Laser photo detachment of negative ions in a RFQ cooler

Within this project a gas-filled radio frequency quadrupole cooler for studying laser photodetachment of negatively charged ions is constructed. Laser photodetachment is a non-resonant process where the extra electron of the negative atomic or molecular ion can be removed if the photon energy exceeds the electron affinity of the respective ion. This can be used in mass spectrometry to selectively suppress unwanted isobars provided that the electron affinity of the unwanted isobar is lower than the isobar under investigation. However, being a nonresonant process the cross sections for laser photodetachment are too low and therefore the achieved suppression by overlapping a laser beam with the fast moving ions of energies up to 30 keV is not sufficient. For this reason the ions will be slowed down in the RFQ cooler to increase the interaction time and bring the efficiency to practical values.

The project is conducted in collaboration with the VERA Laboratory at the Faculty of Physics of the University of Vienna. The setup itself is situated in a laboratory space of the University of Vienna close to the VERA accelerator mass spectrometry facility. In 2013 the construction of the cooler has been finished and the commissioning has been started. In parallel, a detection beamline for the reaccelerated ions has been assembled. To identify the charged particles emerging from the cooler laser photodetachment will be used. The negative ions will be overlapped with a laser beam in crossed-beams geometry and selectively neutralized. The remaining ions will be bent away by an electrostatic bender whereas the neutrals will continue onto a neutral particle detector. This detector consists of a conductively coated glass plate where the impinging particles create secondary electrons, which are subsequently detected by a channeltron electron multiplier. The commissioning of this beamline has been finished and laser photodetachment of selected negative ions and moleculescould be shown.