Isotopomer-selective overtone spectroscopy by ionization detected IR+UV double resonance of jet-cooled aniline

We report the analysis of the NH-stretching overtone absorption of gas-phase aniline C6H5NH2 and C6H5NHD between 6500 and 7100 cm−1. Spectra of jet-cooled aniline are obtained by a new IR+UV double resonance scheme, where vibrationally excited aniline is ionised by resonantly enhanced two-photon ionisation via hot-bands of the S1←S0 transition. Ionisation detection in a time-of-flight mass spectrometer allows observing isotopomer-selective overtone spectra (ISOS), for example of C6H5NHD, which occurs in a mixture with C6H5NH2 and C6H5ND2. For C6H5NH2, a strong Fermi resonance between the symmetric NH-stretching and the symmetric NH-bending vibration is identified, with a coupling matrix element W≈47 cm−1. We also discuss the interplay of stereomutation on the femtosecond timescale and excitation of high-frequency stretching modes.