Mass analyzed threshold ionization spectroscopy of 1-methylindole cation

The vibrationally resolved mass analyzed threshold ionization spectra of jet-cooled 1-methylindole (1MI) have been recorded by ionizing via four vibronic levels. The adiabatic ionization energy of this molecule is determined to be 60 749 ± 5 cm−1, which is less than that of indole by 1842 cm−1. This indicates that the N-methyl substitution causes a greater extent of the lowering in the zero energy level of the cationic ground state than that of the neutral. A few characteristic vibrations of the 1MI cation are observed, where the frequencies of the out-of-plane, in-plane bending and stretching vibrations of the N–CH3 part are found to be 124, 251, and 1492 cm−1, respectively.