Coming to a lab bench near you: Femtosecond X-ray spectroscopy

​Summary: For the first time, researchers have captured the ephemeral electron movements in a transient state of a chemical reaction using ultrafast, tabletop X-ray spectroscopy. The researchers used femtosecond pulses of X-ray light to catch the unraveling of a ring molecule that is important in biochemical and optoelectronic processes.

The experiments are described in the April 7 issue of the journal Science.
“Much of the work over the past decades characterizing molecules and materials has focused on X-ray spectroscopic investigations of static or non-changing systems,” said study principal investigator Stephen Leone, faculty scientist at Berkeley Lab’s Chemical Sciences Division and UC Berkeley professor of chemistry and physics. “Only recently have people started to push the time domain and look for transient states with X-ray spectroscopy on timescales of femtoseconds.”

This light-activated, ring-opening reaction of cyclic molecules is a ubiquitous chemical process that is a key step in the photobiological synthesis of vitamin D in the skin and in optoelectronic technologies underlying optical switching, optical data storage, and photochromic devices.

“The key to our experiment is to combine the powerful advantages of X-ray spectroscopy with femtosecond time resolution, which has only recently become possible at these photon energies,” said study lead author Andrew Attar, a UC Berkeley Ph.D. student in chemistry. “We used a novel instrument to make an X-ray spectroscopic ‘movie’ of the electrons within the CHD molecule as it opens from a ring to a linear configuration. The spectroscopic still frames of our ‘movie’ encode a fingerprint of the molecular and electronic structure at a given time.”

The use of femtosecond X-ray pulses on a laboratory benchtop scale is one of the key technological milestones to emerge from this study.
“We have used a tabletop, laser-based light source with pulses of X-rays at energies that have so far been limited only to large-facility sources,” said Attar.

The X-ray pulses are produced using a process known as high-harmonic generation, wherein the infrared frequencies of a commercial femtosecond laser are focused into a helium-filled gas cell and, through a nonlinear interaction with the helium atoms, are up-converted to X-ray frequencies. The infrared frequencies were multiplied by a factor of about 300.

The researchers are now utilizing the instrument to study myriad light-activated chemical reactions with a particular focus on reactions that are relevant to combustion.

 Source: Science daily

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