Attosecond delays in molecular photoionization (1607.07435v1)

Abstract: We report measurements of energy-dependent attosecond photoionization delays between the two outer-most valence shells of N$_2$O and H$_2$O. The combination of single-shot signal referencing with the use of different metal foils to filter the attosecond pulse train enables us to extract delays from congested spectra. Remarkably large delays up to 160 as are observed in N$_2$O, whereas the delays in H$_2$O are all smaller than 50 as in the photon-energy range of 20-40 eV. These results are interpreted by developing a theory of molecular photoionization delays. The long delays measured in N$_2$O are shown to reflect the population of molecular shape resonances that trap the photoelectron for a duration of up to $\sim$110 as. The unstructured continua of H$_2$O result in much smaller delays at the same photon energies. Our experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.

Attosecond Delays in Molecular Photoionization

The paper entitled "Attosecond delays in molecular photoionization" presents a detailed paper of energy-dependent attosecond photoionization delays in two distinct molecules: N$_2$O and H$_2$O. The authors employ a combination of single-shot signal referencing and the spectral filtering capability of different metal foils to delineate photoionization delays from congested spectra. This paper is particularly significant as it provides both experimental measurements and theoretical insights into the phenomenon of photoionization delays at the attosecond level.

The authors report the observation of remarkably large delays, reaching up to 160 attoseconds (as) in N$_2$O, particularly at photon energies in the 20-40 eV range, in contrast to significantly smaller delays observed in H$_2$O, which were all less than 50 as. These findings are contextualized within a theoretical framework developed specifically to understand molecular photoionization delays. The prolonged delays observed in N$_2$O are attributed to the presence of molecular shape resonances which effectively trap photoelectrons for durations up to approximately 110 as. This trapping results from a potential barrier formed by the molecular and centrifugal potentials that can support quasi-bound states, showing up as shape resonances. Meanwhile, in H$_2$O, the relatively flat nature of the photoionization continua results in much smaller delays.

An essential part of this work involves the successful application of attosecond metrology to decipher photoionization dynamics in molecules, highlighting how such short time scales can provide insight into molecular electronic behaviors that were previously challenging to measure with frequency-domain techniques alone. The experiments relied on attosecond interferometry using an XUV attosecond pulse train (APT) combined with synchronized infrared (IR) pulses, achieving the resolution necessary to discern subtle phase shifts in photoelectron angular distributions.

The notable implications of these results include a deeper understanding of molecular electronic dynamics, particularly the manifestation of shape resonances in the time domain. Whereas shape resonances have been observed in atomic photoionization, the presented work is one of the few that elucidates this phenomenon in molecules through direct temporal measurements. These findings are set within the broader context of attosecond science, indicating potential future applications in probing complex electron dynamics in molecular systems, thus extending the boundaries of ultrafast spectroscopy.

The theoretical framework developed provides a robust explanation of the experimental outcomes and suggests avenues for predicting photoionization delays across other molecular systems. This framework is based on quantum-scattering calculations that are generalizable, offering a potential pathway for broader exploration of ultrafast dynamics in various molecular environments.

Looking ahead, while this paper primarily focuses on gas-phase molecules, the methodologies optimized here open prospects for analogous investigations in liquid and solid phases, enabling comprehensive explorations of attosecond dynamics across different states of matter. Such advancements promise to enhance our understanding of fundamental molecular processes and bridge gaps between theory and experiment in the domain of ultrafast science. Future research might also explore applications related to charge migration and other electron dynamics phenomena, possibly elucidating processes such as intermolecular Coulombic decay and more.

In conclusion, the paper makes a substantial contribution to the field of ultrafast molecular dynamics by successfully applying attosecond techniques to measure and understand photoionization delays in molecules, providing new insights into molecular electronic processes at the most fundamental time scales.