Spectroscopy Probes Attosecond World
Researchers have probed the microworld with femtosecond spectroscopy, enabling them to investigate nuclear motion at its 10- to 20-fs timescale. Now a team from Technische Universität Wien in Vienna, Austria, Steacie Institute of Molecular Sciences in Ottawa and Universität Bielefeld in Bielefeld, Germany, is developing a means to investigate processes inside the atom that take place on the scale of attoseconds. Its efforts mark the first time that such processes have been measured with spectroscopy.
Researchers have demonstrated attosecond spectroscopy by converting 5- to 7-fs laser pulses to x-rays in a gas jet. The technique enables investigations into the subatomic world, including electronic transitions to core levels in excited atoms.
Using the "pump-probe" technique in femtosecond studies, researchers take snapshots of a fast-evolving microscopic process. An excitation, or "pump," pulse initiates the event, and a later "probe" pulse captures its evolution. Ultrafast laser pulses as short as a few femtoseconds are available, but they are not fast enough to measure inner molecular processes. Therefore, using a subfemtosecond soft x-ray pulse and a few-cycle visible light pulse, the team developed a means to produce attosecond pulses.
Such pulses, explained Ferenc Krausz of Technische Universität Wien, one of the authors of the study, enable investigations into the subatomic realm. "One important example is capturing the transition of electrons in excited atoms to core levels, which play a central role in the operation of an x-ray laser."
The efficient, short-wavelength x-ray lasers that may result could find applications across physics, chemistry and biochemistry.
The team uses a home-built laser pulse-compression system that delivers 5- to 7-fs pulses with an energy of 0.5 mJ. The laser pulses are converted into x-rays in a gas jet. To obtain attosecond pulses, a Mo/Si multilayer mirror with a bandwidth of about 5 eV centered around 90 eV and a reflectivity of 60 percent selects fractions of the generated x-ray spectrum.
In the initial experiments, the researchers used the laser pulses and the delayed x-ray pulses to investigate a krypton gas target. A time-of-flight photoelectron energy analyzer detected the emitted photoelectrons, and the pulse duration was deduced from the change of the energy spectrum vs. delay.
The system has some limitations. The conversion efficiency (of laser pulses to x-rays) is very low -- less than 10-7.
"Techniques to increase the conversion efficiency have been theoretically proposed, and first attempts to implement it have been made," said Christian Spielmann, also of the Vienna university and another author of the study. "However, the 'easiest' way to raise the number of x-ray photons is to increase the laser pulse energy."
The researchers also hope to improve the temporal resolution. Currently, the 5-eV bandwidth of the multilayer mirror limits the duration of the pulses to roughly 600 as. Spielmann suggested that the process used to generate x-ray bursts has the potential of emitting pulses down to 100 as.
"With improved technology and a more sophisticated coating design, it seems feasible to increase the bandwidth whilst maintaining high reflectivity," he said. "Together with our partners from Bielefeld, we are working on it."
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