Two color photoionization spectroscopy of jet cooled aniline: Vibrational frequencies of the aniline X̃ 2B1 radical cation

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Abstract
Photoionization-efficiency spectra of free jet expansion cooled aniline in the 1B2 excited state have been obtained by the technique of two color photoionization spectroscopy. The adiabatic ionization potential of rotationally cooled (<5 K) aniline is determined to be 62 265±18 cm−1. By pumping a variety of vibronic transitions, photoionization-efficiency curves are obtained from discrete vibrational levels in the 1B2 state. The spectra generally show a strong propensity for vertical (Δv=0) ionization. As a result, it has been possible to determine the vibrational frequencies of the ground state aniline 2B1 radical cation with good precision for six vibrational modes. Vibrational progressions observed in the threshold curves support these assignments as well as allowing the observation of two additional modes in the ion which do not manifest themselves in the Ã(1B2)←X̃(1A1) electronic spectrum. The magnitude of these vibrational frequencies compared to those known for the ground and first excited state of aniline suggest that the 2B1 ionic state is more rigidly planar than the 1B2 excited state.