The Inside Story on Oxygen
Oxygen molecules, which make up about 20 percent of the earth's atmosphere, serve as a shield (along with nitrogen, ozone, and other molecules) against ultraviolet radiation from the sun by absorbing the radiation before it reaches the earth's surface. Therefore, understanding the absorption process and the lengthy cycle of subsequent chemical reactions that may lead to the regeneration of oxygen is of deep interest to atmospheric chemists.
At the ALS, researchers have used pulsed-field ionization photoelectron spectroscopy (PFI-PE) to study the electronic structure of oxygen cations, created when a short-wavelength ultraviolet ray (vacuum ultraviolet or VUV) ejects one electron from a neutral oxygen molecule (O2). PFI-PE is adapted from a laser technique, but the ALS provides bright beams of VUV light over a wider wavelength range than is available using lasers. Thus, scientists using PFI-PE at the ALS can probe excited electronic states of the cation that are inaccessible to lasers.
The results in these oxygen experiments exceed the highest spectral resolving power ever achieved for molecular photoelectron spectroscopy using synchrotron radiation. The researchers were also able, for the first time, to obtain spectra with clearly resolved peaks for the different rotational states of the cation in excited states with energies inaccessible with lasers.
The advantage of PFI-PE as compared to the more conventional threshold photoelectron spectroscopy (TPE) is its ability to resolve spectral peaks due to different rotations in photoionized molecules in excited states. Above, the comparison of spectra for molecular oxygen cation (O2 with a charge of +1) shows that the pulsed-field ionization method results in clearly resolved rotational structure that is obscured in the traditional technique. The marks on the Q, S, and O scales indicate where theory says the peaks should be. Comparison of measured spectra with theoretical calculations yields information about the inner workings of the molecule; for example, which electrons and molecular orbitals make dominant contributions to the creation of excited states.
Research conducted by C.Y. Ng (principal investigator), M. Evans, and S. Stimson (Ames Laboratory and Iowa State University); C.-W. Hsu (Berkeley Lab); and P. Heimann (ALS), using the photoionization endstation at Beamline 9.0.2.2.
Funding: Office of Basic Energy Sciences of the U. S. Department of Energy.
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