[Paper Review] Real-time observation of electronic, vibrational, and rotational dynamics in nitric oxide with attosecond soft X-ray pulses at 400 eV

Nariyuki Saito, Hiroki Sannohe+8 more|arXiv (Cornell University)|Apr 23, 2019

Laser-Matter Interactions and Applications26 references5 citations

TL;DR

This study demonstrates real-time, all-optical observation of electronic, vibrational, and rotational dynamics in nitric oxide using attosecond soft X-ray pulses at 400 eV generated via high-harmonic generation (HHG). By probing nitrogen K-edge transitions, the method achieves simultaneous resolution of electron and nuclear motions across attosecond to sub-picosecond timescales, marking the first comprehensive measurement of all three degrees of freedom in a single experiment with element and electronic-state specificity.

ABSTRACT

Photoinduced quantum dynamics in molecules have hierarchical temporal structures with different energy scales that are associated with electron and nuclear motions. Femtosecond-to-attosecond transient absorption spectroscopy (TAS) using high-harmonic generation (HHG) with a photon energy below 300 eV has been a powerful tool to observe such electron and nuclear dynamics in a table-top manner. However, comprehensive measurements of the electronic, vibrational, and rotational molecular dynamics have not yet been achieved. Here, we demonstrate HHG-based TAS at the nitrogen K-edge (400 eV) for the first time, and observe all the electronic, vibrational, and rotational degrees of freedom in a nitric oxide molecule at attosecond to sub-picosecond time scales. This method of employing core-to-valence transitions offers an all-optical approach to reveal complete molecular dynamics in photochemical reactions with element and electronic state specificity.

Motivation & Objective

• To achieve real-time, all-optical observation of electronic, vibrational, and rotational dynamics in molecules at attosecond resolution.
• To overcome the limitations of prior transient absorption spectroscopy by enabling simultaneous measurement of all three molecular degrees of freedom.
• To demonstrate the feasibility of using core-level transitions (N K-edge at 400 eV) for probing complex molecular dynamics with high specificity.
• To establish a table-top method for element- and electronic-state-specific monitoring of photochemical processes.

Proposed method

• High-harmonic generation (HHG) is used to produce isolated attosecond soft X-ray pulses at 400 eV, targeting the nitrogen K-edge of nitric oxide.
• A pump-probe scheme is employed, where an extreme ultraviolet (XUV) pump initiates molecular dynamics, and the HHG-generated X-ray probe captures time-resolved absorption spectra.
• Transient absorption spectroscopy (TAS) is performed at the nitrogen K-edge, sensitive to core-to-valence electronic transitions in NO.
• The method leverages the element specificity of core-level excitations and the temporal resolution of attosecond pulses to resolve dynamics across multiple energy scales.
• Time-delayed X-ray pulses are used to map the evolution of electronic, vibrational, and rotational states with sub-femtosecond precision.
• The experimental setup enables all-optical, table-top access to molecular dynamics without requiring large-scale facilities.

Experimental results

Research questions

• RQ1 Can attosecond soft X-ray pulses at 400 eV resolve electronic, vibrational, and rotational dynamics simultaneously in a single molecular system?
• RQ2 How does the temporal evolution of core-level absorption spectra reflect the coupled motion of electrons and nuclei in nitric oxide?
• RQ3 To what extent can core-to-valence transitions at the nitrogen K-edge provide element-specific and electronic-state-specific information on molecular dynamics?
• RQ4 Can high-harmonic generation-based transient absorption spectroscopy at 400 eV achieve sub-femtosecond time resolution for multi-scale molecular dynamics?
• RQ5 What is the role of rotational and vibrational coherences in the observed transient absorption response at the nitrogen K-edge?

Key findings

• The study achieves the first real-time observation of electronic, vibrational, and rotational dynamics in nitric oxide using attosecond soft X-ray pulses at 400 eV.
• Core-level probing at the nitrogen K-edge enables element-specific and electronic-state-specific monitoring of molecular dynamics with sub-femtosecond time resolution.
• The transient absorption spectra reveal coherent superpositions of vibrational and rotational states evolving on attosecond to sub-picosecond timescales.
• The method successfully resolves the hierarchical temporal structure of molecular dynamics, with electronic motion occurring on the attosecond scale and nuclear motions on the femtosecond to picosecond scale.
• The all-optical HHG-based approach provides a compact, table-top alternative to large-scale X-ray facilities for studying ultrafast molecular dynamics.
• The results demonstrate that core-to-valence transitions can serve as a powerful probe for complete molecular dynamics in photochemical processes.