Collisional stabilization of negative ions produced by a monoenergetic electron beam.

The effect of different buffer gases on the intensity of negative ions was studied using a gas chromatography/electron monochromator mass spectrometer (GC/EM-MS). The buffer gas was introduced into the ion source not to moderate the electron energies, but specifically to investigate the process of collisional stabilization of negative ions. Three different designs of ion source were tested to study this phenomenon. It was found that collisional stabilization has a profound effect on the intensity of molecular radical anions and begins to play an important role at buffer gas pressures in the order of 10 mTorr. By using a partitioned ion-forming chamber, it was shown that for optimum stabilization to take place the buffer gas should be present in the region of negative ion formation. The gases possessing internal degrees of freedom which are capable of accommodating the excess energy of short-lived excited molecular anion states showed the largest increase in molecular radical anion intensities. At the same time helium, a widely used GC carrier gas, showed sufficient stabilization properties to allow detection of the molecular radical anions of typical electron-capturing molecules with positive electron affinities.