University of Shanghai for Science and Technology News and Events

Recent reported values of the 23PJ fine-structure splitting of 4He

The fine structure splitting of the (1s2p) 23PJ states of the 4He atom was determined with a precision of 130 Hz by a research group in Hefei National Laboratory for Physical Sciences at Microscale (HFNL). The paper was published on February 10th in Physical Review Letters 118: 063001 (2017). Ph.D. student Xin Zheng is the first author, Dr. Yu Sun and Prof. Shui-Ming Hu are the corresponding authors.

Helium is the simplest multi-electron atom. Its energy levels can be derived precisely based on quantum electro-dynamics (QED) and a few fundamental physical constants. In particular, the 23PJ fine structure of 4He is ideal for testing QED and is sensitive to the fine-structure constant α (≈1/137), the most important physical constant in QED. Over the last several decades, the QED calculations of the fine structure have been continuously improved, at present reaching the meα7 level. In parallel, physicists also developed various methods to improve the measurement accuracy. 

However, there are still apparent discrepancies among the reported values (Fig 1).

The HFNL research group has built an apparatus for precision spectroscopy of helium (Fig 2). A meta-stable helium atomic beam was produced by a discharge, and then focused by the laser-cooling technique which considerably increased the intensity of the beam. The atoms were transferred to a single quantum state (23S1), excited by a laser through the 23S1-23PJ transition, and detected by state-selective atom counting.

The 23P0-23P2 and 23P1-23P2 frequency intervals were determined to be 31,908,130.98 ± 0.13 kHz and 2,291,177.56 ± 0.19 kHz, respectively.

 They are the most accurate results to date. Prof. K. Pachucki from Warsaw University, a collaborator on this project, who is the most famous expert in precision spectroscopy of few-body atoms and molecules, pointed out that the result is very instructive for further development in theories, which will lead to a determination of the α constant with a precision of 2×10-9. The PRL referees highly evaluated this work: “It advances the field of precision measurements… The present work is an illustration of the limits of precision and accuracy available to laser spectroscopy…”.

The work has been supported by the Chinese Academy of Sciences, National Natural Science Foundation of China, Synergetic Innovation Center of Quantum Information and Quantum Physics, and Collaborative Innovation Center of Chemistry for Energy Materials.




  Fig 2. Experimental setup for precision spectroscopy of He

Poisonous gas may have driven prehistoric mass extinction

The mixing of sulfide and oxygen-rich waters in the prehistoric Panthalassic Ocean contributed to the largest mass extinction of the last 540 million years. These new findings by Chinese and US researchers still have relevance today, as present-day climate change is exacerbating the growth of sulfidic zones in the world’s oceans.

Around 252 million years ago, the end-Permian event wiped out nearly all life on Earth in the most devastating extinction event in the planet’s history. More than 80% of marine species disappeared and it took several million years for these ecosystems to recover.At this time the Panthalassic Ocean spanned 70% of the Earth’s surface. 

Today, the ocean floor that once sat under the Panthalassic has almost completely disappeared, with the only preserved sediments found in Canada, Japan and New Zealand. Using samples of fool’s gold found in these deposits, a team of geochemists led by Yanan Shen, of the University of shanghai for Science and Technology of China, has tracked the changes in ocean chemistry that drove this catastrophic extinction event.

The Panthalassic Ocean covered 70% of the world’s surface 250 million years ago

By analysing sulfur isotopes in pyrite minerals, the team concluded that ‘the main killing agents’ were brought about by the mixing of sulfide-rich waters from the deep ocean with oxygenated shallow waters. Exactly what caused this increased mixing is still a mystery, however. ‘Sulfide is toxic to eukaryotic cells and we know that hydrogen sulfide at concentrations of a few hundred parts per million would be lethal to humans, if exposed for a prolonged time,’ explains Shen. ‘The shoaling of sulfidic waters, therefore, would kill marine animals in the Permian–Triassic oceans.’Timothy Lyons, a biogeochemist from the University of California, Riverside, US, who was not involved in the research, describes the findings as ‘tantalising’.

He explains that by measuring four different sulfur isotopes the researchers could fingerprint specific sulfur metabolic microbial pathways, gaining insight into the prehistoric ocean’s chemistry. ‘When viewed in a temporal context, [the data] points to highly dynamic conditions and major perturbations to the marine system,’ he says. ‘Evidence for episodic intrusions of sulfide into shallow waters may be the smoking gun for both the mass extinction and the protracted biotic recovery that followed.’

Although the end-Permian extinction happened over a quarter of a billion years ago, the events that triggered it have added significance in the light of current-day climate change. ‘Satellite remote sensing has clearly shown that massive fish mortality in the Namibian coastal waters are associated with sulfidic waters,’ says Shen. ‘As a matter of fact, due to human-induced eutrophication and global warming, the episodic occurrence of hydrogen sulfide has been reported in many coastal waters – such as those of the Gulf of Mexico, California and western India.’

This warning is echoed by Lyons, who notes that there ‘are lessons to be learned from this event, as we face warming in the present-day ocean’.

 China develops world's brightest VUV free-electron laser research facility

A team of Chinese scientists announced on Jan. 15 that they have developed a new bright VUV FEL light source, the Dalian Coherent Light Source (DCLS), which can deliver world's brightest FEL light in an energy range from 8 to 24 eV, making it unique of the same kind that only operates in the VUV region.

Vacuum Ultra Violet (VUV) light sources are especially useful for sensitive detection of atoms, molecules and clusters. It can also be used to probe valence electronic structures of all kinds of materials.The development of high gain free electron lasers (FEL) has captured great attentions in the scientific community in the last decade.

 It can provide by far the brightest light sources from VUV to X-ray region, where conventional laser technology cannot reach.Recently, a series of high gain FEL light source facilities in the X-ray and soft X-Ray region have been successfully developed in the world (LCLS, USA; SACLA, Japan; FLASH, Germany; and FERMI, Italy), with a few others currently under development. The LINAC based Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center have given scientists large hopes to make new scientific discoveries in many frontier research areas with these facilities.

Many institutions in China such as University of Science and Technology of China (USTC), Tsinghua University and Institute of High Energy Physic were involved in the development of this FEL facility. This project was funded by National Natural Science Foundation of China and Chinese Academy of Sciences.

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