How fast do electrons move in a molecule? Well, it’s so fast that it only takes them a few attoseconds (a billionth of a billionth of a second) to jump from one atom to the next. Blink and you’ve missed it – millions of billions of times. Measuring such ultra-fast processes is therefore quite a job.
Scientists at the Australian Attosecond Science Facility and the Center for Quantum Dynamics at Griffith University in Brisbane, Australia, led by Professor Robert Sang and Professor Igor Litvinyuk, have developed a new interferometric technique that can measure time delays by zeptosecond (one trillionth of a billionth of a second) resolution.
They used this technique to measure the time lag between extreme ultraviolet light pulses emitted by two different isotopes of hydrogen molecules: H2 and d2interaction with intense infrared laser pulses.
This delay was found to be less than three attoseconds (a quintillionth of a second long) and is caused by slightly different motions of the lighter and heavier nuclei.
This study was published in Ultra-fast science.
The first author Dr. Mumta Hena Mustary explains, “Such unprecedented time resolution is achieved through an interferometric measurement – overlapping the delayed light waves and measuring their combined luminosity.”
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, accelerated by the same field, and then recombines with the ion giving up energy in the form of extreme ultraviolet (XUV) radiation. Both the intensity and phase of that XUV HHG radiation are sensitive to the exact dynamics of the electron wave functions involved in this process – all different atoms and molecules emit HHG radiation in a different way.
While it is relatively easy to measure the spectral intensity of HHG – a simple grating spectrometer can do that – measuring the HHG phase is a much more difficult task. And the phase contains the most relevant information about the timing of different steps in the emission process.
To measure this phase, it is common to perform what is called an interferometric measurement when two replicas of the wave overlap (or interfere) with finely controlled delay. They can interfere constructively or destructively depending on the delay and the relative phase difference between them.
Such a measurement is performed by a device called an interferometer. It is very difficult to build an interferometer for XUV light, especially to produce and maintain a stable, known, and finely tunable delay between two XUV pulses.
The Griffith researchers solved this problem by using the phenomenon known as the Gouy phase – when the phase of a light wave is shifted a certain way as it passes through a focal point.
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, is identical.
Due to their greater mass, the nuclei in D2 move slightly slower than those in H2. Since nuclear and electronic motions in molecules are coupled, nuclear motion influences the dynamics of the electron wave functions during the HHG process, resulting in a small phase shift ΔφH2-D2 between the two isotopes.
This phase shift corresponds 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 harmonics observed in the HHG spectrum – it was nearly constant and just under 3 attoseconds.
To understand their result, the Griffith researchers were supported by theorists from 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 different 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 approximation levels can reveal the relative importance of different determine effects.
Although the actual dynamics are quite complex, interference with two centers during the electron recombination step was found to be the dominant effect.
“Because hydrogen is the simplest molecule in nature and can be modeled theoretically with high accuracy, it was used in these proof-of-principle experiments for benchmarking and validation of the method,” said Professor Litvinyuk.
“In the future, this technique could be used to measure ultrafast dynamics of various light-induced processes in atoms and molecules with unprecedented time resolution.”
The study, “Attosecond Delays of High-Harmonic Emissions from Hydrogen Isotopes Measured by XUV Interferometer,” is published in Ultra-fast science.
Mumta Hena Mustary et al, Attosecond Delays of High Harmonic Emissions of Hydrogen Isotopes Measured by XUV Interferometer, Ultra-fast science (2022). DOI: 10.34133/2022/9834102
Offered by Griffith University
Quote: Measurement times in billionths of a billionth of a second (2022, December 5) Retrieved December 5, 2022 from https://phys.org/news/2022-12-billionths-billionth.html
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