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  • Carleton College Researchers Study Molecular Bond Breakage

    A research team led by Professor Tricia Ferrett of Carleton College, Minnesota, has recently completed a highly successful experimental effort to study synchrotron radiation induced bond breakage using a newly designed instrument.

    This new research effort uses photoionization coincidence detection techniques to study the fragmentation of molecules. A typical experiment involves a molecule like freon, CF2Cl2. The monochromatic photon beam is swept through the absorption edge of the Cl 2p core electron. The photon absorption event initiates a subsequent Auger decay, leading to a doubly ionized state with substantially weakened chemical bonds. The molecule then readily fragments via many pathways, depending on the input X-ray energy and the details of the Auger decay.

    In effect, the incident synchrotron beam acts as a kind of molecular 'scissors', resulting in a distribution of new molecular fragments which depends on the photon energy. In order to study these fragments, the Carleton group has constructed an electron-ion coincidence spectrometer of a new design. The emission of a photoelectron or Auger electron occurs rapidly, and is used as the 'start' pulse to measure the time-of-flight of the atomic and molecular fragments, which generate 'stop' pulses in the ion detector. Knowing the flight time of the ions determines their speed, which is used to calculate the mass-to-charge ratio of the ions. This information identifies the ion fragments and their relative abundance.


    The new detector design incorporates a novel ion lens system which substantially improves the efficiency of the ion collection. The resulting spectrometer has the power to resolve atomic isotopes, such as 35Cl and 37Cl. More importantly for these experiments, however, is that the new optics increase the collection efficiency without degrading spectral line shape, so that quantitative measurements of ion yield can be made.

    A 'singles' coincidence spectrum keeps track of all fragmentation events in which a single ion is detected following photoionization. A more difficult experiment involves the detection of an ion pair coming from the fragmentation of a single molecule. The Carleton scientists make correlation plots of the masses (flight times) of the ion pairs in such 'doubles' events, which generate spots corresponding to specific ion-pair fragmentation events. The shape, orientation, and intensity of these correlation spots yields a wealth of information about the bond breakage process. 'Only a few groups in the world have gotten this experiment to work,' Tricia Ferrett points out, 'so we are quite pleased that our system worked the first time we tried it at SRC this past summer.'

    Professor Ferrett has a philosophy that students should both 'learn how to do good science, and have a rich experience.' Her research at SRC upheld that trend by involving four undergraduate students in the latest experimental run. Kris Poduska, a junior majoring in physics, was amazed at how much she learned during the experiments. 'After this summer,' Kris says, 'I know for sure that I would like to continue in experimental physics research.'

    Mounting a new experimental effort is never without challenges. Describing her experience at SRC, Nadine Czoschke, a Carleton senior, related that 'we encountered barriers that we were not sure we could overcome. Now, the equipment is characterized and will be great to come back and do more.' In the future Nadine hopes to teach science in Western Africa while in the Peace Corps.

    Greg Harbers and Lou Madsen both worked as summer interns with Ferrett, and contributed extensively to the design and construction of the new spectrometer. Greg, a Carleton chemistry graduate in 1993, is applying for graduate school in biomedical and chemical engineering. Lou, who helped design the new TOF ion detector, will begin graduate school at Cal Tech in the fall of 1995.

    Aladdin -- Editor Brian Tonner