Trapping ions from a fast beam in a radio-frequency ion trap: The relaxation of the ion cloud and its resulting column density

Abstract

The relaxation of trapped Cl${{}_{2}}^{\ensuremath{-}}$ ions and their resulting column density in a multipole radio-frequency (RF) ion trap have been investigated after loading the trap from an initial fast-moving beam exploiting a mechanism described recently [A. Svendsen et al., Phys. Rev. A 87, 043410 (2013)] where the injection is mediated through the exchange of energy between ions and the oscillating RF field. The temporal relaxation of the energy distribution of the trapped ion cloud was probed by observing the evolution of the resulting time-of-flight distribution of ions after extraction and fragment mass analysis in a quadrupole mass filter. The ion energy distribution was found to be essentially stationary after $\ensuremath{\sim}20$ ms. The resulting column density of trapped ions after relaxation was probed by two-dimensional position-resolved photodissociation of the trapped Cl${{}_{2}}^{\ensuremath{-}}$ ions. A detailed statistical analysis of the ion column density in the ring-electrode trap is given, and by comparison to the experimental data, a value of the maximum adiabaticity parameter of ${\ensuremath{\eta}}_{\mathrm{max}}\ensuremath{\simeq}0.28$ is inferred. It is further demonstrated how the present experimental system allows for time-resolved mass spectrometry by probing explicitly the populations of both parent (Cl${{}_{2}}^{\ensuremath{-}}$) and daughter (Cl${}^{\ensuremath{-}}$) ions as a function of time after closing the trap and after laser irradiation. Finally, it is discussed how the setup can be used to obtain absolute photodissociation cross sections via a tomographic method without assumptions on the decay law for the trapped ions.