectroscopy has also revealed fundamental phenomena, such as the delay in photoemission from solids and atoms, the evolution of tunneling processes in dielectrics, and electron behavior during photoexcitation and autoionization. These discoveries have significant implications for understanding quantum mechanical interactions and refining theoretical models.³

Recent Breakthroughs in the Field

Direct Observation of Electron Movement in Liquid Water

A research team at Argonne National Laboratory captured the real-time movement of electrons in liquid water using X-Ray attosecond transient absorption spectroscopy. The results are published in Science.

This innovative technique allowed researchers to "freeze" atomic motion, isolating and observing the electron's response to X-Ray ionization on an attosecond timescale. Their findings resolved a longstanding debate regarding the interpretation of the 1b1 X-Ray emission doublet in liquid water, demonstrating that it is related to dynamics rather than evidence of two structural motifs.⁷

Combining Attosecond Pulses and Electron Microscopy to Investigate Ultrafast Dynamics of Matter

A team led by physicist Jan Vogelsang and 2023 Nobel Laureate Anne L'Huillier combined attosecond light pulses with electron microscopy, marking a significant breakthrough. Their results are published in Advanced Physics Research.

Vogelsang and his team demonstrated the feasibility of attosecond time-resolved photoemission electron microscopy (PEEM) using current technology. This technique allows for the examination of electron interactions with optical fields at the nanoscale.

Their experimentation focused on zinc oxide, which was chosen for its defined energy states and low binding energy. This enabled the observation of clear structures in the electron spectrum resulting from the interaction with the electric field of another laser pulse.

This achievement resulted from years of incremental improvements in lasers, microscopes, and measurement concepts. The successful integration of attosecond light pulses with electron microscopy has opened new possibilities for understanding ultrafast electron dynamics in various materials.⁸

High Repetition Rate Attosecond-Pump Attosecond-Probe Spectroscopy

Another study published in Science demonstrated two-color attosecond-pump attosecond-probe spectroscopy (APAPS) using a high-intensity attosecond source operating at a repetition rate of 1 kHz.

This technology overcomes the limitations of previous APAPS experiments, which required specialized setups and had low repetition rates (10-120 Hz), by using a compact, out-of-focus geometry to generate intense attosecond pulses.

This enabled the team to analyze the formation of argon ions in real time, showing that attosecond pulses effectively probe the electronic response.⁹

Future Prospects and Challenges

As attosecond spectroscopy advances, researchers explore new frontiers while overcoming challenges to improve measurement accuracy and pulse generation.

Advanced theoretical models and computational techniques are being developed to interpret attosecond spectroscopy data accurately while improving pulse intensity and repetition rate to maintain temporal coherence and stability, enhancing the signal-to-noise ratio and enabling more complex experiments.¹⁰

Despite these challenges, attosecond spectroscopy continues to expand our understanding of the fundamental principles governing the surrounding matter. As researchers refine this technique, the prospects for uncovering new insights into the ultrafast dynamics of molecules, atoms, and materials remain incredibly promising.

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References and Further Reading

1. Borrego-Varillas, R., Lucchini, M., Nisoli, M. (2022). Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics. Reports on Progress in Physics. doi.org/10.1088/1361-6633/ac5e7f
2. Ramasesha, K., Leone, SR., Neumark, DM. (2016). Real-time probing of electron dynamics using attosecond time-resolved spectroscopy. Annual Review of Physical Chemistry. doi.org/10.1146/annurev-physchem-040215-112025
3. Calegari, F., et al. (2011). Principles and applications of attosecond technology. Advances in atomic, molecular, and optical physics. doi.org/10.1016/B978-0-12-385508-4.00008-5
4. Sidiropoulos, TP., et al. (2022). Attosecond core-level spectroscopy reveals the flow of excitation in a material between light, carriers and phonons. Conference on Lasers and Electro-Optics (CLEO).  doi.org/10.1364/CLEO_QELS.2022.FM4N.5
5. Severino, S., et al. (2024). Attosecond core-level absorption spectroscopy reveals the electronic and nuclear dynamics of molecular ring opening. Nature Photonics. doi.org/10.1038/s41566-024-01436-9
6. Sidiropoulos, TPH. et al. (2021). Probing the energy conversion pathways between light, carriers, and lattice in real time with attosecond core-level spectroscopy. Physical Review X. doi.org/10.1103/PhysRevX.11.041060
7. Li, S., et al. (2024). Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water. Science. doi.org/10.1126/science.adn6059
8. Vogelsang, J., et al. (2024). Time‐Resolved Photoemission Electron Microscopy on a ZnO Surface Using an Extreme Ultraviolet Attosecond Pulse Pair. Advanced physics research. doi.org/10.1002/apxr.202300122
9. Kretschmar, M., Svirplys, E., Volkov, M., Witting, T., Nagy, T., Vrakking, MJ., Schütte, B. (2024). Compact realization of all-attosecond pump-probe spectroscopy. Science Advances. doi.org/10.1126/sciadv.adk9605
10. Yuen, CH., Lin, CD. (2024). Rotation in attosecond vibronic coherence spectroscopy for molecules. Communications Physics. doi.org/10.1038/s42005-024-01607-8

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