Attosecond measurement of electrons in water cluster
Attosecond laboratory view: A vacuum chamber, inside of which is a cluster of water ionized by laser pulses, seen on the left. Credit: ETH Zurich / HJ Wörner
Almost all important chemical processes occur in aqueous solutions. In such processes, a decisive role is played by electrons that are exchanged between different atoms and molecules and thereby, for example, creating or breaking chemical bonds. However, the details of how that happens are difficult to investigate because the electrons are moving so fast.
Researchers at ETH Zurich led by Hans Jakob Wörner, professor of physical chemistry, in collaboration with colleagues at the Lawrence Berkeley National Laboratory (USA) have now succeeded in studying electron dynamics in clusters made of water molecules with a time resolution of just a few attoseconds. Their results recently appeared as follow-up publications in scientific journals Natural.
Delay in ionization
In their experiment, the scientists studied how water clusters are ionized by short laser pulses in the extreme ultraviolet. To do so, clusters were first created by squeezing water vapor through a small nozzle under high pressure. The extreme ultraviolet photon energy of the laser pulse then causes one electron from the cluster to be released. This causes a void also known as a “hole.”
The ejection of electrons, however, does not occur immediately after the arrival of the pulse, but rather after a short delay. The delay depends on how the electron holes are distributed throughout the molecular cluster. “Until now, the hole distribution could only be calculated theoretically, because the delay is too short to be measured by traditional methods,” explains Xiaochun Gong, the post-doc in charge of the project.
Attosecond resolution with two laser pulses
Delays actually last only a few seconds, or millionths of a billionth of a second. To appreciate how short attoseconds are, one can make the following comparison: the number of attoseconds in one second is roughly the number of seconds in 32 billion years.
To be able to measure very short periods of a few attoseconds, Wörner and his colleagues split a very intense infrared laser pulse into two parts, one of which was converted to extreme ultraviolet by frequency multiplication in the noble gases. They overlap the two pulses and direct them both to the water cluster.
Infrared pulses modify the energy of electrons emitted by ultraviolet laser pulses. The oscillating phase of the infrared laser pulse can be very precisely tuned using an interferometer. The number of ionization events, measured with the aid of a detector, varies depending on the phase of the oscillation. From those measurements, in turn, the researchers were then able to directly read the ionization delay.
“Because we can determine the original water cluster size for each ionization event using a mass spectrometer, we can show that the delay depends on the cluster size,” says Saijoscha Heck, Ph.D. students in Wörner’s group. Until the cluster size of four water molecules the delay increases steadily to about one hundred attoseconds. However, for five or more water molecules, it remains practically constant. This is related to the high degree of symmetry exhibited by small clusters, which allows electron holes to spread throughout the cluster according to the rules of quantum mechanics. In contrast, beer clusters are somewhat asymmetrical and irregular and hence the holes are localized to some water molecules.
Applications also in semiconductor technology
“With these attosecond measurements we have opened up entirely new research opportunities,” says Wörner. He is already planning follow-up experiments in which he wants to solve electron-hole dynamics both spatially and temporally using additional laser pulses. Among other things, Wörner hopes that this will lead to a better understanding of how radiation damage develops in biological tissues, given that water ionization plays a dominant role in that process.
But Wörner also saw a wide range of possible applications beyond research on electron dynamics in water. For example, to realize faster electronic components, an in-depth understanding of the spatial expansion of electron and hole states and their evolution in time is necessary. Here, the new technique developed by the ETH researchers can be very useful.
The movement of electrons in a liquid is measured in super slow motion
X. Gong et al, Attosecond spectroscopy of size-solution water clusters, Natural (2022). DOI: 10.1038/s41586-022-05039-8
Quote: Attosecond measurements of electrons in water clusters (2022, 19 July) retrieved 19 July 2022 from https://phys.org/news/2022-07-attosecond-electrons-clusters.html
This document is subject to copyright. Apart from any fair dealing for purposes of personal study or research, no part may be reproduced without written permission. Content is provided for informational purposes only.
#Attosecond #measurement #electrons #water #cluster
Comments
Post a Comment