Crazy spectroscopy tricks out quantum physics

Scientists at the Max Born Institute have developed a novel spectroscopic method that simultaneously observes the atomic composition and the spatial structure of molecules. They report on their work in the online edition of Science.

Looking at different material properties at the same time is a matter of course in our everyday lives: Even a small child can sort building blocks according to color and shape at the same time. In the world of atoms and molecules, this is not so easy, because a law of quantum physics says that you can not measure a property without changing it.

To determine the properties of molecules, scientists today have access to a multitude of spectroscopic methods. For example, with rotational spectroscopy, molecular structures can be distinguished from one another because molecules with characteristic frequencies rotate. The analysis with a mass spectrometer "weighs" molecules and their fragments and thus gives information about their atomic composition. So far researchers have only been able to perform such measurements individually or in succession, but not simultaneously. The Correlated Rotational Alignment Spectroscopy, CRASY for short, now allows the simultaneous determination of correlated properties of molecular structure and atomic composition via rotational and mass spectroscopy.

The researchers use an experimental trick to do this: they first stimulate the molecules to spin using an ultrashort laser pulse. With a time delay, they send a second laser pulse, which shoots an electron out of the molecule, thus ionizing the molecule. The rotation of the molecule in space ("rotational alignment") influences the probability with which it is ionized. The researchers repeat this experiment many times, with the molecules having different times to rotate. In this way, the rotational motion of the molecules is mapped to the number of ions and electrons produced. The weight of the resulting molecular ions is determined with a mass spectrometer, the rotation frequency can then be calculated from the time-dependent number of ionized molecules. The researchers thus outsmart the limits of individual spectroscopic methods and obtain coupled information about structure and mass.

"With CRASY, we get much more information than with traditional methods, because measuring two molecular properties at the same time not only doubles the information content, but also squared it out," Dr. Thomas Schultz from MBI. This allows the study of more complex systems. The researchers first used their method to determine the rotational constants for ten stable isotopes of a natural carbon disulfide (CS2) sample. With a single experiment, the researchers recorded all known and three previously unknown molecular constants. "In contrast to conventional rotational spectroscopy, we only need a small amount of material and our samples can also be contaminated," Schultz continued. In the future, researchers want to use this technique to understand reactions in complex biomolecules, such as DNA bases.

Original publication

CRASY: Mass- or electron-correlated rotational alignment spectroscopy

Ch. Schröter, K. Kosma, T. Schultz

Science 333 (2011) 1011-1015