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
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
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 tested....it 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 firstname.lastname@example.org