Electronic absorption spectrum of CO⁺in an ion beam

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.