Researchers have used intense laser fields to reveal unique electron dynamics in liquids, providing new insights into the high-order harmonic spectrum and revealing the importance of the electron mean free path in determining the photon energy limit. The behavior of electrons in liquids plays an important role in many chemical processes that are important to living things and the world at large. For example, slower electrons in liquids have the potential to cause DNA strands to break.
Intense laser pulses (red) hit a stream of water molecules, inducing ultrafast dynamic changes in electrons in the liquid. Image source: JoergM.Harms/MPSD
However, electron movements are difficult to capture because they occur in attoseconds (five trillionths of a second). Because advanced lasers can now operate on such timescales, scientists can glimpse these ultrafast processes through a range of techniques.
An international team of researchers has now demonstrated that it is possible to use intense laser fields to probe the dynamics of electrons in liquids and retrieve their mean free path - the average distance an electron can travel before colliding with another particle.
Zhong Yin of Northeastern University's Synchrotron Radiation Innovation and Intelligence International Center (SRIS), co-first author of the paper, said: "We found that the mechanism by which liquids emit a specific spectrum (i.e., a high-order harmonic spectrum) is clearly different from the mechanism in other material phases such as gases and solids. Our findings open the door to a deeper understanding of ultrafast dynamics in liquids."
The research details of the research team were published in the journal Nature Physics on September 28, 2023.
High harmonic generation technology
The use of intense laser fields to generate high-energy photons, a phenomenon known as high-order harmonic generation (HHG), is a technique widely used in many different fields of science, such as for detecting the movement of electrons in materials or tracking chemical reactions in time. The phenomenon of harmonic generation in gases and, more recently, in crystals has been studied extensively, but little is known about the generation of harmonics in liquids.
The research team, which also includes scientists from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg and ETH Zurich, reports the unique behavior of liquids under intense laser irradiation. Until now, almost nothing was known about these light-induced processes in liquids, which are ubiquitous and present in every chemical reaction. In contrast, scientists have made great strides in recent years in exploring how solids behave under irradiation.
Therefore, the experimental group at ETH Zurich developed a unique instrument dedicated to studying the interaction of liquids with powerful lasers. The researchers discovered a unique behavior in which the maximum photon energy obtained by HHG in liquids is independent of the laser wavelength. So, what factors cause this phenomenon?
Revealing the upper limit of photon energy
To answer this question, scientists discovered a hitherto undiscovered link.
"The distance an electron can travel before colliding with another particle in a liquid is a key factor in imposing an upper limit on the energy of a photon," said study co-author Nicolas Tankogne-Dejean, a researcher at MPSD. "We were able to retrieve this quantity - called the effective electron mean free path - from experimental data thanks to a specially developed analytical model that takes the scattering of electrons into account."
By combining the results of experimental and theoretical studies of HHG in liquids, the scientists not only identified the key factors that determine the maximum light energy, but also provided an intuitive model to elucidate its basic mechanism.
Yin added, "Measuring the effective mean free path of electrons in the low kinetic energy region is very challenging, and this study does just that. Ultimately, our collaborative efforts establish HHG as a new spectroscopic tool for studying liquids, and therefore an important cornerstone in exploring the understanding of electron dynamics in liquids."
This research is a continuation of Yin's previous work.