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Sunday, December 18, 2022

Measuring times in billionths of a billionth of a second

ULTRAFAST SCIENCE

Schematics of the experimental setup for Gouy phase interferometer. — IMAGE: SCHEMATICS OF THE EXPERIMENTAL SETUP FOR GOUY PHASE INTERFEROMETER. THE SCHEMATIC OF THE EXPERIMENTAL SETUP IS SHOWN IN FIGURE 1. THERE ARE TWO MOVEABLE MOLECULAR JETS SEPARATED IN SPACE NEAR THE FOCUS OF THE DRIVING INFRARED LASER BEAM. THE PHASE DIFFERENCE BETWEEN HHG EMISSIONS FROM THE TWO JETS INCLUDES A CONTRIBUTION FROM THE GOUEY PHASE (THE ONLY CONTRIBUTION WHEN THE SAME GAS IS FLOWING IN BOTH JETS) AND AN ADDITIONAL PHASE SHIFT DUE TO THE DIFFERENT INTRINSIC HHG PHASES OF DIFFERENT SPECIES. TO EXTRACT THAT INTRINSIC PHASE SHIFT, THE HHG SPECTRA ARE MEASURED FIRST WITH THE SAME GASES IN BOTH JETS AND THEN WITH DIFFERENT GASES SWITCHED BETWEEN THE JETS. THIS PROCEDURE ALLOWS TO COMPLETELY REMOVE THE CONTRIBUTION OF THE GOUY PHASE AND THE EFFECT OF SLIGHTLY DIFFERENT INTENSITIES AT THE JETS’LOCATIONS. view more
CREDIT: ULTRAFAST SCIENCE

How fast do electrons inside a molecule move? Well, it is so fast that it takes them just few attoseconds (1 as = 10-18 s or one billionth of billionth of a second) to jump from one atom to another. Blink and you missed it – millions of billions of times. So measuring such ultrafast processes is a daunting task.

Scientists at the Australian Attosecond Science Facility and the Centre for Quantum Dynamics of Griffith University in Brisbane Australia, led by Professor Robert Sang and Professor Igor Litvinyuk have developed a novel interferometric technique capable of measuring time delays with zeptosecond (a trillionth of a billionth of a second) resolution.

They have used this technique to measure the time delay between extreme ultraviolet light pulses emitted by two different isotopes of hydrogen molecules – H2 and D2 – interacting with intense infrared laser pulses.

This delay was found to be less than three attoseconds (one quintillionth of a second long) and is caused by slightly different motions of the lighter and heavier nuclei.

This study has been published in Ultrafast Science, a new Science Partner Journal.

The first author Dr Mumta Hena Mustary explains: “Such unprecedented time resolution is achieved via an interferometric measurement – overlapping the delayed light waves and measuring their combined brightness.”

The light waves themselves were generated by molecules exposed to intense laser pulses in the process called high harmonic generation (HHG).

HHG occurs when an electron is removed from a molecule by a strong laser field, is accelerated by the same field and then recombines with the ion giving up the energy in the form of extreme ultraviolet (XUV) radiation. Both intensity and phase of that XUV HHG radiation are sensitive to exact dynamics of the electron wavefunctions involved in this process – all different atoms and molecules emit HHG radiation differently.

While it is relatively straightforward to measure spectral intensity of HHG – a simple grating spectrometer can do that – measuring HHG phase is a far more difficult task. And the phase contains the most relevant information about the timing of various steps in the emission process.

To measure this phase, it is usual to perform a so-called interferometric measurement when two replicas of the wave with finely controlled delay are made to overlap (or interfere) with each other. They can interfere constructively or destructively depending on the delay and relative phase difference between them.

Such measurement is performed by a device called an interferometer. It is very difficult to build an interferometer for XUV light, in particular to produce and maintain a stable, known and finely tuneable delay between two XUV pulses.

The Griffith researchers solved this problem by taking advantage of the phenomenon known as the Gouy phase – when phase of a light wave is shifted a certain way while going through a focus.

