High-resolution spectroscopic techniques, such as microwave fluorescence excitation, and double-resonance spectroscopy, have been widely used to measure the rotational constants of neutral molecules and hence to determine their structures. In the case of polyatomic cations, 7] the use of such techniques faces various difficulties, even for cations in ground electronic states. The main difficulty lies in preparing a high concentration of a state-selected gas-phase cation. A neutral molecule excited to a high Rydberg state can be ionized by an electric-field pulse (pulsed-field ionization, PFI). In zero-kinetic-energy (ZEKE) photoelectron spectroscopy, the electronic spectrum of the corresponding cation is recorded by detecting electrons generated by PFI as a function of the excitation wavelength. The ZEKE method can resolve rotational lines in each vibrational band and hence allows structure determination for gas-phase polyatomic cations. However, its application in studying excited states is rather limited due to various experimental difficulties. In mass-analyzed threshold ionization (MATI), ions rather than electrons are detected. Since its spectral resolution (around 10 cm ) is not as good as that of state-of-the-art ZEKE (ca. 0.1 cm ), it has been used mostly for vibrational spectroscopy of molecular cations. An important advantage of MATI over conventional ionization techniques such as electron ionization (EI) and resonance-enhanced multiphoton ionization (REMPI) is the resonant formation of a molecular cation in a particular vibronic state. We have utilized this advantage to study photodissociation (PD), that is, MATI–PD, of vibrationalstate-selected molecular cations. Internal conversion to the ground electronic state—one of the main relaxation mechanisms for a molecular cation in an excited electronic state— often determines the spectral resolution of a PD spectrum. For example, a lifetime of 1 ns corresponds to a spectral resolution of 0.003 cm , sufficient for rotational resolution. Photodissociation spectra for the ffiA1 !~ XE3/2 transition of CH3I + measured by monitoring the CH3 + product ion have been reported by several groups. Most of the studies used EI or REMPI to generate CH3I + . However, rovibrational analysis of the spectra was difficult due to severe spectral congestion originating from the nonresonant preparation of the precursor ion. Recently, we showed that a much better PD spectrum can be obtained by utilizing CH3I + selectively prepared in the ground vibronic state by MATI. Also, the simpler rotational structure of each vibrational band in MATI–PD compared to EI–PD allows reliable rotational analysis in favorable cases such as transitions to e-type vibrational states of ffiA1. In a MATI study of the ground electronic state of CH3I + , each vibrational main band (MB) was accompanied by several satellite bands (SBs). Application of the symmetry selection rule suggested that the MB consists mostly of CH3I + in three different rotational K states, while each SB is in a single K state. This means that not only vibronic state selection, but also rotational K selection is possible with MATI if the rotational state of a Rydberg-state neutral species is not altered by PFI. In a subsequent MATI–PD study performed by subband-selective excitation, the rotational K quantum number in each SB predicted by the symmetry selection rule was confirmed. Furthermore, the presence of a single K state in each SB resulted in well-resolved rotational structures, which led to reliable rotational analysis. For CD3I + , SB-selective MATI–PD is difficult because SBs in the MATI spectrum of CD3I are not as well-separated as those of CH3I. Herein, an effort was made to separate the SBs further. We show that, in SB-selective MATI–PD, the K quantum A method is devised better to resolve the subbands of the ground vibronic band in the mass-analyzed threshold ionization (MATI) spectrum of CD3I. By selective photodissociation of CD3I + in these subbands, high-resolution spectra for the ffiA1 !~ XE3/2 transition are recorded. Spectral analysis confirms our previous suggestion that these subbands are due to cations in different rotational K states; this demonstrates the capability of MATI to generate rovibronically selected ion beams. By using the rotational constants of CH3I + and CD3I + obtained by spectral analysis, the zero-pointlevel geometries of the cations in the ~ XE3/2 and ffi A1 states are determined. To the best of our knowledge, this is the first time that the capability of MATI–PD to determine the geometry of a gas-phase polyatomic cation in an excited electronic state is demonstrated.