A reflectron-type time-of-flight (TOF) mass spectrometer is useful as an apparatus for the study of photodissociation reactions of cluster ions. In the present study, we have developed an apparatus using a linear-type tandem reflectron. Duncan et al. also reported a different type of apparatus for observation of images of the fragment ions from mass-selected ions . The apparatus was designed to observe the images of the photofragment ions from mass-selected ions under velocity map imaging condition. Performance of the apparatus was examined by the ultraviolet photodissociation experiment of M+Ar (M = Mg, Ca) cations.
Cluster ions MArn+ including M+Ar were produced by laser vaporization, and they were mass-selected in the TOF mass spectrometer, which was equipped with two reflection regions operated by pulsed electric fields. The M+Ar parent ions first ran through the 2nd reflection region (located upstream-side), and then they were reflected by the 1st reflection region (located downstream-side). The reflected ions were irradiated with linearly polarized dissociation laser (266 or 355 nm) at the middle of the two reflectrons. Photofragment M+ ions were then reflected by the 2nd reflection region and ran through the 1st reflection region. Finally, the fragment ions were detected by imaging detector. The optical images on the screen were accumulated using a CCD camera.
The photodissociation process of Mg+Ar at 266 nm has been studied extensively so far . The similar process can be expected for Ca+Ar at 355 nm. In Ca+Ar, electronic transition of E state (excited state) ← X state (ground state), which has a character of one electron excitation localized on Ca+ (4pz ← 4s, where z axis lies parallel to the bond axis), was induced by laser excitation, followed by direct dissociation into Ca+(2P3/2) + Ar. Total kinetic energy releases (TKERs) were deduced from the velocity distributions, and finally the binding energies of M+Ar in the ground state were determined to be 1330 ± 100 cm-1 for Mg+Ar and 760 ± 160 cm-1 for Ca+Ar, both of which well reproduce the energies determined in previous studies [3,4]. Anisotropy parameters BETA at the most probable velocities of Mg+ and Ca+ were also obtained to be 0.91 ± 0.06 for Mg+Ar and 1.37 ± 0.15 for Ca+Ar, which deviate from the value for the pure parallel transition, BETA = 2. These deviations can be discussed by correlation between the amount of TKER and rotational motions before dissociation.
 Maner, Mauney, and Duncan, J. Phys. Chem. Lett. 6, 4493 (2015).
 Hoshino, Misaizu et al., Chem. Phys. Lett. 630, 111 (2015), and references therein.
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