Femtosecond X-ray spectroscopy

By Benedette Cuffari, M.Sc.Sep 8 2017

X ray spectroscopy is a widely used method to characterize a compound based on its characteristic X ray emission spectrum¹. When energy is supplied to an atom, the electrons in the atom absorbs the energy and the electrons become excited and jump to a higher energy level, until finally returning to their original orbital.

During this process of returning to the initial ground state, the energy previously absorbed from the photon will be emitted as a photon with a wavelength characteristic of that atom¹. Therefore, depending on the elemental composition, every compound will have a distinguishing X ray emission spectrum that could be detected by X-ray spectroscopy¹.

Rearrangements of molecular structures are ultrafast and can happen in a matter of femtoseconds, which is one millionth of one billionth of a second. The determination of the electronic structures of transient states of such reactions could therefore serve a great deal in understanding this transition².

As rationalized by the Woodward-Hoffmann rules, these rapid bond rearrangements involve a symmetrical conservation in the electronic journey from reactants to products. The light-activated, ring-opening reaction of cyclic molecules is a commonly occurring reaction in biochemical and optoelectronic devices². For example, this ubiquitous reaction is an important step in the synthesis of vitamin D in the skin, and in several optoelectronic technologies underlying optical switching, optical data storage and photochromic devices².

A research team from the University of California’s Department of Chemistry reported the use of femtosecond x-ray absorption spectra to track the shifts in carbon electronic states during the light activated ring-opening transformation of 1,3-cyclohexadiene (CHD) into hexatriene³. The light activated ring-opening reaction of CHD is a fundamental prototype of photochemical reactions that occurs in femtoseconds³.

This process is thought to occur through an intermediate excited state minimum known as the pericyclic minimum³. Stephen Leone’s team from the University of California was also able to take snapshots of the electronic structure of the transient intermediate state during this reaction using a femtosecond (fs) soft x-ray spectroscopy on a table top apparatus².

Leone’s team used an ultraviolet pump pulse to trigger this light activated ring-opening reaction of CHD. To produce this pulse, high-harmonic generation was used, which involved infrared (IR) frequencies of a commercial femtosecond laser to be focused into a helium (He) filled gas cell that converts the IR rays to x-rays². The non-linear interaction of the IR rays with the He atoms causes the frequencies of IR rays to be multiplied by a factor of 300 to obtain x-ray pulses. The progress of the reaction was then probed at a controllable time delay using x-ray flashes³.

Using time resolved x-ray spectroscopy, the wavelengths of x-rays absorbed by CHD were measured at a given time delay following the UV light exposure. A series of theoretical simulations modeling the ring-opening process and the interaction of x-rays with the molecule during the transformation were performed to decode the spectroscopic fingerprints that were obtained after the experiment². The results from the core to valence spectroscopic signature of the pericyclic minimum and the time-dependent density functional theory calculations revealed the overlap and mixing of the frontier valence orbital energy levels³. The valence electronic structures were also found to emerge within 60±20 fs following UV photoexcitation and decays at 110±60 fs³.

Leone’s team successfully took snapshots of the conversion of CHD to hexatriene at the femtosecond margin, which elucidated how the electrons within the CHD molecule transitioned as the ring opened to form a linear structure^2,3.

The team of Researchers believe that their work will further the understanding of the coupled evolution of molecular and electronic structures². They are also looking to study other light-activated chemical reactions that are relevant to combustion processes using the femtosecond x-ray spectroscopy².

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References:

1. Bonnelle, C. "Chapter 7. X-Ray Spectroscopy." Annual Reports Section 'C' (Physical Chemistry). The Royal Society of Chemistry, 01 Jan. 1987. Web. https://pubs.rsc.org/.
2. "Coming to a Lab Bench near You: Femtosecond X-ray Spectroscopy." Phys.org. 6 Apr. 2017. Web. 10 Apr. 2017. https://phys.org/news/2017-04-lab-bench-femtosecond-x-ray-spectroscopy.html.
3. "Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction", Science (2017). Web. http://science.sciencemag.org/content/356/6333/54.

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