15min:

S. T. GIBSON, S. J. CAVANAGH, M. N. GALE, E. H. ROBERTS AND B. R. LEWIS, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia (email: Stephen.Gibson@anu.edu.au); J. R. GASCOOKE, School of Chemistry and Physics, The University of Adelaide, Adelaide SA 5005, Australia.

The 532~nm photodetachment spectra of O^- and OH^- have been measured using a negative-ion beam spectrometer which incorporates a velocity-map imaging (VMI) lens, located co-axially within the ion beam. The VMI technique offers a number of advantages for the recording of photoelectron spectra, including the simultaneous detection of all kinetic energies and the complete angular distribution of photoelectrons.

To date, the energy-resolution reported for photoelectron spectra using imaging techniques have been limited to E/E \ge 2% with improvement in energy-resolution achieved only through the use of slow electrons. Our spectrum for O^- was recorded with electron energies near 0.87~eV with E/E \le 0.5%. This is a significant achievement for this technique, providing spectra with considerable detail, where individual fine-structure and some rotational transitions are resolved. Measurements above threshold provide more stringent tests on the usefulness of near threshold theories of photodetachment.

The VMI image of O^- is visually similar to OH^-, with a propensity for the electron to be ejected at 90^o to the laser polarization (asymmetry parameter -1), reflecting the detachment of an electron from the p-orbital of the oxygen atom. Detail in the spectra reveal different angular distributions for individual transitions, reflecting the nature of the fine-structure transition and the interference between partial waves.