Zeptosecond Birth Time Delay in Molecular Photoionization

Abstract: Photoionization is one of the fundamental light-matter interaction processes in which the absorption of a photon launches the escape of an electron. The time scale of the process poses many open questions. Experiments found time delays in the attosecond ($10^{-18}$ s) domain between electron ejection from different orbitals, electronic bands, or in different directions. Here, we demonstrate that across a molecular orbital the electron is not launched at the same time. The birth time rather depends on the travel time of the photon across the molecule, which is 247 zeptoseconds ($10^{-21}$ s) for the average bond length of H$_2$. Using an electron interferometric technique, we resolve this birth time delay between electron emission from the two centers of the hydrogen molecule.

• The paper demonstrates that electron emission in H2 shows a 247 zeptosecond birth delay due to photon travel across the molecular orbital.
• It employs electron interferometry with a setup akin to a double-slit experiment to resolve angular shifts in electron momentum.
• The findings challenge conventional photoionization models and pave the way for refined studies of quantum dynamics in complex systems.

Zeptosecond Birth Time Delay in Molecular Photoionization

The paper entitled "Zeptosecond Birth Time Delay in Molecular Photoionization" explores the phenomenon of time delay in molecular photoionization, a fundamental process where a photon interacts with a molecule, leading to the ejection of an electron. The research focuses on the intricacies of timing involved in this process, particularly investigating the electron emission delay that occurs as light travels across a molecular orbital, specifically the hydrogen molecule (H$_2$).

Summary of the Findings

Photoionization has traditionally been examined in terms of attosecond time scales, with research showing time delays in the ejection of electrons from different orbitals or electronic bands. This study advances the understanding by demonstrating that electron emission is not simultaneous across a molecular orbital. Instead, the electron's emission timing is influenced by the travel time of the photon across the molecule, which for H$_2$ is approximately 247 zeptoseconds (zs = 10$^{-21}$ s).

Employing an electron interferometry technique, the authors resolve the emission timing discrepancies in electrons emanating from the two centers of the hydrogen molecule. This interferometric approach draws a parallel with a classical double-slit experiment, enabling the authors to measure and infer the birth time delay from the angular shifts in electron interference patterns.

Methodology and Experimental Design

The research team conducted the experiment using one-photon double ionization of H$_2$ with circularly polarized photons at 800 eV energy. The ionization resulted in fast-moving electrons sharing the excess energy after overcoming the double ionization energy threshold of H$_2$. By constraining their analysis to electrons carrying over 96% of the excess energy, the study effectively isolated the single-particle quantum interference effects crucial for assessing birth time delays.

The researchers collected 3D momentum data using a COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) reaction microscope, allowing them to trace electron momentum through momentum conservation principles. Analyzing angular distributions as a function of internuclear distance revealed interference patterns analogous to double-slit experiments, with angular shifts correlating to birth time delays.

Interpretation and Implications of the Results

The reconstruction of the interference patterns enabled the determination of the birth time delay across the molecule. Notably, this delay manifests not as a Wigner delay incurred during the electron's travel to the continuum but rather due to differentiated initiation times across the molecular orbital.

The implications of this study extend beyond molecular photoionization. Given the fundamental nature of photoelectron emission processes, these findings may influence broader understandings of electron emission dynamics in more complex molecular systems, as well as in solids and liquids. The study provides baseline data for potential theoretical explorations and experimental confirmations in conditions differing from the hydrogen molecule.

Speculation on Future Directions

Future developments in this research domain may involve exploring birth time delay phenomena in more structurally complex molecules, applying sophisticated theoretical models to account for electron correlation effects. Considering nondipole effects, informed by the findings around the hydrogen molecule, may refine the models further to predict electron dynamics precisely in diverse settings of photoionization.

In conclusion, this study signifies a critical advancement in delineating the timing in molecular photoionization. By unraveling the zeptosecond delay across molecular orbitals, it challenges the conventional dipole approximation, prompting refined methods to address spatial dependencies in light-matter interactions. As experimental techniques and theoretical frameworks evolve to capture these ultrasmall temporal shifts, a deeper understanding of quantum processes at similarly minuscule scales will likely emerge, propelling advancements in both fundamental physics and applied science domains.