Speaker
Description
The recent capability of creating soft-X-ray attosecond pulses at free-electron lasers has opened the possibility of performing element-specific measurements of electronic dynamics on their natural time scale. I will present recent results from four user beam times at LCLS. Combining attosecond soft-X-ray pulses and circularly polarized infrared pulses, we have measured attosecond photoionization delays between the N1s and C1s shells of azabenzenes (pyridine, pyrazine, s-triazine) [1]. This study demonstrates the unique advantages of core-level chronoscopy, which make the technique scalable to complex systems: the photoelectron spectra remain atomic-like, the measurements are element specific and the photoelectron waves originate from a point-like source. This study moreover identified two unknown mechanisms regulating photoionization delays in molecules: (i) the substitution of C-H groups with N atoms increases the confinement of shape resonances and (ii) the increasing symmetry decreases the coupling between photoelectron partial waves. Both mechanisms lead to an increase of the photoionization delays, which was experimentally observed. In the second beam time, we demonstrated X-ray photoelectron spectroscopy in an attosecond pump, attosecond probe setting. This new technique was used to observe the recently proposed core-hole coherent spectroscopy [2]. Preparing coherent superpositions of core-hole states in N2O and 2-nitropyridine, we have observed electronic dynamics with periods of 1.1 and 0.83 fs, respectively [3]. Remarkably, the decoherence of the electronic wave packet is faster in the case of N2O than in 2-nitropyridine, which reveals the different topologies of the respective potential-energy surfaces. In the third beam time, we demonstrated attosecond double-resonant core-hole spectroscopy. Photoexciting nickelocene at the carbon K-edge, probing it at the nickel L-edge and detecting the Auger electrons, we identified the signatures of nickel core-hole screening and electronic coherences on sub-femtosecond time scales [4]. In the fourth beam time, attosecond pump, attosecond probe measurements have been performed on aqueous urea solutions. The ionization-induced dynamics of urea solutions have been probed by time-resolved X-ray absorption spectroscopy at the oxygen K-edge. These results demonstrate the considerable potential of attosecond spectroscopy in the soft-X-ray domain, illustrating the key advantages of element- and site-specific probing.
[1] J.-B. Ji, Z. Guo et al., Phys. Rev. X 15, 041031 (2025)
[2] E. Rodriguez-Cuenca et al., Phys. Rev. Lett. (2024)
[3] F. Vismarra et al., in preparation
[4] F. Vismarra et al., in preparation
[5] S. Lee et al., in preparation
| Scientific Topics | AMO Physics |
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