For their experiments the researchers used two different isotopes of molecular hydrogen – the simplest molecule in nature. The isotopes – light (H2) and heavy (D2) hydrogen – differ only in mass of nuclei – protons in H2 and deuterons in D2. Everything else including the electronic structure and energies are identical.

Due to their larger mass the nuclei in D2 move slightly slower then those in H2. Because nuclear and electronic motions in molecules are coupled, nuclear motion affects the dynamics of the electron wavefunctions during the HHG process resulting in a small phase shift ΔφH2-D2 between the two isotopes.

This phase shift is equivalent to a time delay Δt = ΔφH2-D2 /ω where ω is the frequency of the XUV wave. The Griffith scientists measured this emission time delay for all the harmonics observed in the HHG spectrum – it was nearly constant and slightly below 3 attoseconds.

To understand their result the Griffith researchers were supported by theorists at Shanghai Jiao Tong University in Shanghai, China, led by Professor Feng He.

The SJTU scientists used the most advanced theoretical methods to comprehensively model the HHG process in the two isotopes of molecular hydrogen including all degrees of freedom for nuclear and electronic motion at various levels of approximation.

Their simulation reproduced experimental results well, and this agreement between theory and experiment gave the team confidence that the model captured the most essential features of the underlying physical process, so adjusting the model’s parameters and levels of approximation can determine the relative importance of various effects.

While the actual dynamics is quite complex, it was found that two-centre interference during the electron recombination step is the dominant effect.
“Because hydrogen is the simplest molecule in nature and it can be modelled theoretically with high accuracy it was used in these proof-of-principle experiments for benchmarking and validation of the method,” Professor Litvinyuk said.

“In the future, this technique can be used to measure ultrafast dynamics of various light-induced processes in atoms and molecules with unprecedented time resolution.”

JOURNAL

Ultrafast Science

DOI

10.34133/2022/9834102

METHOD OF RESEARCH

Experimental study

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Attosecond Delays of High-Harmonic Emissions from Hydrogen Isotopes Measured by XUV Interferometer

Securing stable green energy under future extreme weather conditions

Climate is changing, and this will affect future energy systems based on solar and wind. But how? A new research project at Aarhus University will focus on this with a Sapere Aude grant from Independent Research Fund Denmark.

Grant and Award Announcement

AARHUS UNIVERSITY

Associate Professor Marta Victoria — IMAGE: ASSOCIATE PROFESSOR MARTA VICTORIA view more
CREDIT: CLAUS LILLEVANG, INDEPENDENT RESEARCH FUND DENMARK

In February 2021, the state of Texas was hit by wave of cold weather lasting two weeks. The event was without precedent, and the Texan energy supply, which is primarily dependent on wind, coal and gas, was completely unprepared for this kind of extreme weather.

Due to the cold weather, gas pipes and some wind turbines froze, and since this was accompanied by a period with low wind speeds, it caused two-week-long power outages in 4.5 million homes. The incident cost USD 24 billion and 262 people lost their lives. It was the worst winter-weather disaster ever in the US.

The European energy network is better interconnected and prepared to deal with such events, but in a future with increasing climate change and more extreme weather, such events will very likely occur again.

So how do we ensure a stable future energy system, if this will be based on energy sources that may well be sustainable and renewable, but that are also dependent on local weather conditions?

A new research project at Aarhus University will now try to explain.

"We can't build an energy system that takes account of everything. That would be extremely costly. So, what do we do if there is a long period without wind and without sun, and with a high demand for energy, and how can we adapt the system without over-dimensioning it? We need to find out what we have to be prepared for when the weather changes as a result of climate change," says Associate Professor Marta Victoria from the Department of Mechanical and Production Engineering at Aarhus University, who is heading the project.

She has just received a prestigious Sapere Aude-grant from Independent Research Fund Denmark (DFF) of DKK 6.2 million. Her project is called EXTREMES (Highly renewable energy systems under extreme weather events) and it aims to describe periods of extreme weather using statistics, and to design strategies to ensure that an energy system with a very high proportion of solar and wind energy is reliable, even under extreme weather conditions.

