Electronic absorption spectrum of CO+in an ion beam

Abstract

We have constructed a tandem mass spectrometer system in which a CO⁺ ion beam is irradiated co-axially with an argon ion laser beam. Excitation of rotational components of the A ²Π ← X ²Σ⁺ band system is detected by measuring changes in the charge-exchange cross-section of the CO⁺ ions with CO molecules. The sensitivity of the charge-exchange detection technique is analysed. Kinematic compression of the velocity spread of the ions results from acceleration of the ion beam to a few kilovolts. As a result the Doppler width of the absorption lines is reduced to 100 MHz. We have used the high resolution to study the electronic absorption spectrum of ¹³CO⁺ and have observed the ¹³C nuclear hyperfine structure in a number of rotational components. Analysis of this structure yields values of hyperfine constants for both the ground ²Σ⁺ and excited ²Π states. The ground state isotropic and anisotropic constants are consistent with a simple description of the electronic structure, in which the unpaired electron and the positive charge are localized in an sp orbital on the carbon atom.