School Seminar Program

Extreme velocity-map imaging of photoelectrons

Dr Stephen Gibson
Atomic and Molecular Physics Laboratories

Negative ions do more than just make you feel good(a), they also play a role in a number of key physical processes, including electrical discharges (plasmas), and in facilitating chemical reactions, such as those responsible for aerosol formation, that in turn limit global warming and climate change.

The physics of negative ions concerns loosely bound quantum systems that exist only due to electron correlations. When exposed to light an electron may detach, in a photoelectric effect that is a cornerstone of quantum physics. The departing electron experiences only a short range force, carrying with it information about its former environment, the electron orbital, and the energy-structures of both the anion and remaining neutral species. In some cases, electron loss will cause a molecule to break apart, tracing a chemical reaction. Here the electron has information about the energy barrier that controls the rate of the reaction.

In AMPL we have constructed a unique and versatile spectrometer that interrogates negative-ions using laser light, imaging the resultant detached electrons. Its performance exceeds all other particle imaging spectrometers of its type in the world, providing a most detailed, “extreme”, view of photodetachment and chemical reaction dynamics. Research with this instrument links quantum physics to reaction chemistry.

This talk will demonstrate the exciting new level of spectral clarity achievable using imaging techniques, highlight some of the discoveries exposed by the high-resolution imaged spectra, and outline some future plans that exploit our unique experimental capabilities.

Steve Gibson graduated from the University of Adelaide in 1984 with a PhD in Physics, in the research area of vacuumultraviolet (VUV) molecular spectroscopy. This was followed by a postdoctoral appointment at Argonne National Laboratory, Illinois USA, studying photoionization/photoelectron spectroscopy, with Joe Berkowitz. In 1986 Steve joined the ANU, where his research continued in VUV photodissociation spectroscopy, working with Brenton Lewis. The key achievements have been the development of high-resolution coherent VUV light sources (also with Ken Baldwin), and the writing of a sophisticated computer program to solve the coupled-channel Schrödinger equation for molecular dissociation. These developments provided the first evidence of asymmetric line-shapes in molecular spectra. The model calculations have broad application to many spectroscopies and research areas. In more recent times Steve has established a fast-beam negative-ion spectrometer (with Steve Cavanagh and Brenton Lewis) for photodetachment and photofragment studies. This research is progressing from fundamental studies of the photodetachment process to studies of important chemical processes, through transition state spectroscopy.

(a)A popular web topic, although not a scientifically verified fact.

Snacks will be available after the seminar in the RSPE tearoom

Date & time

Thu 15 May 2014, 4–5pm


Leonard Huxley Lecture Theatre


Staff, students and public welcome