A RYDBERG-ATOM IONIZATION SOURCE FOR NEGATIVE-ION MASS-SPECTROMETRY

An ion source for negative ion mass spectrometry has been developed. The ionization method is based on the collisional electron transfer from a high-Rydberg atom (A**) to a target molecule which possesses a positive electron affinity. In this ionization process, negative ions are directly produced via resonance capture of the ''quasi-free'' Rydberg electrons: A** + M --> A+ + M-. In contrast to negative chemical ionization, the Rydberg-atom ionization proceeds at a low pressure of almost-equal-to 10(-3) Pa. The method was applied to several molecular systems, such as 1-decanethiol, 3-nitrobenzyl alcohol and o-nitrophenyl octyl ether.