Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlin
A physicist discusses how to visualize a molecule changing shape.
It is the dream of many a chemist to watch a movie of a molecule undergoing structural change. So how can we achieve this? One way is to use the relationship between a molecule’s absorption spectrum and its structure to deduce how the structure changes over time. However, a drawback of this technique is its reliance on prior knowledge of the molecular absorption spectrum.
Faton Krasniqi at the Max Planck Advanced Study Group in Hamburg, Germany, and his co-workers present an alternative idea: using photoelectrons ejected from molecules excited by X-ray free-electron lasers to determine molecular structures that change over time (F. Krasniqi et al. Phys. Rev. A 81, 033411; 2010).
They explain how electrons that are ejected and directly detected without any further interaction with the molecule interfere with electrons that scatter off the surrounding atoms in the molecule, thereby creating holographic patterns. These patterns encode the molecule’s three-dimensional structure. As an example, the researchers present calculations through which they reconstruct the six-membered phenyl ring in a chlorobenzene molecule.
This approach of holographic structure retrieval promises powerful insight into time-dependent molecular dynamics in the next few years. It is an idea that is well founded in earlier experiments at synchrotrons. Many of the required experimental techniques — such as the ability to position molecules in space using moderately intense laser fields — have recently been demonstrated. The Linac Coherent Light Source (LCLS), an X-ray free-electron laser at Stanford University in California, has been up and running since last year. Now it’s showtime!