An ion imaging apparatus has been developed in order to measure velocity and angular distributions of mass-analyzed fragment ions produced by photodissociation of mass-selected gas phase complex ions. The apparatus consisted of a double linear-reflectron time-of-flight mass spectrometer in which the 1st and the 2nd linear reflectrons were placed facing each other. The performance of this apparatus was evaluated using imaging measurement of Ca+ photofragment ions from photodissociation reaction of Ca+Ar complex ions at 355 nm photoexcitation. The focused ion images were obtained experimentally with the double linear reflectron at the voltages of the reflection electrodes close to the predictions by ion trajectory simulations. The velocity and angular distributions of the produced Ca+ ([Ar] 4p1, 2P3/2) ion were analyzed from the observed images. The binding energy D0 of Ca+Ar in the ground state deduced in the present measurement was consistent with those determined theoretically and by spectroscopic measurements. The anisotropy parameter β of the transition was evaluated for the first time by this instrument.
(a) Schematic diagram of the newly developed experimental apparatus. Gas-phase Ca+Ar complex ion was produced in vacuum by laser vaporization of a Ca rod and subsequent reaction with Ar gas. The Ca+Ar ion was photodissociated by irradiation with a ultraviolet laser, and the recoil velocity and angular distribution of Ca+ fragment ion were obtained simultaneously with an image detector.
(b) A typical image of the Ca+ photofragment ion. The angular distribution was discussed with respect to the electric vector direction of the linearly polarized dissociation laser, E.
K. Okutsu, Y. Nakashima, K. Yamazaki, K. Fujimoto, M. Nakano, K. Ohshimo, and F. Misaizu,
"Development of a linear-type double reflectron for focused imaging of photofragment ions from mass-selected complex ions,"
Rev. Sci. Instrum. 88, 053105 (2017). (8 pages).