Spectroscopy of Negative Ions

Abstract

This paper is concerned with the spectroscopy of negative ions — ions which have either a permanent or temporary excess of negative charge. Negative ions which retain their extra electron for an indefinite period of time are considered stable: additional energy is required to remove the electron. Negative ions which hold on to their extra electron for 10-6 seconds are metastable: this period of time is long enough for the ion to be useful for a laboratory experiment. Negative ions which last 10-12 sec. are a noticeable entity decaying predominately by electron emission. Shorter lived species, 10-15 sec., having a fleeting moment of existence, can be readily observed as a broad resonance in electron scattering measurements. An atom which is brushed by an electron as it passes by in 10-17 sec. is not granted the distinction of being called a negative ion.