A simple resonance enhanced laser ionization scheme for CO via the A¹Π state

We investigate the laser ionization process taking place when the CO molecule is exposed to vacuum ultraviolet (VUV) radiation resonant with the CO A¹Π(v = 0) ← X¹Σ⁺(v = 0) transition around 154 nm, along with the ultraviolet (UV) and visible (Red) radiation used to generate VUV by four-wave difference-frequency mixing. By measuring the CO⁺ ion recoil and a room temperature gas spectrum, it is possible to assign the ionization process as 1 + 1' + 1'' REMPI where the one-photon steps refer to the VUV, UV, and Red radiation, respectively. Resonance enhanced ionization of rotational states around J = 12 arise due to the overlap of the fixed wavelength UV (∼250 nm) with the R band-head of a transition assigned to CO E¹Π(v = 6) ← A¹Π(v = 0) with a term value of 104 787.5 cm^-1. The REMPI process is efficient and polarization sensitive and should be useful in a wide range of studies involving nascent CO.