A photoion–photoelectron coincidence study of (CO)2 and (CO)3

Abstract

The photoion–photoelectron coincidence (PIPECO) spectra for (CO)+2 in the wavelength region of 620–990 Å have been obtained at different nozzle stagnation pressures (P0). The ionization energy for (CO)2 to CO+(X̃)⋅CO is determined to be ≲12.73±0.05 eV (974±4 Å), indicating that CO+(X̃)⋅CO is bound by more than 1.29 eV. The PIPECO measurements also provide evidence that CO+(Ã,B̃)⋅CO are bound with dissociation energies >0.3 eV. At P0≤200 Torr and a nozzle temperature (T) of 120 K, nozzle expansion conditions which minimize the formation of (CO)n (n≥3), the intensities for the CO+(Ã,B̃)⋅CO PIPECO bands are found to be negligibly small compared to that for the CO+(X̃)⋅CO PIPECO band. This observation supports the conclusion that the excited CO+(Ã,B̃)⋅CO ions are dissociative, with dissociation lifetimes<42 μs. This conclusion is contrary to that of the preliminary study which is based on the PIPECO spectrum for (CO)+2 measured at P0=350 Torr and T=120 K. Assuming that the radiative lifetimes of CO+(Ã or B̃) and CO+(Ã or B̃)⋅CO are identical, we estimate that the dissociation lifetimes for CO+(Ã)⋅CO and CO+(B̃)⋅CO are ≲4 μs and ≲50 ns, respectively.