HIGH RESOLUTION VELOCITY-MAP IMAGED PHOTODETACHMENT SPECTRA OF O$^-$ AND OH$^-$

Abstract

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 $\Delta E/E \ge 2\%$} 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 $\Delta E/E \le 0.5\%$. 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 $90^irc$ to the laser polarization (asymmetry parameter $\beta\sim-1$), 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.