"How long will the worst period of low wind or solar production be? How many times a year will this happen? What is the worst period we can expect within a decade? Or a century? And what conditions should we adapt the system to? As a rule, there’s a backup, but in the project we’ll try to take into account situations where several adverse events happen at the same time, and where spatial and temporal fluctuations can be hard to predict," she says.

The project starts in spring 2023 and will run for four years.

Sapere Aude grants are awarded by Independent Research Fund Denmark to talented early-career researchers to enable them to develop and strengthen their own research ideas and establish themselves as research directors with the foundation's financial assistance.

This year, Independent Research Fund Denmark has awarded Sapere Aude grants to 41 researchers following 356 applications. Of these, 12 recipients are from Aarhus University: The 41 researchers share a total of DKK 247 million.

Plastic pollution kills sea urchin larvae

Peer-Reviewed Publication

UNIVERSITY OF EXETER

Sea urchin development — IMAGE: SEA URCHIN DEVELOPMENT IN UNCONTAMINATED SEAWATER AND VARIOUS CONCENTRATIONS OF PVC NURDLES view more
CREDIT: DR EVA JIMENEZ-GURI

Sea urchin larvae raised in high levels of plastic pollution die due to developmental abnormalities, new research shows.

Scientists put fertilised urchin eggs in seawater with varying levels of plastic, and compared the effects of newly made PVC pellets (called nurdles) with fragments collected on beaches.

In all three concentrations tested (1%, 5% and 10% of plastic in seawater), PVC led to significant abnormalities and all urchin larvae died.

Beach-collected fragments at 10% concentration also killed the larvae, which developed no proper shape.

Lower concentrations of beach-collected plastic did not kill the embryos, suggesting newly made plastic – which still contains high levels of additives that can leach out – are more harmful.

Although the concentrations tested in the study are rare in the oceans, they could occur after spills of plastic or in areas like the tide line where pollution accumulates.

The research team – from the Anton Dohrn Zoological Station and National Biodiversity Future Center (Italy), and the University of Exeter (UK) – previously found plastic additives can harm sea urchin larvae, and the new study develops this and reveals how this harm is caused.

“The larvae affected by plastic pollution showed developmental abnormalities including malformation of the skeleton, neural and immune cells,” said Dr Eva Jimenez-Guri, from the Anton Dohrn Zoological Station and the University of Exeter.

“They also showed ‘radialisation’ – meaning they lacked proper symmetrical structure, and were instead largely formless and therefore unable to survive.

“In these larvae, mitochondria (the ‘powerhouses’ of cells) didn’t work properly, and they showed signs of oxidative stress, which damages cells.”

The study reveals the genetic processes behind these abnormalities.

In the case of new PVC nurdles, the damage was caused by high concentrations of zinc that leached into the water.

Larvae exposed 10% PVC pollution developed their gut outside their body, while the 5% and 1% levels also lead to fatal abnormalities.

The beach-collected samples – gathered in Cornwall, UK – did not release high levels of zinc, as most of the additives they contained would already have been released in the sea.

However, such particles are known to collect a variety of organic pollutants, and the release of these pollutants explains the abnormalities seen in this study.

“Our findings point to clear and specific detrimental effects of marine plastic pollution on the development of sea urchin larvae,” said Dr Jimenez-Guri.

“We have identified the genes that are affected by these pollutants, and we know many animal species rely on the same genes for key early stages of development.

“So it’s possible that plastic pollution could cause similar abnormalities in other species, and we are already investigating this in the next stage of our research.

“While the levels of pollution assessed in the study are not common in the ocean, marine plastic pollution continues to increase at a rapid rate – with potentially serious consequences for marine life.”

Dr Jimenez-Guri’s work was funded by a Marie Skłodowska-Curie fellowship.

The paper, published in the journal Science of the Total Environment, is entitled: “Plastic leachate-induced toxicity during sea urchin embryonic development: Insights into the molecular pathways affected by PVC.”