Attosecond-scale measurement of Wigner time delay in molecular photoionization
The photoelectric effect is one of the most fundamental interactions between light and matter, widely used in investigating ultrafast dynamics in atoms, molecules and condensed matter. It has been in the spotlight of research for over 100 years and most of its natural aspects are well understood. However, the fundamental questions about how long the photoionization process takes and how to identify the specific mechanisms responsible for the measured time delay are open and debated.
The controversy stems from the fact that time is not a quantum operator. Therefore, there are no well-constructed dynamic observations that can be used to characterize such a photoemission delay. The concept of Wigner time delay, established seventy years ago by Eisenbud and Wigner (and later Smith) for scattering processes, has been extended to characterize the timing of the photoionization process. The Wigner time delay is defined as the energy derivative of the phase shift of the transmitted photoelectron wave packet. This means that photoionization time delay can be constructed by the phase shift.
A research team led by Prof. Yunquan Liu presented the “double-pointer attoclock” scheme, in which two-tone bi-circular fields were used to investigate the phase and amplitude of emitting wave packets in atomic multiphoton ionization (2018). Recently, this research team transferred this scheme from atoms to molecules. The results of the study have been published in the Ultra-fast science.
Experimentally, they measured the orientation-dependent photoelectron corner stripes of asymmetric CO molecules in bi-circular fields. They then developed a semi-classical quantum-orbit non-adiabatic Monte Carlo model (MO-QTMC) to unravel the orientation-dependent behavior of molecular Coulomb interaction and molecular orbital structure on photoelectron angular distributions. They extracted the sub-Coulomb barrier phase of emitted electron wave packets and reconstructed the asymmetric Wigner time delay of photoemission.
The “double-pointer attoclock” scheme with sculpted circular fields shows the promising potential application in exploring the time-resolved photoionization process and measuring the orientation-dependent Wigner time delay of polyatomic molecules.
How the mechanism of photoionization can provide insight into complex molecular potentials
Zhenning Guo et al, Probing Molecular Frame Wigner Time Delay and Electron Wavepacket Phase Structure of CO Molecule, Ultra-fast science (2022). DOI: 10.34133/2022/9802917
Provided by Ultrafast Science
Quote: Attosecond Scale Measurement of Wigner Time Delay in Molecular Photoionization (2022, June 24) Retrieved June 25, 2022 from https://phys.org/news/2022-06-attosecond-scale-wigner-molecular-photoionization.html
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