Threshold photoelectron spectroscopy is a useful tool for measuring vibrational levels of rare gas dimer ions. In this short review, features of threshold photoelectron spectroscopy are introduced. After that, the observed vibra-tional levels of Ar
2+ are described in detail. Explanations for the vibrational levels of KrXe
+ measured by threshold photoelectron-photoion coincidence technique are also given. In the subsequent study, it is revealed that the II (1/2g) states of Kr
2+ and Xe
2+ completely dissociate via optical transitions to the repulsive I (1/2g) states, while that of Ail does not dissociate. Precise analysis of the time-of-flight spectrum for the Kr
2+ fragment from the Kr
2+ II (1/2u) state is also shown. The potential energy curve of the I (1/2g) state and the lifetime of the optical transition from the II (1/2g) state to the I (1/2g) state are obtained from this analysis.
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