Over 99% of the atoms in the known universe exist in the ionized "plasma" state, and the light they emit and absorb carries most of our knowledge about distant stars and nebulae. Oxygen is the third-most abundant element in the universe, and its ions play important roles in stellar as well as terrestrial atmospheres. Metastable states of O⁺ are produced by the photoionization of oxygen in the earth's ionosphere and strongly influence the chemistry of the thermosphere, a tenuous layer of the atmosphere where the space shuttle and other satellites orbit. A detailed understanding of the structure of ions and their interactions with photons is critical to many other fields, including the study of laboratory plasmas and their technological applications. Until recently, most of this understanding was based on theoretical calculations, with few experimental benchmarks.

Keeping It Real

Centuries ago, the purity of a piece of gold would be tested by observing the color of the mark it left when rubbed on a black flint-like stone called a touchstone. These days, chemical and spectroscopic techniques have made such primitive methods for determining elemental composition obsolete, but modern science has not outgrown the need for experimental "touchstones" to verify the quality of its theories and calculations, upon which many diverse disciplines may depend. For example, a detailed understanding of plasmas—ionized gases—is relevant to a wide range of areas from astrophysics (star "birth" and "death") and atmospheric science (chemistry of Earth's upper atmosphere and the aurora borealis) to more down-to-earth applications such as microchip etching, flat-panel displays, and water-purification systems. Independent measurements provide a crucial reality check on the complicated system of equations that describe the interactions of photons and ionized matter, helping to ensure that new theoretical developments are not fool's gold, but the genuine article.

Photoionization of an ion occurs primarily by two mechanisms. Direct photoionization can occur at any photon energy greater than the ionization potential of the ion. Indirect photoionization is a two-step process involving excitation of the ion's electronic core and the subsequent ejection of an electron as the ion "relaxes." Because the photon's energy is completely absorbed in the initial step, indirect ionization occurs only at discrete photon energies exceeding the ionization potential and corresponding to differences between the ion's energy levels. The superposition of the probabilities of the two mechanisms gives the total ionization cross-section, a characteristic profile whose energies, widths, and shapes provide a powerful probe of the internal electronic structure and dynamics of ions.

However, such measurements are difficult, requiring intense vacuum ultraviolet radiation to get a detectable signal from low-density ion beams. The first absolute measurements of the photoionization of a metastable ion, O⁺, were carried out at the ALS, where a fast O⁺ ion beam was merged with a counterpropagating photon beam whose energy could be continuously scanned. Two metastable excited states of O⁺, the ²P and ²D levels, are significantly populated in the ion beam relative to the ⁴S ground state and have mean lifetimes of 14 seconds and 1.3 hours, respectively, significantly longer than the ion flight time. The ion–photon interaction produced O²⁺ photoion products, which were separated from the main beam and counted. The ion and photon beam fluxes were measured simultaneously. The initial metastable-state fraction (57%) was measured in an independent experiment.

The absolute photoionization cross-section measurements for a mixture of metastable and ground-state O⁺ ions were compared with the predictions of two independent theoretical calculations. While there is correspondence between the energy positions and strengths of some of the cross-section peaks, the differences between the theoretical and experimental profiles indicate sensitivity of the theories to the set of mathematical functions used to represent the electronic states of O⁺.

Research conducted by A.M. Covington, A. Aguilar, I.R. Covington, M. Gharaibeh, C.A. Shirley, R.A. Phaneuf (University of Nevada, Reno); I. Álvarez, C. Cisneros, G. Hinojosa (Universidad Nacional Autónoma de México); J.D. Bozek, I. Dominguez, M.M. Sant'Anna, A.S. Schlachter (ALS); N. Berrah (Western Michigan University); S.N. Nahar (Ohio State University); and B.M. McLaughlin (Harvard Smithsonian Center for Astrophysics).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); DOE Nevada Experimental Program to Stimulate Competitive Research (EPSCoR); National Science Foundation (NSF); Consejo Nacional de Ciencia y Tecnología (CONACyT), México; Dirección General de Asuntos del Personal Académico (DGAPA), México; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil; Ohio Supercomputer Center; Institute for Theoretical Atomic and Molecular Physics (ITAMP), Harvard Smithsonian Center for Astrophysics; and Engineering and Physical Sciences Research Council (EPSRC), UK. Operation of the ALS is supported by BES.

Publication about this research: A.M. Covington, A. Aguilar, I.R. Covington, M. Gharaibeh, C.A. Shirley, R.A. Phaneuf, I. Álvarez, C. Cisneros, G. Hinojosa, J.D. Bozek, I. Dominguez, M.M. Sant' Anna, A.S. Schlachter, N. Berrah, S.N. Nahar, B.M. McLaughlin, "Photoionization of Metastable O⁺ Ions: Experiment and Theory," Phys. Rev. Lett. 87, 243002 (2001).

ALSNews Vol. 206, August 28, 2002