Selective detection of C by laser photodetachment mass spectrometry

In this paper, we demonstrate how laser photodetachment mass spectrometry (LPMS) can be used to selectively detect ¹³C⁻ ions in the presence of ¹²C⁻ ions in a low energy ion beam. An isotopically enriched beam of carbon ions consisting of equal amounts of ¹³C⁻ and ¹²C⁻ ions was extracted from an ion source. The ions interacted with a laser beam in a collinear geometry over a distance of 70 cm. Residual atoms produced in the photodetachment process were detected in a neutral particle detector placed downstream of the collinear interaction region. By making use of the Doppler effect we were able to selectively photodetach ¹³C⁻ ions. The number of detected ¹³C atoms was 13 times larger than the number of detected ¹²C atoms. The population of the excited, weakly bound ²D excited state of the C⁻ ion was depleted by the use of a second laser. This significantly reduced the background accompanying the signal arising from the photodetachment of the ⁴S ground state C⁻ ion. Different applications of the LPMS method will be discussed in the paper.