Electronic Fluorescence Spectra of Gas-Phase Positive Ions

Abstract

We describe here a novel technique to observe electronic spectra of gas-phase positive ions. The ions are formed by electron impact on a supersonic beam of neutral molecules, and the fluorescent radiation from the ions is dispersed. The two particu­lar properties of supersonic beams that we exploit are:

• (a.) The density of molecules in a beam can be high, yet they all travel in the same direction in a collision-free environment. Collisional deactivation of the ion by fast ion-molecule re­actions is therefore absent.

• (b.) In the expansion, random motion of the molecules is converted into forward directed flow, producing a beam of internally cold molecules; the rotational temperature can be less than 1°K.