A survey of negative ions in mass spectra of polyatomic molecules

A s urvey was made of negative-ion production in 27 different molecules covering a wide variety of chemical compounds. In nine compounds no ions were found. For the other cOlr>pounds rough measurements of the appearance potentials and relative abundance of the ions are reported . About 60 different ionization processes are observed in 18 compounds. Formation of ion s by dissociative resonance attachment is t he most common process, but there are several cases of ion pair formation and of attachment without dissociation. Perchlorylfluoride gives by far t he greatest abundance and variety of ions. with lanthanum boride was substituted for the pure tungsten filament that is used routinely as an electron source. This resulted in an ion source temperature of about 150 0 C instead of 250 0 C. Negative ions formed by attachment processes were first detected by repeated sweeping of portions of the spectrum while varying t h e electron energy from 0 to 20 v, and negative ions formed by ion-pair processes were detected at 100 v. Data for ioniza­ tion-efficiency curves were then obtained by varyin g t he electron energy in 0.2-v step s from 0 to 20 v and in larger increments up to 100 v. Curves were plotted on a lineal' scale. The r elative efficiency for collecting negative ions produced by resonance capture as compared with positive ions depends criticall y on the operating conditions. The collecting efficiency of the 1800 mass spectrometer has been estimated by a method used by Marriott and Craggs (2). Lozier (6) made measurements of the relative abundance of positive and negative ions from oxygen in a nearly field-free space and found the ratio 0 - 10 +=0.04 at 35 v. This instrument gave a ratio of about 0.0125 and the collecting efficiency is taken to be 31 percel~t. This paper reports observed appearance potentials without any correction for effects of contact poten­ tials and the electric field across the ionization chamber. A comparison with published resul ts in­ dicates t.hat the correction would be less than 1 v. 3. Results