Mass-selected Ion-molecule Cluster Beam Apparatus for Ultrafast Photofragmentation Studies: CF₃I·I^- as an example

We describe the design and implementation of an apparatus to investigate the charger transfer process and the resulting fragments after photoexcitation of the ion-molecule cluster. The apparatus includes the generation of the ion-molecule clusters in source chamber, separation and selection of the parent ion-molecule cluster in time of flight (TOF) chamber and photofragments detection after excitation of ion-molecule cluster in detection chamber. In our apparatus, the mass filter is employed to select the parent anion cluster which ensures only parent anion cluster exists to be photoexcited in the detection chamber. In addition, the linear-plus-quadratic reflectron (LPQR) is used to resolve the anion photofragments with the focus of the reflectron for a certain energy unchanged while the potentials applied to the reflectron varying. The performance of the apparatus is shown in experiments of ion-molecule cluster CF₃I· I^- photoexcited by 266 nm UV beam. Anion fragments I^- and CF₃I^- are detected and identified with neutral fragments collected together. The results are compared with existed calculation and photoelectron spectrum of CF3I· I^-. Detection of CF₃I^- directly verifies that the charge transfer inside CF₃I·I^- cluster happens when it is excited by 4.66 eV photons and implies the existence of charge-transfer excited state of CF₃I·I^-.