Selective detection of 13C 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 70cm. 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.