We have applied mass analyzed threshold ionization (MATI) spectroscopy to record the vibrational spectra of the indoline cation by ionizing through several vibrational levels in the S-1 state. The adiabatic ionization energy is determined to be 59 26965 cm(-1). The present experimental results show that only one stable configuration is involved in the photo-excitation processes, as predicted by ab initio calculations. Comparing the electronic transition and ionization energies of indoline with those of aniline and N-ethylaniline suggests that the cyclization of the five-membered ring leads to a redshift in both quantities. When the intermediate states involve large amplitude vibrations, the intense bands that appeared in the MATI spectra correspond to similar vibrational patterns, indicating that the structure of indoline in the D-0 state resembles that in the S-1 state. The puckering of the five-membered ring, the ring flopping (butterfly), the out-of-plane ring twisting, and the N-inversion of indoline in the D-0 state are observed with frequencies of 92, 187, 235, and 706 cm(-1), respectively. These spectral bands may be regarded as the characteristic features of the indoline cation for molecular identification. (C) 2003 American Institute of Physics.