Chemists get hint of things to come with successful beamline test

July 21, 1995

By Lynn Yarris, [email protected]
Users of the chemical dynamics beamline of the Advanced Light Source (Beamline 9.0.2) got a taste of things to come during a recent experiment in which the ALS electron beam was accelerated to little more than half its full energy (1.0 GeV rather than 1.9 GeV). The low energy photons (8-12 eV) that resulted gave the users a preview of the selective chemistry they will be able to perform this fall when the beamline's current 8.0-centimeter-period undulator (U8) is replaced with a new 10.0-centimeter-period undulator (U10).

"The use of low energy photons enabled us to take a blowtorch rather than a sledgehammer approach to photochemistry," says Arthur Suits, a chemist in the Chemical Sciences Division and member of the beamline research team that includes Nobel Laureate Yuan T. Lee. "Instead of bombarding molecules with high energy electrons to create fragments for analysis, we can use soft photons to selectively ionize the products we want to study."

The 8-12 eV photons that were produced especially for this proof-of-principle experiment are at the valence energy of most radicals--atoms and molecules that possess one unpaired electron and play a vital role in combustion and atmospheric chemistry. (The valence energy is where the chemical activity of an atom or molecule takes place.) By tuning their photons to just above this valence threshold, the beamline researcher team was able to separate hydrocarbon molecules from alkylamine compounds without fragmenting larger molecules. This gave them a "near-zero background" detection capability.

Despite the abundance and potential importance of alkylamine compounds, scientists have until now lacked the tools to selectively study the dynamics of their photochemistry. When the U8 undulator is replaced with the U10 undulator --scheduled for September--researchers on the chemical dynamics beamline will have regular access to photons at energies as low as 5 eV. These photons will be delivered in an intense beam (1016 photons per second flux) of bright white light. A rare-gas filter will give the white undulator light unprecedented spectral purity.

"This will be a novel application of synchrotron radiation made possible because of the brightness of the light (a thousand times brighter than conventional synchrotron light beams)," says Suits. "Rather than serving just as a probe of stable systems, the radiation can be used for selective ionization and product detection in primary photodissociation reactions. From this, we expect to get quite a bit more information on the detailed dynamics of these processes and study systems that were previously inaccessible."

Currently, Suits and the other members of the beamline research team are using one endstation. A second branch and endstation for the chemical dynamics beamline, designed by Cheuk Ng of the Ames Laboratory at Iowa State University, is now being installed and should be complete in time for the new U10 undulator. This second branch will feature a 6.65-meter monochromator that will provide users with the highest resolution of any scanning monochromator in the world at its spectral region. The branchline will also be equipped with a state-of-the-art electron spectrometer and several lasers, including a custom-made high-resolution infrared laser.

In addition to Suits, who is the technical director for the chemical dynamics beamline research team, others working on this project include postdoc Xueming Yang, who designed the first endstation and performed the alkylamine experiment, and David Blank, a graduate student in Lee's research group. Phil Heimann of the ALS staff is the coordinator for this beamline and is overseeing the installation of its new components.