Laser Optogalvanic Detection of Molecular Ions

This Letter reports the first optogalvanic detection of molecular ions (${\mathrm{N}}_2^{+}$, C${\mathrm{O}}^{+}$). The present results indicate a new optogalvanic mechanism involving direct alteration of ion mobility by laser excitation, due to a difference in charge-exchange collision rates for excited- vs ground-state ions. The technique provides a sensitive probe of ions in the cathode dark space. Ion kinetic energies are obtained from Doppler shifts and internal energy distributions from rotational line intensities.