2.3 Time-resolved XUV-science
Project coordinator(s): A. Rouzée, S. Patchkovskii

"CRASY Spectroscopy Plays with Quantum Physics "

C. Schröter, T. Schultz

Scientists at the Max-Born-Institute developed a novel spectroscopic method for the simultaneous measurement of molecular structure and composition. They reported their work in Science.

In our daily experience, the observation of multiple material properties is a trivial task: Even a small kid will have no trouble to sort his building blocks according to shape and color simultaneously. But in the world of atoms and molecules, every observation must conform to the laws of quantum physics, which state that an observation always changes the observed system. Therefore, simultaneous observation of multiple molecular properties is a tricky proposition.

Scientists can play with a large set of spectroscopic tools when they wish to analyze specific properties of molecules. Rotational spectroscopy, for example, can resolve different molecular structures, because each molecule rotates with a characteristic set of frequencies. Mass spectrometry can determine the mass of a molecule and its fragments, and therefore offers information about the atomic composition of the sample. To date, experiments such as rotational spectroscopy or mass spectrometry were only performed separately. The method of Correlated Rotational Alignment Spectroscopy (CRASY) now allows the simultaneous ("correlated") measurement of both, atomic composition and molecular structure.

Figure: CRASY experiments resolve mass and structure of inseparable compounds, such as the depicted CS2 isotopes.

To perform this experiment, the scientists used an experimental trick: They first used an ultra-short laser pulse to initiate a rotational motion in each molecule of a molecular ensemble. After a short time, a second laser pulse was used to remove an electron, i.e., to ionize the molecules. The mass of all molecular ions was then determined in a mass spectrometer. The rotational motion turns the molecules in space ("rotational alignment"), and thereby affects the probability to ionize. When the molecules are allowed to rotate for different amounts of time, the rotational motion is directly reflected in the number of detected molecular ions and the rotational frequencies can be calculated. With the simultaneous determination of rotational frequencies and masses, the researchers overcame the limits of the individual spectroscopic methods and obtained correlated information on molecular structure and atomic composition.

"CRASY experiments contain much more information than conventional spectroscopic experiments, because the information content scales as the product of that in individual experiments", claims Thomas Schultz from the MBI. This should permit the investigation of increasingly complex systems. The researchers first demonstrated their technique with the analysis of rotational constants for ten isotopes in a natural carbon disulfide sample. In a single experiment, they were able to reproduce all rotational constants in the literature and to determine three additional constants, which were previously inaccessible by spectroscopic measurements. "As compared to conventional rotational spectroscopy, we only require minute amounts of sample and the sample can be highly impure", continues Schultz. In the future, the researchers plan to use CRASY experiments for the analysis of photochemical reactions in DNA bases.

SKS11 C. Schröter, K. Kosma, T. Schultz
CRASY: Mass- or Electron-Correlated Rotational Alignment Spectroscopy
Science 333 (2011) 6045
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